C121E That Persists

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ConorToot

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I have a 2012 RAM 1500 SLT, 2wd 4.7L V8 with 188,000 miles on it. It runs pretty well except for some pesky and nagging little things that are annoying. The one that bugs me the most and I need some guidance on is the ESP light, or stability control light that is always on, which prevents me from using a feature I use quite often, or used to use quite often, and that's the cruise control. When I read the faults with AlfaOBD, it gives the C121E error, Comparative Brake Pressure Sensor or something like that.
So far, I've replaced and adjusted the brake switch, I put new calipers and rotors on the front, and I put new speed sensors on the back. I had a shop flush the brake lines not too terribly long ago. I haven't noticed any softness on the brake. Maybe there is some, it goes down a short distance and is firm. If I continue to press firmly, it will give a little after a few seconds. I can try again to get any remaining air out of the lines, but I bled them several times when I put the front brakes in. Is there a way to test if the error light is caused by the master cylinder. Like I said, everything works but the light is on. I'll clear the errors with AlfaOBD, it will turn off, but within an hour or two, it will be back on. I also went through the entire process of doing the ABS and ECU Initialization. So, I've done the drive test, and each of the steps that it requested, but the light stays on.
Any suggestions?
 

62Blazer

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Did the issue start occurring before or after any of the work you did on the brake system? Not 100% clear if all the work you listed was done after the fact in an effort to fix the issue, or if you did any of the work and then the code started coming up?
If the light didn't come on until after the work was done I would suspect air in the lines. I know you said you bled them but need to verify if you need to do the special procedure that bleeds the ABS module and requires a scan tool that can do that.....I'm not 100% on this 2012 but it is pretty common on many vehicles in order to get air out of the ABS module.
 

indept

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In pre computer cars they had a switch between the 2 hydraulic brake systems so if either side lost pressure it would illuminate the brake light. Back then the switch was "ballanced" between both systems and it would "lock" in 1 side or the other when tripped. So after you fixed the leak you had to bleed each side back & forth until the switch reset. Not sure if that switch still exists on modern systems but it's worth checking into.
 
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ConorToot

ConorToot

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My apologies for not responding in a long time. I live in Houston and it seems every year we have the hottest June on record in addition to me teaching myself how to be an HVAC tech since some company put the tiniest condenser on my house's AC unit. Plus, I had termites eat the vinyl floors, load bearing members, and roof structure of the eastern side of my house that sent me into damage control mode.
To answer the questions - the light was on before I did any brake work. As of this past week, I've now replaced all of the calipers with calipers that have steel pistons as my previous ones with phenolic pistons shattered the pistons. Apparently, because we drive through flood water often in the gulf coast, the phenolic pistons will shatter if water is driven through when they are hot. That's every other week here. I also replaced the disks, pads, front lines, speed sensors (and verified they work with AlfaOBD), I've checked the steering wheel angle and it's dead on correct, and I've bled brakes several times.
The only thing I can think of is the brake bleeding although after replacing the rear brakes last week, the pedal is very solid when it had a bit of a drift previously. I would stop at a light and after 40 or so seconds, if I didn't press harder, I would start to scooch forward. This went away with the last bleed. However, bleeding brakes on my own might not be getting the job done. It's damn near impossible to hold down on the brake and tighten the fitting on the caliper at the same time. I use the bottle with fluid and hose method to bleed them. To watch for air bubbles, I use an old phone with a security camera app and aim it at the clear tubing I use to fit back into the bottle with clean fluid. It seems to work other than I still have the light, which might indicate something other than air in the line. I just don't know what else it could be because it all seems to work. If I use Alfaobd, I can test all of the valves one by one, and each valve is heard clicking as they are said to do when working. I've also test run the pump for a second and it kicked right on. Every week or two, the light will go off on my drive to or from work, but this doesn't last the whole ride as it illuminates again before I get to my destination 30 minutes either way.
When I was testing the new rear brakes last week, the thing stops amazingly fast, evenly, and without any skidding when I tested hard braking from about 75. And when it was raining last week, when I stomped the gas on a wet road the light flashed and the traction control at least attempted to work.
So, everything seems to work except that light stays on and I lose cruise control until it is off.
 
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ConorToot

ConorToot

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read both of these


Thank you. I have seen but th of these before, but on one of them it says to separate the HCU from the ECU and test. I don't know what those acronyms are or how to separate them.

