so i wanted to know why after replacing the thermostat why is the gauge not working it always reads high

also
after driving when i put it in park it revs up to about 1500 rpms up and down up and down
whats going on there?

 

welcome to the forum. eng revs up and down, check throttle position sensor, and map sensor.
need to explain after replacing the thermostat does the gauge work on not work. if it works does it read high all the time? what did it read before you changed the thermostate. Also did you bleed the coolant system after changing the thermostate?
Does your cooling fan go on when the temp gets hot? 2 fans, cooling, and a/c fan.

 

The gauge works but always read high but there is no over heating issues. I didnt replace the thermostat the owner previous to me did about 10 000 miles ago . My fan goes on when its hot for up to 10-15 mins then stops. On cooler days it stays on 5 mins or so. thanks any help is greatly appreciated!

 

Mostbpaets stores sell different temperature thermostats. My civic did re samething and it was all a matter of going with a lower temperature thermostat to fix the problem. The closest parts dealer to you should have a website just go onlyne and do a quick registration whichbia free and look at the different temp thermostats. Try going with the lowest temped one and the problem with it running a little hot should be fixed.