I have a problem with my w204 c63...fan come on after a few minutes of driving and spins at hi speed..only goes off wen I switch the car off...so I changed the thermostat..car still doing same thing..heat gauge shoots up to 120 so I stopped..but car never really heated up..so I restarted the car..temp gauge dropped but was erratic a bit then settled above the 90 mark..but fan was still on..I then read a thread abt changing the coolant temperature sensor..which I did..car still doing same thing..fan comes on at hi speed after few minutes of driving and onky goes off wen I switch the car off...i actually don't know wat to do next..or maybe take it to mercedes to diagnose the fault...and also wen diagnostics are run ..no faults appear

Is your Aircon on because my fan runs continuously with AC on but turns off if engine cold and AC switched off.
BTW I have my AC always on.

Hi...well I drove the car with the aircon on and also with it off..still does the same thing...after a few minutes of driving I think once the engine heats up then the fan comes on comtinuosly tolli switch off...I dont know if the new thermostat I put in is faulty but the problem is that wen u run diagnostic it doesn't pik up any errors or faults

Is it the main radiator fan or the oil cooler fan that comes on? What are the coolant and oil temperatures (the digital readout) when the fan comes on?

Hi...no it's the radiator fan..the main fan that comes on....I haven't done a digital reading but looking at my temp gauge it looks like it's somewhat above the 91.5...but then again it fluctuates now and then..rite up to 120..then comes bck down to normal after I switch off and restart...but the engine doesn't really heat up..no signs of it boiling either..so today I decided to get my radiator flushed..gonna put it bck tomorrow morning and see if the fan still comes on after a while and stays on..I hope not...and remember it showed signs initially of a faulty thermostat which I changed and I also changed the coolant temperature sensor..with no luck

There are several coolant temperature sensors in the engine (and based on the symptoms you're describing it's not likely that it is the thermostat). If there are no codes, you'll need to take it to a dealer or an authorized service center.

Thanks bud..looks like that's wat I need to do

If you're shutting the car off and restarting, and the temperature immediately goes from 120 back to 91, that's most likely an electronic fault somewhere and not a cooling system issue.
The only thing I'm aware of that would cause that kind of fluctuation in temperature is a big pocket of steam moving through the system. When that happens, the temperature sensor will read much cooler when the steam pocket passes by. Does your heat work consistently when this is happening?

Start by checking the coolant temp with an IR thermometer and see what the temperature actually is when you are getting all these crazy numbers on the digital display.

Like others have said, there are multiple coolant temp sensors. The one that turns the fan on and off is not the same one that provides input for the temperature gauge.