For the last two or three winters, my 05 has been unreliable at temperatures below -20C (-4F) or so. It'll start fine, and it acts normal as I start to drive it, but sometimes as it warms up and hits the quarter mark on the temperature gauge the temperature will suddenly drop off to nothing. I know that the engine is warm because I still get (lots!) of heat. Stopping the car is unwise unless I plan to park it, as starting it is usually impossible.

I've taken it to my mechanic and he's replaced the temperature sensor a couple of times as well as the PCM. The one time he was able to catch the problem in action he said that the temperature was being reported to the control system as -75C, which explained the starting problem as it wasn't sending the right fuel mixture to the engine to start it properly.

The problem sounds electrical to me. Since it happens as the car warms from very cold temperatures, I was suspecting moisture (from melting ice) getting somewhere and causing a short. My wife's uncle (who used to be a mechanic) suggested that he had seen similar behaviour when air bubbles were in the coolant and collected around the temperature sensor, but I haven't been able to chase this avenue yet.

Any other suggestions?

The ECM supplies a 5V source to the ECT sensor (a thermister) through a pull-up resistor as well as a ground reference. The resistance value of this thermister varies, initially high when cold and falling as the temperature rises. The ECM actually measures this in terms of the voltage drop across the ECT sensor on this circuit. So, the ECM interprets a high voltage drop as a low temperature value. In your case, -75 degrees C represents an extremely high voltage drop, i.e., 5V, probably caused by a break or very poor connection on the ECT sensor circuit. The wiring connector at the sensor, due to exposure to elements, is often the culprit but it could be anywhere including the ECM proper. There is also a remote possbility that another 5V source is shorted to the ECT sensor circuit. So, yes, your problem has to be of electric/electronic nature.

your mechanic need to measure the resistance between the pcm and the ect sensor. more than likely he'll find that there is a wire open, probably the 5v ref. wire.

its probably right at the ect connector, broken inside of the insulation.

(I realized that I mistakingly said PCM in my original message; I meant that the ECM had been replaced.)

Interesting. The last time we were in, my mechanic did notice that the soldering looked bad at the sensor and re-soldered it, but both my wife and I have seen the problem once since then. I'll double-check the wiring and connections when I have a chance. Thanks for the info.