Q: Does a stuck thermostat set off a Check Engine Light?

Hey there. A stuck open thermostat can cause the engine to take too long to warm up, not reach proper operating temperature at all, possibly run rich, and MAYBE set that fault code.

The engine electronics are programmed to see the engine operating in a specific temperature range. If the engine is running too cool, it can fool other sensors - like the coolant temperature sensor - into thinking the engine is cold, therefore needs more fuel.

A cold engine needs more fuel to run properly. Once operating temperature is reached, the fuel mixture is leaned out. The oxygen sensor may not reach its proper operating temperature as well if the engine is running too cool.

So yes it is possible that the thermostat can be the root cause of your problems but they can also be completely separate. Code 1220 can be caused by the oxygen sensor itself, improper signal to the oxygen sensor from the engine control unit, a vacuum leak, or even a sticking or worn idle control valve.

The Check Engine Light may have turned off due to the ECU not seeing the fault that turned it on for the last several ignition cycles - starting it, driving, and shutting the engine down. That does not necessarily mean the car "fixed itself." It could just mean the ECU did not see the problem occur for the last few ignition cycles. Start by having your thermostat replaced and then have any other issues diagnosed and repaired from there.