Update from today:
It rained last night and this morning. When I took my lunch break, the roads were still damp. I was at a red light that turned green and my heavy foot hit the gas. When the wheels tried to spin, I heard a brief grinding noise and noticed the traction control light flashing. The wheels did t spin because the traction control kicked in just briefly but when the light stopped flashing, it stayed off when it was on all of the time previously. After work, I took the 30 minute drive home and continued to drive a little further to see if the stability control light would come back on as it usually does. It didn't! Then, I took it to my local machine car wash to run it through once. When the machine was done and the light turned green, I put the truck from neutral into drive and the wheels tried to spin, the traction/stability control light flashed, but then stayed on as it now remains on. During the rest of the drive home, I tried to turn off the traction control and see if I could get a little scratch on take off to see what it would do, but it won't spin the wheels.
I'm wondering if there is something wrong with the wheel hub like a leaking seal that allowed some water or gunk to get in where the wheel speed sensor gets it's count from. My neighbor has a Dodge Journey that was reported as having a problem with defective rear seals that this is a common problem. I ran the codes for him, but something different than the code that I had came up. I've run checks on all of the codes for the computer on my truck and other than an AC deflector out of position code, I'm not getting anything but the C121E code. I've also bought, but haven't replaced yet, the rear brake hoses and one of the lines. I have to get the second line because the aftermarket hoses and lines are separate and the stock ones are a single unit as best I can tell. My thought was that maybe one is clogged or kinked somewhere and not allowing the pressure to disbilurse evenly when the brake is pressed. However, with what happened with the light today and it turning off and back on after a very slight slip of the wheel, I'm now more focused on the wheel speed sensor system. It's a bit baffling though because I've connected a performance computer up to the OBDII and ran graphs after clearing the code/light and drove it until it came back on. The wheel speed sensors all showed very similar readings without any defect like one suddenly dropping off line and going to zero or spiking. It might be time to take the rear hubs off and see if I can check down in them for any water intrusion or to see if there is anything noticeably wrong, but I just don't think that this is it because the traction control kicks in when it is supposed to and it was when the traction control engaged that the fault cleared and reappeared. I will also need to know more about how the traction control works because I really don't know a damn thing about how it stops rear wheel spin.
 
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ConorToot

ConorToot

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Is it still the same trouble code?
The trouble code continues to return. The light turned off today but came back (read longer thread from 10/11/23). It seems to be most effected when the rear wheels spin when starting from stopped position. I've replaced the brake switch, front and rear calipers, discs, speed sensors, checked and confirmed function of the steering wheel angle position sensor, bled the abs system several times, checked for other associated codes (no other related codes present). The stability or traction control light is on and no other lights are on (no ABS light, ESP, or brake lights are illuminates). If I punch it when the roadway is wet to spin the rear tires, the light flashes and prevents rear wheel spin. Since I put new calipers (the good ones with steel pistons and not phenolic ones that shatter in Gulf Coast Texas) and racing style discs with venting to dissipate heat, I've stomped the brakes from 70mph and it brakes very evenly with no pull and very, very quickly when I tested them. I did not feel the ABS kick in, but I didn't want to lose control since I know that light is on and ABS might not work. I only tried this on a dry day because again, I know not to rely on the ABS system when that specific code and light are present. Totalling the truck in the attempt to find the cause of an error code isn't on my bucket list or short list of great ideas.
 
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ConorToot

ConorToot

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In pre computer cars they had a switch between the 2 hydraulic brake systems so if either side lost pressure it would illuminate the brake light. Back then the switch was "ballanced" between both systems and it would "lock" in 1 side or the other when tripped. So after you fixed the leak you had to bleed each side back & forth until the switch reset. Not sure if that switch still exists on modern systems but it's worth checking into.
Thank you. I have seen no reference to any switch. I use alldata for specifications for the truck since it's an extremely reliable paid for service. I wish there were a switch because I've bled the brakes many times using both the normal method and the method with the ABS pump running to remove any air that might get in that system. It's not super easy for me to do since I do all of my repair work alone.
 
