2009 Honda Civic Sdn - Fan Runs Periodically When Engine Off

Dear Car Talk.
My daughter has a 2009 Honda Civic 4dr sedan with AC and about 85,000 miles on it. Several weeks ago after returning from work, she called and said the radiator fan was still running even after she shut the engine off. Having had some experience in maintaining cars for many years I thought a moment and said, “The fan will run until the engine cools down.” The next morning the battery was dead. (Lost major points on that one.)
I took her car to my trusted mechanic who spent three days trying to duplicate the problem to no avail. I returned the car to her and it ran ok for another week. Meanwhile, I checked online and found several UTube videos describing this problem for this model car, which pointed to the Fan Control Relay.
When the problem recurred, I took it back to my mechanic who, after some persuasion, replaced the relay. Several days later, the fan stayed on again.
I took it to a Honda dealer who acted as though he’d never heard of this problem before, He kept it for three days. He couldn’t reproduce the problem but suggested I replace the fan control relay (which I told him I’d already done).
Several days later, she called and said both fans were on. I took her car home and pulled the fan control relay, both fans stopped.
I left the car was outside all night. (It gets up to the 80s in the daytime and down to 60 at night.) The next morning, I put the relay back in and both fans turned on. I drove the car 5 miles to my mechanic again, turned off the engine and opened the hood: Both fans sat idle with malicious grins on their faces.
Oh Car Guru, what do I do next?


If you have a scan tool,hook it up when it happens again. If the temperature reading doesn’t correspond the way it’s supposed to, replace temp sensor. Or you could just replace the sensor anyways. They are pretty cheap and easy.

Should have said, already replaced tenp sensors.

Forget to say “Thank you “.

Not a common occurrence, but maybe the primary wire to the relay has a short to power, or ground shorted to ground. You can look for a wire diagram for this circuit and actually see what components could cause the fan(s) to turn on and test each.

Have you checked the ignition switch to see if the key comes out while it’s still in the “on” or “accessory” position?

…or does this model even use a manual key? If it’s a push button system, maybe she isn’t shutting it off?

This comment is based on general knowledge, not this particular make and model.

There may well be 2 temp sensors on this car, one in the engine cooling system probably near the thermostat and the other in the radiator. The fans are controlled by the radiator one. If you replaced the engine one it won’t help your problem.

Wild ass guess for something to try. Turn off the ac and defrost before you turn off the car. Many cars turning on the ac kicks the fan on, without delving int circuits I would try this band aid workaround.

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The only thing left that hasn’t been replaced that controls the cooling fans is the PCM/ECM.