Closed Loop fuel control

My check engine light has come on and the code came back Insufficient Coolant temp. Everything seems ok, heats up good holds temp steady May be down on gas mileage a little. Any Ideas or help greatly appreciated.

Are you able to read Data/PID temps?

No I am not
 

If the code was a P0125, the thermostat is stuck partially open and taking too long for the engine to warm up to place the system intoclosed loop.  Change thermostat and clear the code.  It should be OK.