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ConorToot

ConorToot

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read both of these


Okay, here is something that I noticed that also might shed light on the issue. I'm not getting the ABS light. I was looking through YouTube videos and it seems that almost every video was traction control and ABS lights as well as the link you sent on C121E. I'm not sure how significant that is, but the brakes stop super evenly when I tested them hard stopping from about 70. They stopped super fast and there was zero pull to either side, so the idea of comparative pressure issues seems unlikely as the reality is that they must be getting very similar pressure to stop evenly. If there were an issue, they should pull to that side when the brakes are pressed hard. So, I'm not sure if this isn't a false positive meaning that some sensor is t reading properly when the brakes are working perfectly fine.
 

GTyankee

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some comments from the Dodge folks

Complaint --- scan test and check cause of abs light on and scan abs light found code
Cause ------- C121E - brake pressure comparative performance
Check and found the brake pressure sensor switch working intermittently
Correction -----order new switch
replace switch
===================================

C121E-BRAKE PRESSURE SENSOR COMPARATIVE PERFORMANCE
  • When Monitored: With the ignition on.
  • Set Condition: When the Anti-Lock Brake Module indicates that the Brake Pressure Sensor Signal is irrational.
AIR IN BRAKE SYSTEM
BRAKE SWITCH SIGNAL OPEN
INTEGRATED CONTROL UNIT

==========================

 
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ConorToot

ConorToot

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Today (10/12/23), I had time after work to take another look at the truck and wasn't so exhausted that I went straight to bed after work as I had the previous two. I bled the blasted brakes again. I checked the codes and this time I saved the log files from AlfaOBD for anyone who might know something to look for within the logs and current readings that might pinpoint the issue. When I was done bleeding the brakes, I cleared the C121E error, the ESP light went from illuminated to not illuminated, I disconnected the computer, cycled the vehicle power, put it in reverse to get out of the drive, and last, I put it in drive and after two or three feet, the light came back on and the cuss words lit up the neighborhood from the front of my house. Holy crap, I really got upset.
After sitting in the truck stewing for about an hour as it got dark, I opened AlfaOBD, turned on the truck key and went through the myriad of body computer settings before I moved on to other modules. What I did was something I didn't expect the result to be as telling as it was. When I first bought my truck with 123k miles on it in 2018, it pulled to the right significantly. I had it aligned and returned with a complaint that it still pulled to the right. The tech assured me it was dead on, showed me the readouts, showed me the alignment, etc. I knew that I probably needed tires, maybe new tie rods, bearings, or something. I couldn't afford it then and almost lost the truck to repossession later.
Since last February, I've been making pretty decent money with a new job and degree. The truck was paid off, and what I spent on payments is going back in parts. New upper and lower ball joints with grease-able fittings were added, new calipers with steel pistons, racing style discs, speed sensors, brake switch, fuel dingus thingy that broke, stabilizer bars and sway bar bushings, Android stereo, Android rear view mirror, and tires, tie rods, and tie rod ends with with grease fittings were on the upcoming list after I replace the radiator that just started leaking last week.
So, I didn't get the alignment because I'm still getting front end parts installed. However, tonight, I went into AlfaOBD and reading through noticed that I could turn off the ESP, which would turn off the dash light, and would allow me to drive my truck because I know how to drive and don't need the truck to drive me. When I turned off the ESP feature, the light went away. YAY! Then I drove it. It F$*@*&#*ING drove straight as a well aimed arrow. I should be happy. I'm not because that means that Chrisler put something in the truck that has burned through brakes, lowered my fuel efficiency, caused me to doubt the poor dude who first did my alignment, and pulled my tires into racing slicks for years.
What, why, and how come my truck is pulling to the right with a good alignment, two sets of new tires over five years, and how do I get a truck that doesn't try to take control but will let me drive for a change?
By the way, I love how it drives now, except that the cruise control doesn't work.
 

