'04 GT Temperature gauge

[BWCAVibe]

Hello,

'04 GT, 103K miles. Towards the end of this winter (which was real long for us here in Northern WI...and still is..snow this morning), the temperature gauge started to not climb above about 1/3rd, even after driving for hours. Even this weekend, at 60 F, the gauge did not rise above 1/3rd. Gas mileage did not seem to be effected at all. The check engine light code confirmed it is either the t-stat or the temp sensor. Coolant is mixed to -35 F. Typically, the gauge is just below half. The coolant was changed before winter, but the t-stat was never changed. I know I should have had that done at that time, but did not. I will be changing the t-stat. I bought a coolant temperature sensor also. Question is if I change the t-stat when the engine is cold, and the t-stat is "open" when I take it out, and the engine is lets say 50 F (outside temp 50 F), that pretty much means the check engine light is due to the t-stat, not the temperature sensor? I guess I could change both at the same time to be safe, as the sensor is only about $20. If I do change the temp sensor, is a normal plumbing teflon thread tape ok to use on the threads, or due to heat, is there an automotive version of the teflon that I should use?

Thanks

[kumquat]

Sounds extremely unlikely to be the sensor. This is 100% classic "thermostat stuck open."

Anyway, most sensors come with sealant already on the threads so no need to use anything. If you do need to use some, teflon tape is fine.