DSTC Service Required, then limp mode, then total failure

[gregsl]

Hello everyone.
This week, my 2011 S80 had the dreaded DSTC Service Required show up, with the squiggly lines and everything. The car would soon go into limp mode. This happened twice, I pulled over both times, re-started the car and the DSTC was back on (warning off), and drove the rest of the way ok. I took it to the dealer and they told me the battery was corroded and too small for the vehicle -- ok, so maybe not enough voltage. After they overcharged me for that , I shook his hand and got into the car, and the DSTC warning instantly came on. So they took the car in for another four hours to figure out the issue, testing various modules and diagnosing all the electronics. They said they are 95% they pinpointed it to the steering wheel module causing the electronics to go wonky. They wanted $1,800 for it, including the four hours diagnosis time. No thanks. So I go to pick up the car and... it won't start. They give me a loaner and two days later they say oh, it starts now, but it's still going to be a problem. I decide to take it to a local Volvo independent guy. On the way, the DSTC warning came back, then into limp mode for a mile, then total failure. Every warning message came up, and I couldn't get it started - telling me to put it in park or check the indicator -- just an electronic mess. I got it started a couple of times but couldn't move it from the parking brake. After letting it rest for 10 minutes it finally worked again, with only 'engine service required' still showing, and I got it to the mechanic. He said it could be a module, and since the dealer did all the diagnostics we can start with the steering module, but he also said that the steering wheel module should only make the DSTC light go on, maybe limp mode, but NOT cause the car to shut down totally like it has twice.

I guess my only option is to have the steering module replaced (at half the cost with the local mechanic) and hope the dealer was correct? Has anything like this happened to anyone else before? Could it be the steering module to cause this? I have a feeling the module will be replaced and the same issue will return a week later. But, I don't have any more options being that the dealer did a full diagnostic.

Thanks so much for even reading this far. Hopefully someone can help.