Fault Code U4121

[Rob Stough]

I can’t find what this fault code is but I think it’s linked to the following symptoms:
When it starts and it's cold, the symptoms are:
• Windshield wipers constantly running at normal speed: not adjustable or stoppable.
• Steering wheel controls don't work.
• Seat heaters don't work.
• Trailer controls flashing constantly.
• Instrument cluster unlit with no meters or gages functioning.
• Headlights on: not adjustable.
• Blinkers don't work.
• Windows, locks, stereo, navigation, temperature controls all work.
• Truck is driveable, just without instruments or blinkers.
Once it warms up enough, all non-functioning things go back to normal.
• Once instrument cluster comes back on, the check engine warning light comes on solid, and is cleared with the OEM reader manually.
• As long as check engine light is on, remote start is disabled.

 

[hotrod45]

It sounds like it could be a connection in a wiring harness. I'd move cables and connectors around to see if you can either find it or fix it.

 

[Elevated 2013]

It sounds like it could be a connection in a wiring harness. I'd move cables and connectors around to see if you can either find it or fix it.

I agree with hotrod. I almost think it might be a connector that might be outside of the cabin and related to the BCM since it seems temperature related. It may have a touch of water/moisture in it and the moisture might be expanding the connector just enough to back it out of a tight connection when it’s cold. When it warms up, the moisture contracts and allows the tight connection to happen again. It’s definitely BCM related based on what the symptoms are though.

 

[Rob Stough]

I agree with hotrod. I almost think it might be a connector that might be outside of the cabin and related to the BCM since it seems temperature related. It may have a touch of water/moisture in it and the moisture might be expanding the connector just enough to back it out of a tight connection when it’s cold. When it warms up, the moisture contracts and allows the tight connection to happen again. It’s definitely BCM related based on what the symptoms are though.

I can’t find a BCM part online to buy. It’s a 2011 6.7L Longhorn Diesel. Does it not have a BCM? I can find PCM, but that’s it.

 

[Elevated 2013]

It should have a body control module as that’s what controls the door locks, wipers, windows going up and down, instrument panel cluster. Unfortunately I couldn’t tell you a part number but it should be under your dash or behind a kick panel I would think. I’m not as up on the Rams as I am other vehicles. I’m still learning where some things are but the theory behind my suggestion is probably pretty sound. Hopefully someone with a 2500 will chime in.

 

[Jimmy07]

I can’t find a BCM part online to buy. It’s a 2011 6.7L Longhorn Diesel. Does it not have a BCM? I can find PCM, but that’s it.

Your truck doesn’t have a BCM. It has a TIPM (the fuse box in engine bay), which is basically the fuse box and bcm combined. Double check that fault. There’s no “U” faults that start with 4.