1994 was the last year that you could read codes off of an 850 w/out a code reader. My first question to the dealer would be what codes did he pull. The 850's computer control is more sophisticated than that on the 240. Most problems like you describe would set a code. A failed engine coolant temperature sensor (ECT) would likely set a code, but the failure mode would be the computer (ECU) would get a message from the sensor that it's either impossibly cold or impossibly hot. If the sensor was simply off a bit (like telling the ECU the engine temp was 150F when it was really 220F) it might not set a code. $300 to change the ECT seems pretty high to me. Call another dealer and ask the fee for doing the same. I'm guessing the $300 includes other things.