I replaced the thermostat about three weeks ago because the gauge was not moving at all and I had no heat. This worked fine but now on two occasions the gauge has been stuck in the red zone and the check engine light came on. I turn off the car and the fan runs for about 5 minutes and then its ok again. What is happening now?

 

Either the coolant temp sensor is bad or it is overheating.

 

it does not appear to be overheating .... is it a simple to change the sensor

 

It is not all that bad to replace the sensor. You have to remove the top of the thermostat housing to make it real easy. If the thermostat is older replace that and the sensor.