All,

A couple of days ago, the check engine light turned on on my 1995 850 that has a little over 130K miles on it. The code pulled from the A2 is 1-2-3. From what has happened in the recent days, the check engine light definitely has something to do with the Coolant temperature sensor. I am posting a message here to make sure I will take care all the problems.

Here is the story:
1. Just before the light went on, I noticed there's a surge in rpm, from normal 800 to about 1200.
2. The temperature gauge acting erratically, moving between the 3 o'clock position and the 5 O'clock position (the lowest point the needle can go.)
3. Sometimes, it took a little effort to start the car and the car was running rough at the start (seemed not all cylinders were firing). It would be Ok after a couple of minutes. Temperature here in NY is about 40F during the day.
4. Every time when the temperature gauge's acting, there's a surge in rpm.
5. Seems that the car lost some coolant; but there is no obvious leak.
6. I cleared the code this morning. After several rounds of driving, it came back on. After stopping for traffic lights, I felt the hesitation when I tried to resume driving. Again, the hesitations were accompanied by the temperature "drop" (by the temperature gauge).
7. Another wired thing is that the fan kept running for a few minutes after the car's keyed off. It never happened before.

About a couple months ago, the car was overheating for a few minutes (on Highway) due to a big leak in the radiator. the radiator, tubes, and tank were replaced afterwards. Since the thermostat (volvo) was pretty much new, it was not replaced; neither was the temperature sensor.

So, my question is: besides to get the sensor (and thermostat) replaced, what else should I also check?

Thank you in advance for your inputs.

I apologize for this long posting.

Paul