I had a weird experience last week. I was on the freeway and the it got really cold in the car, looked at the coolant temp and it was 55 Celsius (around 125F) and going down!!! It wasnt that cold outside (-15C). When I stopped the car the temp would go up slowly, but on the road there was no way to keep it warm!!! Next morning it was -30C outside and hopefully everything was back to normal.

I check the computer and there was no code... my guess is the thermostat is sticking open?!? A week later everything still works fine. Ill be replacing the thermostat when I have the time... they are usually pretty cheap and i guess its cheap insurance. BTW the car as 152k km (90k miles)

I would do the same.. Its a good thing it didnt get stuck closed.....

Might double check the sensor and make sure the connection isnt bad, but since you said it got cold in the car, then id agree with your idea...

Certainly could happen. Thermostats start to get more and more "lazy" the older they get. The thermostat is actuated by a "wax motor". It really is a closed container of wax that expands with themperature forcing the metal plunger in the container out and actuating the thermostat. The wax is doped with copper shavings to adjust it activity rate and temperature reaction. Over time, the wax container in the thermostat gets contaminated with the coolant in the system and becomes less and less active. It could have stuck slightly open so that the stat couldn't control.... The next time you started the engine it probably warmed up due to low speed/idle and the state moved past where it was stuck and it didn't stick again.....Replacing it is a good idea while it is still on your terms.