Service Trailer Brake System?

[sdeeter19555]

Where should I start to look for the cause of this message?

 

Hit twice yesterday, no trailer (and haven't had one behind it for months).  I do use the 5th wheel wiring for high-mount brake/tails and backups on a backrack.  But all that wiring is soldered and in good shape.

 

No codes are stored that a typical code reader can see.

 

Thoughts on where to start looking?

[C/K Man]

You need a scanner that can read the ABS/stability/trailer brake systems.  These are 'C' codes.  If there isn't a trailer hooked up the problem could be with the plug if it isn't with your 3rd. brake light. 

[sdeeter19555]

Would a slipping converter or otherwise failing transmission cause this to pop up?  It did it a couple times that day, and has not returned; just had the transmission done a week ago for a bad converter. 

[davester]

No, the transmission isn't likely to be involved w that.

You need to get codes from that system, and then diagnose those.

 

If they unplugged the ITBC, either above the spare tire, or the switch in the driver's kick panel, and didn't fully plug them back in, that can cause this to pop up.

[newdude]

On that generation of truck, the trailer brake relay, the module or the dash switch are probably top 3.  Wiring is a possibility also.  If your ABS light is on that can also cause trailer brake faults.  

 

Definitely have the codes scanned with a reader that can read all modules on the vehicle.  That system does set codes.  

Edited November 18, 2025 by newdude