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ConorToot

ConorToot

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some comments from the Dodge folks

Complaint --- scan test and check cause of abs light on and scan abs light found code
Cause ------- C121E - brake pressure comparative performance
Check and found the brake pressure sensor switch working intermittently
Correction -----order new switch
replace switch
===================================

C121E-BRAKE PRESSURE SENSOR COMPARATIVE PERFORMANCE
  • When Monitored: With the ignition on.
  • Set Condition: When the Anti-Lock Brake Module indicates that the Brake Pressure Sensor Signal is irrational.
AIR IN BRAKE SYSTEM
BRAKE SWITCH SIGNAL OPEN
INTEGRATED CONTROL UNIT

==========================

I've replaced the brake switch, verified it with a meter several times, removed it many times and reinstalled it out of lack of confidence that it was adjusted correctly. I doubt its that. I've bled the brakes so many times that I could build a fort with the used brake fluid bottles in my garage. I've done the standard, ABS, standard technique several times. Per someone's recommendation, I've tried engine on, engine off / power on, engine off / power off. Several other ideas were tried. I've run the pump through AlfOBD to try to get an air bubbles moved toward the caliper, etc. The peddle is hard as a rock, when stopped, it doesn't budge or soften, a good amount of lines were replaced. So, I doubt air is in the lines.
Integrated control unit, now this is something new. I will have to look up what it is, what all it does to see if something else isn't working, thus indicating a defective unit, and where this unit would be located.
Is the Integrated Control Unit part of the TIPM?
 
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ConorToot

ConorToot

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So, something strange happened. I replaced the switch again and noticed that the brake lights worked in reverse. There was a thread started regarding this strange development. I ended up rewiring the switch. After the replacement, it didn't mater which switch was installed, the lights worked opposite the way they should.
I also rebuild the ABS control module by disassembling it, reflowing the solder joints, and reassembling it. I doubt that this had anything to do with the fault, but it was a possibility and since I'm most familiar with electronics, this was one of the more simple options that I could undertake with confidence.
Steering wheel angle position has been essentially ruled out. I had an alignment and the tech is one that I can extend a relative amount of trust. When I asked that he have the angle set at zero and make sure that the steering wheel is actually at straight forward, he explained his method and why, which fit what I required, meaning that it was forward and at zero.
I ran more recordings of AlfaOBD monitoring wheel speed and it continues very consistently without any spikes or flat lines in the measurement. So, wheel speed sensors seem okay.
My next step might be the inertia computer. It's a pain to get to, but its one of the only things left.
Does anyone have any tips for checking it?
Beyond that, I might drain and flush all of the brake fluid and replace with all new brake fluid in case mine is contaminated. Any tips on making tha an easy process?
 
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ConorToot

ConorToot

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Today, being broke but with plenty of brake fluid, some new bleeder valves to replace the ones I've worked enough to round out, and nothing but time, I decided that I'm going to bleed the brakes until I'm sick of it, or until I run out of the quart of brake fluid. I ran out of fluid. I did the job manually with the truck on and buy bleeding each brake whether it needed it or not by doing three slow presses of the brake peddle and by connecting a tight fitting hose to the bleeder valve with it cracked and engine running. When I went to tighten the bleeder valve, I used a long extension clamp in the spreader configuration to keep pressure on the brake peddle and then crawled under the running truck to tighten the valve. After clearing the C121E code and performing a bleed on the right rear, left rear, right front, the left front calipers, I took the truck on a drive to see if the code had gone away. In the past few weeks, I would clear the code and it would stay off for a day or so, this makes me believe that replacing all four calipers and bleeding the brakes every weekend for months has me getting close to finally rooting out the problem. I also rebuild the ABS control module by removing it, de-soldering the 50 pin connector, re-flowing every solder joint possible, cleaning the unit as well as possible and re-soldering it back together then cleaning the old gasket and making a new one with an RTV silicon product made for this type of work. It still works somehow! The C121E was not as quick to come back as I stated. Today, now that I have a Phoenix Automotive head unit installed with AlfaOBD loaded directly onto it, I decided to try the AlfaOBD ABS brake bleed method. After doing the manual method as stated above, it warned me to keep the brake pressed at all times, then go open the front right (I assume it means passenger side for US vehicles). Thus, I did, and it ran the pump. Then it said to close the valve and I ran my old butt over and closed it. Then it had me open the right rear side and I ran over like a good servant of the computer that I seem to be turning into, and I opened the right rear. It ran the pump again and said close the right rear, and I followed the orders of my electron eating boss computer and I closed it. Then it said it was done with my service and the brakes are good. I took the truck immediately on a test run and before I made it 20 yards to the stop sign, the C121E error popped back up in the form of the MIL light on my dash. The computer boss and I went through this six, maybe seven times. Each time I was careful to follow exactly what it said and I put a bottle of brake fluid with hose attached to the end of the bleeder to make sure it didn't suck in air and each time, I didn't make it twenty yards before the damn light with the truck on wavy tracks showed up again and upon checking each time it was the C121E error that has haunted me for over a damn year now. I'm about to conclude this mystery by disengaging the ABS within the truck with AlfaOBD and live dangerously with me, a lowly human at the controls of a vehicle that I could not ever be possible of controlling according to the car manufacturer executives that decisded that the computers they put in the vehicles must be better at driving than the people that buy the vehicles. In most cases, I think they are correct because people are pretty dumb on average, but I think I can drive my truck without the aid of the Chrysler brains installed in the defective TIPM module that never really worked correctly.
I'm not sure what to do at this point. I do know that I don't have the funds to have any specialist work on it, just lowly me with my degree, my experience with electronics and hydrualics from working on F-18 aircraft, my years of experience working with vehicle electronics for public tranist vehicles and my ability to figure out things without much assistance. However, I am apealing to anyone who has had this issue.
Oh, recently I also replaced the front bearings/hubs with Timken ones that included new tone rings and speed sensors. They have the same number of teeth that the OEM ones did, so there was no need to change anything and I've graphed them while driving so I'm sure that they are working as well as the back ones. From periodically graphing them, I haven't seen any spike or drop to zero at any point so I'm confident that the C121E error is not coming from them.
With the wheel speed sensors working, the brake lines so clear of any air or aging brake fluid, and nothing more that I would suspect as the culprit, I can only deduce that my truck has a witch inside somewhere that holy water must be sprayed on to get her to melt. Unfortunately, I haven't found any online sources for ridding my truck of witches or where she is hiding.
 

