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Ford Focus Heat gauge

my 2000 focus has an erratic heat gauge, the coolant is good, the engine temp has been fine, the fan just got replaced and is now fine, the fuses all checked out… i have noticed that when i turn the ignition prior to starting the engine i can get an accurate reading of the heat gage, but when i start the engine the gauge drops around the time the fan clicks on, and then throughout the drive the gauge will rise to the middle, or go to the red overheating area or do nothing at all. What could possibly cause this??? and where do i look to see it…

I suspect a faulty coolant temperature sensor. Some are located near the thermostat. There may just be a poor electrical connection though.

Also, it could be a failing or sticking thermostat. When was the last time you had a coolant flush-n-fill?

I’m guessing that when you say the engine temp has been fine that you’ve measured it in some other way. I.e. how do you know the gauge is faulty? - b/c your description just sounds like the car is getting overheated and the gauge is telling you that.

In which case I’ll second checking the thermostat and doing a flush.

I will go along with the thermostat, stuck open. Car accelerates, temp drops. Car stops, temp rises.

yes, i will empty the coolant check out the thermostat and put more coolant back in, and see if it helps, the temp was measured for me, and i can always check the accurate temp by turning off the car and switching to the 2nd spot on the ignition, thankyou all for reponse to my post.-telisssa

thankyou for your help, before i check the thermostat i want to test the connection on my heat gauge sending unit, where is that located? and is that the same as the coolant temp sensor?