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I have a 2007 Mazda6 with the 3.0L V6 that I have owned since new.

This (extraordinarily hot) summer I noticed for the first time that the radiator fan runs for 5-10 seconds after I turn the car off, even with the keys out of the ignition. Initially I thought this might be due to the extraordinary heat, but we've had a few cool days and I've continued to observe the behavior, even after short drives on which the engine is barely reaching its normal operating temperature. Now I can also hear it turning on shortly after the engine is started – something I don't remember hearing ever before (but also something I may not have noticed).

I had the coolant changed a few months ago. No signs of overheating in any condition.

My naive guess is that some temperature sensor has failed. Any tips on diagnosing or debugging the behavior?

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  • It's "normal" for a thermostatically controlled fan to run after the engine is switched off - in fact it may even start running a short time after switch off, as the residual heat in the engine transfers to the coolant. But from your description it seems like the fan might be running all the time, if it starts up when the engine starts. You could check that by looking at the fan, while the engine is idling. In that case a failed sensor is the likely cause. - unless you have had some other electrical work done recently.
    – alephzero
    Sep 12 '18 at 15:27
  • @alephzero: Yes, it seems to start up shortly after the engine starts, even if the engine is idling. No electrical work ever done to the car.
    – feetwet
    Sep 12 '18 at 15:35
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This behaviour is by design. The coolant fan will run when the coolant exceeds a certain temperature irrespective of if the engine is running or not. Some vehicles even have supplementary electric water pumps set to run after engine shut down to keep coolant circulating.

The reason for this behaviour is to prevent hot-stops forming in the engine and to stop the coolant system exceeding the temperature where rupturing seals becomes a possibility.

You'll typically see coolant fans run when there is little or no airflow across the radiator (i.e. when the car is stationary or travelling at low speed in traffic.)

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  • But it makes no sense that "by design" the fan always switches off just a few seconds after stopping the engine. Certainly on my car, if he coolant temperature is high enough for the fan to be running, usually it runs for a minute or more after switch off (and as I said in the comment, it may even start a minute or two after switch off).
    – alephzero
    Sep 12 '18 at 18:10
  • It has nothing to do with if the car is switched on or off, it’s all to do with coolant temperature.
    – Steve Matthews
    Sep 12 '18 at 18:50
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Turns out this is a very common problem with all first generation Mazda 6 cars: The Fan Control Module (FCM) has a high failure rate. I replaced my FCM and it solved this problem. Things I learned along the way:

  1. First make sure the fans themselves are not bad: They should spin freely by hand. If they do not, or if they draw more than 10A when running, then their bearings or motors are bad and that high current alone can fry the FCM.

  2. Don't buy a cheap FCM: They can be found for under $25, but those who spend less than $50 report high and rapid failure rates on the replacements.

  3. Replacement can be done in under half an hour with only a 10mm wrench and a screwdriver (to release all the electrical connectors). Lots of videos show the process. (I referenced this one.)

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