Mister Luck

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Any of these you missed ?
  • Incorrect adjustment of brake pedal height
  • Master cylinder malfunction
  • Brake booster malfunction
  • Incorrect installation position of stop lamp switch
  • Malfunction of the stop lamp switch
  • Brake drag
  • ASC-ECU malfunction
 

Mister Luck

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Did you mention somewhere that the truck is paid off or almost paid off ?
Have you checked to see if you have a gps transponder connected in line with your OBD2 plug ?
 
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ConorToot

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Did you mention somewhere that the truck is paid off or almost paid off ?
Have you checked to see if you have a gps transponder connected in line with your OBD2 plug ?
Yeah, I bought it used with 123k miles on it and now have 207k.
I've continued to work on it. It's now been a year and a month or more since I started trying to repair the defect causing the C121E. So far, this is what I replaced in as close of a chronological order as I can recall:
- brake lamp switch
- All four wheel speed sensors
- front brake calipers, disks, pads, lines, flushed the brake fluid.
- brake lamp switch again.
- rear brake calipers, disks, pads, lines.
- flushed the brake lines again.
- front wheel hubs including tone rings and front wheel speed sensors (again).
- replaced the bleeder screws.
- flushed the brake lines again.
- rebuilt the ABS control module and tested it, it's working well.
- replaced the brake lamp switch again (#3).
- flushed the brake lines again.
- replaced all eight copper seal washers on the banjo bolts and replaced the bleeder screws and used thread sealer.
- flushed the brake fluid again.
- replaced the master cylinder tonight (2/22/24)..
- flushed the lines again (2/22/24) and bled the brakes twice, then performed the ABS brake bleed with AlfaOBD.
- on the test drive, I cleared the code, backed out of the driveway, put it in drive, withing ten feet, the light came back on. I cleared the code, put it in drive and it was on within ten feet. This happened five or six times and I wasn't pressing the gas or the brake. It would roll ten feet and, "bling", I'm the a-hole that thought he could fix Chysler's mistake. So, I drove it to an empty road, got it up to sixty, took my hands off of the wheel and slammed the brake as hard as I dared. Everything in the truck flew towards the windshield, the ABS pump kicked on, the truck came to a stop so fast that I went back in time, and it didn't falter and dart right or left. It stopped really, really short and was straight the entire time. Nothing locked up and nothing skidded. I did this three more times clearing the code before each attempt and watching the light taunt and mock my efforts each time by illuminating the traction control light within single digit feet of where I cleared it. I drove it to the hardware store to get some bug spray and it drove like a Chrysler that had a lot of upgrades and replacement parts. It all seemed fine, except the C121E code mocking me like the Dominoes Noid. Two days ago, I took it to have my brother-in-law take a look as he was/is a mechanic with fifty years of experience. He wasn't able to work on it because he's done enough to help me with my life and I don't want to ask for anything more, other than we took it to a parking lot and he drove it stomping the brakes repeatedly to get a feel for what it was doing. Because the brakes work really well no matter how it's stopped, whether stomping the pedal or easing it to a gentle stop, but we both noticed that if the pedal is pressed really hard after stopping, it will sink. Both of us thought it was a bypass leak in the master brake cylinder. He thought it might be the steering angle sensor or calibration because we were looking at it on the graphing feature of AlfaOBD, but that's because the readings were 0 at center, then slightly right of zero it would jump to 1433 abd decrease to 830 as it was turned right. Turning the wheel left of zero it would rise to 610. I didn't think about it then, but we were looking at the reading in the graphing feature while testing ABS module items. The next day, I connected to the steering wheel module and verified that the steering wheel is at zero when it's centered and goes to -599 when turned fully left and 601 when turned fully right. That's pretty much as dead on as I can imagine. So, I seriously doubt that the steering wheel angle is the problem.
