My wife had the C1117 code on her 15 Tahoe and the dealer replaced the dash switch to fix. This post makes me think your u1556 code is the problem and might the trailer brake connector - Service trailer brake system error fixed.

 

Replaced modual no change!! I have seen service bullitin
About low voltage , both are showing 12.9,

 

See attachment I have nothing on the U-code but I think that the C1117 fix very well may take care of that also. Note the order of fixing problems.

The trailer brake module referred to (TBCM) is the one near the spare tire. If a new module is installed it must be programmed.

 

How are these programmed,I’ve been told the tbcm does not need programmed

 

Thank you kindly sir we shall c!!! I was understanding that modual didn't need flashed but the ibcm below it did ?? Darn it confused again!!

 

There are 2 devices near the spare tire, the relay, and the TCBM module, at least in my 2008 LMM. It is the module that needs programming, the relay needs no programming, just plug and play.

 

Thank you sir!!

 

https://www.duramaxforum.com/threads/trailer-brake-control-module-j95-rpo.1013721/

See post #1