Temp sensor would also trigger the fan to be ON all the time... so if it's not doing that then that takes the temp sensor out of the question.
Look at your temp gauge when this is happening, if the temperature has gone down a lot when the idle goes up, then it goes warmer when the idle goes down then it most likely is the thermostat. My car was doing this just recently, problem fixed after I replaced the thermostat. Now it only stays high to warm up, then goes down to around 1000-1100 all the time.
Look at your temp gauge when this is happening, if the temperature has gone down a lot when the idle goes up, then it goes warmer when the idle goes down then it most likely is the thermostat. My car was doing this just recently, problem fixed after I replaced the thermostat. Now it only stays high to warm up, then goes down to around 1000-1100 all the time.