There is no leaking fluid and everything is tight without any air getting in. I suspected air getting in so I've run about ten to twelve quarts of brake fluid through it and replaced the bleeder screws three times including using thread sealant on the most recent replacement. Also, I recently replaced every copper washer on the banjo bolts. Everything is hydraulic tight. It doesn't lose any fluid after driving it to and from work daily and I've kept a close eye on that.
I only have the brake booster and the ABS pump left to replace and that all of the brake system. I'm not spending any more on this problem because while I was attempting to repair it, I replaced anything that was worn out or not working like new. Below is all of the other items that I found that were worn and needing replacement:
- Front lower control arm ball joints replaced with Precision ball joints that have grease zurts.
- Front stabilizer bar bushings and stabilizers including bushings.
- Inner and outer tyrods.
- Front shocks with Rancho brand shocks.
- Front upper control arms with Mevotech forged control arms, ball joints with grease zurts, and bushings.
- Rear upper and lower adjustable control arms (my axle was a bit off and needed adjustable control arms to straighten).
- Rear shocks replaced with Rancho shock absorbers.
- 3" lowering springs in the rear to level the truck.
- A set of Midway tires which I highly recommend for trucks that are mainly on roadways.
- Front wheel hubs replaced with wheel hubs that have Timken bearings because I found a loose wheel when I had the tires replaced.

The suspension system is pretty much new. My alignment guy, who does an amazing job (I think he might be working out of a chop shop though) he said that I should consider changing the front lower control arms because I might have a bad bushing. I laughed because if I did, that would be everything. I asked why he thought there was a problem. He said that although he and three other guys couldn't get it to budge, one of the bushings he suspected might have a separation in it. I told him I would think about it, but not any time soon.
All of this work and all of these parts were due to the C121E fault code that is still there. I really have nothing more to give to the truck and I'bmve already gone way too far for a truck with 207k miles on it. I should have sold it because it's little crap like a dash light that is going to be on until the metal gets recycled in a scrap yard that will haunt me until I donate it to a charity that doesn't need another problem and would rather have the scrap value. This fault code will be the death of a truck because I don't want it anymore and I really don't care if there is a solution. It would have to be my only option if I get another Chrysler, Dodge, RAM or whoever designed this travesty. I've had a Ford truck, it sucked and drove like a crippled pig. I've had a GMC, but it burned to the ground, luckily three days after I sold it, but it wasnt much better before I sold it. I refuse to buy a Toyota because I was forced to drive Toyota when I worked for a former employer. I'm over 6' tall and Toyotas are not designed to fit people that tall comfortably nor are they designed for people who steer left handed. My credit is shot, so Nissan is out of the question. That leaves the bus. I'll be preferring to take the bus as my primary transportation soon. That's good though because I work for a public transit authority and I get free unlimited rides for life.
 

Mister Luck

cassis tutissima virtus
Joined
Aug 25, 2020
Posts
1,651
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585
Location
WEST COAST
Ram Year
2016/2017
Engine
5.7 V8
I’ve read your latest post so far three times.

Did you reuse the brake fluid reservoir on the master and did you replace the vacuum line off the master brake cylinder with a new one and use a clamp ?
 
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