After needing to do some front end work because of a road kill situation my cooling fan is acting up.. Basically I had to replace my radiator, a/c condenser, and intercooler after getting damaged. I replaced all the parts filled the coolant back up and did the usual process of letting it run and continuously topping it off until the system finished circulating. However now every time I run the engine the cooling fan stays running on high for atleast five minutes after I shut the car down. It does it wether I ran the engine for 2 minutes or 20 minutes, also its below freezing where I am so the engine shouldn't be getting hotter than usual.. I also noticed the fan comes on pretty quickly after starting the engine even when cold. It did not do any of this prior to replacing the radiator. Any ideas why this is happening?

 

Most likely the coolant temperature sensor has failed or the wire to it is broken. The ECU responds by switching the fan on as a precaution and letting it run for minutes after switching off ignition (even if the engine wasn't even started). You can check this by entering the hidden menu and looking at the coolant temperature: instead of a plausible temperature value the display shows ttt (three lower case 'T').

If the thermostat housing is of the newer type, the temperature sensor can be replaced by itself with minimal effort and cost. If the housing is of the older type (i.e. not allowing separate access to the integrated temperature sensor), the whole thermostat housing has to be replaced. In the latter case, make sure you get a housing of the newer type (preferrably directly from BMW as there are reports of issues with aftermarket ones).