I have a 94 civic ex sedan and I replaced the thermostat about a week ago and now the temp sensor is all over the place. I replaced the thermostat cause every now and then the car would try and overheat, then it would be fine after like 10 minutes. The new thermostat was put in and the temp read fine at running temp for about a week and now everytime it gets warm it stays way above running and almost never goes back to running temp, it constantly says overheating and then will drop close to running but then back up and will do that for the whole drive, at idle it will sit still and when it’s reading very hot and I kill the motor and then key on the ignition it will read differently than when it was running.

 

Start the car when its cold, let it warm up for awhile, feel the rad hoses, as it warms, one should be warmer that the other. once warmed up to operating temp, both should be hot to touch. If not, the water pump is having trouble

 

This is a very classic case of "air in the coolant system", especially if you just replaced your thermostat, im guessing you drained the coolant along with it, which introduced air pockets.

To fix this, you should buy a spill-free funnel (found at Walmart or Amazon) in order to burp the coolant system of the air bubbles.

Good luck friend!

EDIT: I re-read your post, and you said that you had this symptom before replacing the thermostat. In that case, I'd follow youstolemybeer's advice on it being a faulty water pump.

 

This could also be headgasket leaving the chat.