Why is the "check engine soon" light on? on 2005 Ford Explorer

I had the code read and it says it has something to do with the engine not reaching the proper temp. Any idea why this is happening?

3 answers
Possibly a bad coolant temp sensor? Does the car overheat?
Bad thermostat.
I think it should be a P0128 (Low Coolant temperature). If it takes the vehicle too long to achieve operating temperature, this code can set. Usually the thermostat is stuck partially opened. I recommend replacing it with a factory thermostat.
I have also had oxygen sensors cause this problem as well, will need equipment to test them.
Good luck