Skip the auto shop - Our mechanics make house calls

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

asked by on

I'm pretty sure my thermostat is stuck on open, and ive had my CEL on for the past month and a half. The code my car gives me is 1221 which is supposed to be the O2 sensor. Today my CEL went off but the temperature still took quite some time to reach operating temperature. Give it to me straight doc

A: Hey there. A stuck open thermostat can caus...

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.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Loud cranking, shaking, RPM decrease upon start - 2004 BMW 325is

Hello. This problem would be considered very serious. I would recommend not driving the vehicle at all until it has been inspected by someone to determine the extent of the damage. Sounds like that are usually either harmful to the...

Q: Overheating at the traffic light or while AC is on

You may have a restriction in your radiator or condenser that is preventing the cooling of the respective fluids that go through these parts. Restrictions will cause hot spots and therefore will not allow sufficient cooling of the refrigerant or...

Q: Can the power steering return hose problems cause the Check Engine Light to come on?

Hello there, you have several different issues going on with your 2004 Toyota Corolla. Without knowing what the Check Engine Light is, I cannot determine if it has anything to do with the engine shaking or not. The power steering...

Related articles

P0052 OBD-II Trouble Code: HO2S Heater Control Circuit High (Bank 2 Sensor 1)
P0052 code definition HO2S Heater Control Circuit High (Bank 2 Sensor 1) What the P0052 code means This code is seen when the Engine Control Module (ECM) tries to control the...
P0240 OBD-II Trouble Code: Turbocharger Boost Sensor B Circuit Range/Performance
P0240 code definition Turbocharger Boost Sensor B Circuit Range/Performance What the P0240 code means P0240 is an OBD-II generic code triggered when the Engine Control Module (ECM) detects the intake boost...
What are the Car Pool Rules in Hawaii?
Hawaii is widely regarded as a land of vacation and relaxation, and as such, its scenic roads and routes are far better known than the state’s freeways. But, as with all...