It took me a long time to finally look under my car after these symptoms suddenly appeared one day while my wife was driving our 2001 S80 T6: STC service required message showing, temp gauge reading cold (though coolant was hot), main odometer not incrementing, trip odo dashes, CEL on. Car seemed to be in limp mode and was very hard to start when cold (well below freezing) and would surge after starting and shift poorly even when well warmed up.

So suspecting the ABS controller, I removed that without getting any improvement, just more warning lights. When I began the process of getting at the coolant temp sensor to check that I dropped a tool and finally saw the two wires hanging down under the car touching the ground. Of course, it was really one wire severed by dragging on the ground and it was the one to my right, rear Oxygen sensor. After that revelation, I found this post.

https://volvoforums.com/forum/volvo-...-others-28742/

After splicing the wires back together, there is no real change except that both odometers started working immediately. I find it very strange that that an 02 sensor would fix that, but I also find it a little hopeful. But still, the idea that the bare wires on wet ground sent a short back to the ECU and fried it is the most reasonable explanation. Before I pay a shop with VADIS and get that confirmed or denied, I hope someone can answer these questions:

Does disconnecting the battery for a long period clear ALL codes, including ones I can't see with my simple OBDII scanner? And so, should that get the car out of limp mode at least until the car detects an issue which puts it back there?

If the above is not true, and assuming the "missing" O2 sensor was really my only issue, will the car, after some period of driving (and how far) decide all is well and clear the codes (and so, limp mode) on its own?

Any other ideas are welcome. I know these symptoms sound much like a classic bad ABS controller (except speedometer always worked), but I have the controller with the number beginning with 8 which supposedly almost never fail and the coincidence with the O2 cable seems much to big to blame anything else.

Thanks in advance!