Why is the "check engine soon" light on?

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