P26AB – Engine Coolant Bypass Valve Diagnosis

[Oxygen454]

2013 Ram 1500 3.6L​

Fault Code: P26AB – Engine Coolant Bypass Valve / 3‑Way Heater Coolant Valve Stuck or Malfunctioning

Summary of Symptoms​

The fault began as an intermittent issue. Clearing the code would temporarily resolve it, but the CEL returned within a few days. Eventually the code became constant.

Parts Tested​

Aftermarket 3‑Way Coolant Valve (Amazon – ~$100)​

• Installed for testing; did not operate correctly, waste of money.
• Reconnected the wiring harness to the original valve for comparison.
• leaving the amazon one connected to the coolant lines, I connected the harness to the old sensor and the sensor was moving as commanded and requested by the computer. So physically the valve appears to be working correctly but the reporting side to the ECM say otherwise. I could see it fully open but reporting was 10%.

Original Valve Behavior​

• The valve physically moved and responded to ECM commands.
• Problem: ECM commanded 85% open, but the actual position reported was only 2%.
• This suggested the valve was opening, but the position feedback signal was incorrect.

New Mopar Valve (OEM – ~$440)​

• Installed and tested.
• Result: Identical readings to the original valve.

Live Data Observations​

New Valve (same as original)​

• Desired Position: 82%
• Actual Position: fluctuates between 9–10%

Additional Behavior​

• The actual position signal is choppy, bouncing between 9–10% before stabilizing.
• After a few minutes of failing to reach the commanded position, the ECM (or valve?) drops the command to 0%, as if it aborts the attempt.
• Wiggling wiring harnesses produced no change in signal behavior.

Key Note​

• With the truck in ON (ignition) but not running, the valve receives no command, which appears normal.

What would be my next step in diagnoses? I don't think the new sensor from Mopar is also toast? I'm thinking something else is wrong... I know there is a ton of threads on this and have been reading them. Part of me is hoping someone with a lot of knowledge can chime in with some help.


Pin Circuit Wire Color Gauge/Size Function
1 F947 PK/LG 20 IGN RS
2 K501 BR 20 ELECTRIC COOLANT PUMP CONTROL
3 F856 YL/PK 20 5 VOLT SUPPLY
4 K473 OR/YL 20 COOLANT VALVE SIG
5 K900 DB/DG 20 SENSOR GROUND
6 Z908 BK 18 GROUND

I could only test the ground and ignition power but all seems fine as anything is functioning, just not quite right. Not sure if I can test the computer ground without causing any issues.

 

[Oxygen454]

Just had a thought, if the ECM needs a software update to rectify this, can I do it myself or do I need a dealer? And how much...

 

[Slinge]

could try disconnecting battery cables for a computer reset ?

 

[Burla]

You have an unlocked PCM, so you should be able to. I think there is a tsb, dealer may have to flash it for free, but if you show up with after market that's on you.

 

[Oxygen454]

You have an unlocked PCM, so you should be able to. I think there is a tsb, dealer may have to flash it for free, but if you show up with after market that's on you.

What do you mean by aftermarket? The 3 way coolant valve is the new one from Mopar so its OEM. (the Amazon one is gone)
As far as an unlocked PCM, not sure I guess so? My Topdon scanner is pretty good at scanning but is not fully bi directional. I was in the process of purchasing AlfaOBD and OBDLink MX+