Hi, I have a 2007 Cobalt LS 2.2 with code u0121

The T/C, ABS and brake lights are lit up but no codes are set. Its only when you run a enhanced diagnostic with the ABS scanner the the U0121 shows up. There is no /xx code after this either.

If I let the car sit long enough without moving everything will clear. Once I start driving for a few seconds they start to light up in sequence a few seconds apart. If I've only been driving for 10-15 seconds or so and stop, the dash will clear within 5 sec or so.

I checked/reseated the ABS fuses, brake fluid is full, parking brake not engaged. I pulled the connectors off the the controller and ECM, dont see any missing pins or pushed in, nothing was correded looking. Checked wiring harnesses and don't see any chaffing etc..

U0121 = Lost Communication With Anti-Lock Brake System (ABS) Control Module

How can I be sure its the ABS controller that is bad or something else like a relay, terminating resitor, the ECM, etc...?

I've ordered the DVD Service Manual however it won't be here until next week sometime.

 

Update....

This morning the tach and gas gauge went to zero and came back and the cruze control shut off as well.
I thought I'd seen a glitch on the tach a couple times but wasn't sure since they were so small. Won't be able to check to see if there are codes until tonight.

Thought I read somewhere about a reflash for this problem?

 

Guess I'll just post as general reference that might help someone else.

Upgraded my AutoEnginuity hardware to support what I need.
The only codes being set is the U0121 by multiple modules all saying they can't talk to the EBCM.
The scan tool is also unable to connect to the EBCM.

Its looking like the EBCM may be bad. I can communicate with all the other modules except the EBCM.
There are no other codes being set other than the U0121 on realted modules not being able to talk to
the EBCM.

I guess as a last check I need to ohm the system. I see the GMLAN leads JM4 pins (36 and 37) and (21 and 22). Still working my way through that and what I should have for voltage(key on) and what pins.

Not sure what to think about the gas gauge, tach and cruze at the moment. Coincidense or maybe the EBCM started to short internaly??? I dont know, hasn't done it since.

 

I have power and ground where they are suppsed to be and the GMLAN doesn't have any opens or shorts from the DLC to the EBCM.

When the ABS lights go out I can communicate with the the EBCM(Electronic Brake Control Module) again. I live monitor shows all the wheel speed sensors reporting correctly until 12mph. Then the ABS and T/C lights come on and loose communications with the EBCM again. Once the lights go out it can talk again.

Enhanced Powertrain
U0121 Lost Communication With Anti-Lock Brake System (ABS) Control Module

Transmission
U0121 Lost Communication With Anti-Lock Brake System (ABS) Control Module

Body Control Module
U2108 CAN Bus Error ABS

Looks like an EBCM problem. The car has 50k on it and I'm told from sources that GM may replace this. I'll see what they can do for me under goodwill warrantee or whatever
and compare costs. If I buy a new EBCM I need the dealer to program it anyway, they charge ~$80 for that.