Volvo Repair: volvo 850 obd II, obd ii, temp gauge

Question
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Followup To
Question -
I have a 96 850 ,2.3 turbo.
cel is on. code is for coolant sensor.
The problem is that sometimes the engine will rev to
3k rpm. At the same time the temp gauge will drop to
the bottom and the fan will run.
After the engine is hot, it will be hard to start. If
I let it sit for a few minutes, it will start and run fine. When it doesn't start, the temp is at the bottom and the fan is running.
I replaced sensor and still have problem.
I have seen a similar condition on gm veh's, and it is usually a bad ecm. I'm not sure exactly what to check on this model.
I suspect a bad ecm. If I buy another unit,does it have to be flashed prior to installation?  Or, is it basically "plug and go"?
By the way, spark is good when engine won't start and fuel pressure is good. I did a valve job on this car about a year ago and replaced the timing belt at the same time. The engine has 175 k
miles and still runs great otherwise.
Any info on this issue would be greatly appreciated.  Thanks, Ken
Answer -
Hello Ken,  Don't buy anything yet! The problem is the temp.sensor! Replace the t/stat too we found that they go hand in hand.Both need to go or the lite will stay On.This was a common problem,Its a DIY alittle tight fit but not painful.Thank you and good hunting!  PKD
PKD  Thank you for the reply. I originally thought that the temp sensor was the problem and replaced it. Are You suggesting that the new sensor is faulty? I will take you advice and replace the t-stat, although I don't really see the connection between the t-stat and the problem. Thanks, Ken

Answer
Hello Ken,  The connection lies in the 'Comebacks' we got before we learned the t/stats were sometimes staying open or closes and that affected the aux fan and the cel monitor.Now I am working on a '96 850 with a/c activation problems and the fan is erractic and so is the a/c compressor.If the temp.sen.fails the ecm will read as too hot and prevent the a/c comp. from working. Now the only non-related item is your temp guage operation,my guage seems not affected.AS far as 'being flashed memory' there should be a label on the ecm and the drivers door 'a' or 'b' piller area if completed.These cars are now been out here for a time and we are seeing more 'never before seen' things start to appear .Ecm's may be startin to 'go goofy'I'm replaceing mine because of the multiple symtoms BUT the temp sensor will be the frist step.It may be 'lying' to the ecm or the ecm is getting flaky.  Learningly,PKD