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?
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
  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: knocking sound from the back of the car

Hello there, many faults will cause a knocking noise in your 2002 BMW 325i. The most common is the driveshaft, drive axles, differential gears, transmission mount, or the rear struts. A qualified Technicians such as one from YourMechanic will be...

Q: Car overheating, but fan still runs

Hi. Several things can cause that condition. A stuck closed thermostat, blocked radiator, a faulty water pump, or low coolant all need to be checked. The fans turning on is normal when the engine gets too hot. However, if the...

Q: The truck starts then stalls or shuts off

This could be a wide variety of potential issues ranging from a vacuum leak, bad idle air control valve, low fuel pressure, bad coil packs, or many other possibilities. When this happens you should have a Check Engine Light on...

Related articles

How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
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...
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...