Q: Check Engine Light on again after thermostat replacement.

If the check engine light is coming on and a code like P1217 is in the computer then the computer is seeing an overheat conditions from the coolant sensor telling it your engine is overheating. This can be caused by a defective coolant sensor, radiator partly clogged, fans not working, thermostat or internal engine head problems. The system should be checked with an infrared temperature probe to see if the temperature of the engine is close to that the computer is seeing from the coolant sensor. If the coolant temperature sensor input to the computer is within about ten degrees of what the infrared tested is reading on the outside of the sensor then this would be okay. The problem may be somewhere else in the system and would need further diagnostics to check if other components are working properly. The fans will need to be working along with the water pump pumping coolant through the radiator and the radiator cooling the engine as designed. I recommend having the vehicle inspected by a certified mechanic to diagnose your Check Engine light and come up with any repairs needed.