I have had this car for 2 years now. After about 8 months I noticed the previous owner (or more likely his mechanic) smashed the check engine bulb, while I was fixing the odometer gear. Code 123 keeps coming up but not all the time. I usually comes on after a long highway run. I thought I had fixed it but its back with a vengance.

At this point the temp sensor has been changed 3 times and all of the old sensors tested normal after removal. I have tried 3 ECUs (951, 561, and a EGR version)and all throw the same code. All three ECUs work fine in my 89. I have replaced the temp sensor wiring harness from the sensor to the firewall. I have cleaned and checked the manifold grounds. I have tested the resistances over and over again at the ECU plugs.

Today the light came on after about 2 hours of highway driving. I stopped checked the code and cleared the ecu. The light came back on after about 2 minutes. When I got home I measured the resistances at the ECUs. They were both .21 ohm, which is low. The usual failure for temp sensors is high resistance, so I am wondering if I should believe the sensor. Could I have a hot spot in the head?

I did just replace a severely warped head because the previous owner used sealer on the headgasket that blocked some of the cooling passages. Does it make sense that some of the passages may still be blocked?

Does the ECU go into limp mode when it has a 123 error? I am wondering because the car is getting over 30mpg even with the light on.

Its getting to the point that I don't know what to do? I have spent many hours on it, as well as several very experienced Volvo mechanics.

sorry for the long post, but any suggestions would be much appreciated.