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?

Asked by for the 2005 Ford Explorer
Possibly a bad coolant temp sensor? Does the car overheat?
2 more answers
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