OK, I'll try to keep my saga brief. I've been searching this and other boards for clues, but have reached the point where I need to ask for insights.

A couple of weeks ago, my 2001 S80 T6 with 204K miles showed a STC message, the temp gauge showed only cold and the car began running very badly. I'll have to take my wife's word for it that all these things came on simultaneously since she was driving. The trip odometer is dashes and the main odometer stopped incrementing too.

Reading how the ABS controllers are notorious and that a bad one can cause seemingly unrelated running problems by sending bad info to the ECU, I removed the ABS controller, and disconnected the battery for an hour hoping that would reset the ECU and only the brakes would be an issue. Am I right that the ECU would recover from limp home mode after being depowered? In any event, there was no improvement in the dash displays or running behavior. My OBDII reader showed no codes after this, but I know there can be codes my simple reader won't show.

I plan to resolder the ABS control anyway when I have time so it is still out of the car and of course the message now says "Brake failure, stop safely ASAP". Driven the car a couple of hundred miles since then without noticable change.

The car continues to show no temp on the gauge and the fan runs on for a couple of minutes after the engine is turned off. It is quite hard to start and harder the colder it gets (5 degrees is the lowest so far, but after much cranking it finally caught and started.) It always runs badly, but worse immediately after you get it started, with much surging. Engine seems to hunt constantly even after warmed up (and it is warmed up since there is plenty of heat.)

So now I'm thinking either the ECU is bad and accounting for everything or I have the unlikely coincidence of the ABS controller's STC circuit failing at just about the same time as the my coolant temp sender went bad and all the running issues are caused by the car having no idea what temp the engine is at.

I hope to get at the temp sender this weekend and test it, but honestly I'm leaning toward a bad ECU at this point and I know that's an expensive replacement which may not be worth it. I'm hoping someone may have seen something similar or have other ideas since after a long line of Volvos, I've had this car for a couple of years and like it very much.

Thanks in advance.