I have this problem with my cooling fan not shutting off after I turn off my car. Here is the deal:

When I don't use the heating system, everything is fine.

When I turn the knob to AUTO or Manual heating, the cooling fan starts to run immediately, and stays on for a while even after I turn off my car, and drains the battery and making it difficult to start the car due to low battery.

Does anyone know the possible cause for this? Any tips on how to fix this myself?

I was wrong about the problem. I believe the cooling fan stayed on because of the thermostat is not functioning properly. When I started my car today (it was -5 Celsius outside), the temperature gauge says the car was heated already and the fan started running immediately. I am guessing sensor/thermostat was frozen, but don't understand when it stays on after I turn off my car.

Please help!!!

You will probably get a check engine light soon. When the ECM sees a problem with the thermostat or temp sensor, it runs the cooling fan as a failsafe measure. Replaceing the t/stat and sensor will probably fix your problem.By the way, volvo thermostats usually crap out shortly after 60k.To quick check the ECM side of things, unplug the temp sensor (located in the thermostat housing) and start the engine and measure the voltage across the harness side of the conector. You should get 5 volts. If so, the wiring to the ECM is probably ok. Most of the time, the t/stat sticks open, causing the problem you are having. I usually recomend replacing the t/stat and sensor together as you have to remove the t/stat to get to the sensor anyway.