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

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

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!
  1. Home
  2. Questions
  3. Does a stuck thermostat set off a Check Engine Light?

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: Where is the best place I can get a 2004 BMW 325i timing belt and water pump?

You're wise to be taking care of this now. If the timing belt fails while you are driving, serious engine damage will result. Additionally, since the water pump on the 325 is driven by the timing belt, this is a...

Q: Bad thermostat causing radiator leak.

Just to be informative the radiator and the overflow tank are two separate parts that both store coolant and if one is leaking the other is not leaking. Also a bad thermostat will not cause a radiator or an overflow...

Q: Check Engine Light is on - codes P0313, P0299, P0304

Hi there. Based on what you've described, this could be a combination of faults between 2 or more things; however, I don't believe the coils and spark plugs are at fault. One of the codes is set during a low...

Related articles

How Long Does a Heater Control Valve Last?
Keeping the right amount of coolant in a car is essential in keeping the engine at the right temperature. Failing to have the right amount of coolant or even bad elements...
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...
P2103 OBD-II Trouble Code: Throttle Actuator Control Motor Circuit High
P2103 means there is a fault with the throttle actuator control motor circuit, likely due to a defective electrical component or part.