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The fourth generation Volkswagen Jetta, also called the MK4 or the Bora, is Volkswagen's family sedan.

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What tells the cooling fan to turn on?

My fan turns on before my engine warms up. It does turn off after 20-25 seconds of running but it’ll start back up again before engine heats up. What tells the fan that the engine needs cooling and it “the fan” needs to turn on?

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Hi @tcanderson ,

The Engine Management System turns the fan on / off.

There is a coolant temperature sensor connected to the EMS which provides coolant temperature information (different resistance readings for different temps) and the EMS uses that info along with info from other sensors to control the fan as well as other functions in the engine such as idle speed, knock control system, oxygen sensor circuit, exhaust gas recalculation and fuel tank venting.

There are two coolant temperature sensors. One for the EMS and one for the temp gauge.

Scan the OBDII port with a scantool (in the USA AutoZone will do this for free apparently) and check for any error codes as this may help to find the problem

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