Hot damn!!! I went to the junkyard again and bought two more ABS modules ($15 each), one of which came from a wrecked HHR. Installed the one from the wrecked HHR, and as you can see, no more ABS/Brake/ESC lights are on and no codes are present!!! Woo hoo!! And guess what!? I don't even get the VIN mismatch so I'm not sure I will need to program it. :) Update Edit: I did actually have to have the ABS module programmed. I was also able to repair the ABS module wiring harness connector with one that I got from the junkyard. Now, we still have a lot of snow so I wasn't able to drive it around for a good test, but this is the first time I've started without the C0550 error code. Thanks to you all for your help!!! I have a few things I'm going to tidy up before I'm done. For example, there is a vibration that annoys me when it is in gear and stopped, don't really notice it when in Park. And there is a rattle coming from the muffler area also when in gear, but not usually noticeable in park. Getting close. Thanks again!!

 

But your gate turned into a jar.

 

HA! Yeah, I had my "gate ajar" while ensuring the battery was disconnected while uninstalling and installing the ABS modules. It's getting down to single digits again this weekend and maybe some snow, so hopefully I fixed the gate ajar issue.

 

Another update: Turns out the ABS module did need programming since the VIN mismatch code came back upon next ignition cycle. I thought it was gone since it cleared and stayed cleared while driving, but came back next time I drove it. Called my local Chevy dealer and they had a guy who has been working on Chevy vehicles for around 20 years and he knew how to program my the old 2010 ABS module. :) There was another service guy who didn't know how to do it so it must be dependent on the mechanic's experience. I had the ABS module programmed and an alignment done; all work completed successfully which means the ABS module is good and all my suspension work I did seemed to be done correctly. :)

Now, I have a couple more things that I want to sort which I've just been putting off. There is a vibration that happens mainly when in Drive or Reverse and stopped with brake applied. The vibration isn't really noticeable when in Park or Neutral or when driving down the road. I also have an intermittent rattle that is coming from somewhere in the exhaust pipe which is also mainly noticeable when stopped in Drive or Reverse. I'm hoping when I fix the vibration, it will also fix the exhaust rattle.

Based on my limited experience (I'm an IT guy during the day), I suspect one or more of the motor/transmission mounts. I've asked ChatGPT and Gemini (just to see what they say as I know they are frequently wrong), and they both say it's probably one of the upper mounts which hold a lot of the vertical weight, passenger side or driver side under the fuse box, but I've also read some posts online that state the lower rear transmission mount could also be at play. What do y'all think I should start with?

 

First place to look is the air box mounting bracket. Then make sure the rear brakes are properly adjusted. Then heck all 4 hubs. Then check the front rotors for warpage.
That is assuming you have already checke4d the flex pipe and the LCA bushings are good.