Does the thermostat need replaced?

Have you verified that the thermostat is stuck open? Do you have hot heat inside after driving the car for a while? Is the top radiator hose hot or just warm? If the guage is accurate, chances are the thermostat is stuck open. How many miles are on the car?

Could be a bad temperature gauge or sensor. Could indeed be a bad thermostat. Could be a loose wire or a blown fuse.

The AC is almost certainly a different issue. The check engine light might be a third issue, but get the temperature gauge or the thermostat fixed first.

--scott

I would be surprised that the engine stayed cool enough not to even register a temp at all on the gauge. Especially in summer.

Feel the hoses and see if they are hot. If they are then check the wiring on the temp sensor and / or replace the temp sensor.

Steve B.

I suggest you give a better description of the A/C and "check engine light" problems including whether their mentions are supposed to be somehow related to the temp gauge problem.