My 2008 i6 heats fine after cold start now in these cold days. It reaches the temperature I set on the AC very quickly.

But when it has reached the preset temperature (e.g. 22° C) it stops heating and never starts again. So it get's really cold in the car. I have to manually adjust the temperature to a new value (e.g. 23° C) and the heating starts again, but then stops again after reaching the new setting. It's quite annoying to adjust the temp setting every 10 minutes or so.

Anyone an idea which sensor is not working properly? There is no fault in the car, I have checked it for any error code.

Thanks a lot for your help.

Firstly I would check the cabin filter is OK, when was that last changed ? FL2 MY10 TD4 GS traded in at 2 years
FL2 MY13 TD4 GS Current

Thanks for your reply, the filter was last changed in spring. Will check again if everything is still ok.

My own experience seems to be with the AC on in Winter and on full Auto if the car is not moving forward on a open road it does tend to feel colder and I sometimes feel I need to turn it up. Once moving gain then its fine. FL2 MY10 TD4 GS traded in at 2 years
FL2 MY13 TD4 GS Current

I drive every morning 60km on the motorway to work.

When I start @home the car starts heating as expected. Thanks to my petrol engine it heats quite quickly and I feel very comfortable after a few minutes. But about 10 minutes later it get's cold in the car while im still driving constant speed on the motorway (140 km/h). It's not because the heating has stopped heating since the internal temperature has reached the preset value, it's because suddenly the AC system lets cool air into the car. Even with temperatures outside below 0°.

OK, first check that filter and the air ducts are clear of any Autumn leaves for a cheap fix.

I assume you are not getting moisture build up in the car, if you were it could be that the AC drain need cleaning out as it won't be dripping under the centre of the car. FL2 MY10 TD4 GS traded in at 2 years
FL2 MY13 TD4 GS Current

Have you tried turning off the automatic functions.
Experiment with taking control of temperature, airflow distribution, & fan manually. That should help you sort out what is not working.
If I am right in reading your profile, you have had the car long enough to experience how it should work.

I have found that although most of the time the heating/cooling works away quietly keeping me at the right temperature, It does sometimes send a quick blast of cooling into the cab (sunlight sensor?) . Also I sometimes have to manually send air up the windscreen in very damp and humid conditions. Otherwise it works perfectly.

I did wonder at first if it was to do with thermostat effecting water flow to heater, but on re-reading your post it seems unlikely. FL2 XS SD4 Auto 2010 2012-2017 (21k - 91k miles) (MY2011)
FL2 Metropolis SD4 Auto 2014 2017- (16k - 76k+ miles) (MY2015)

I do always use the automatic functions, will try manual control tomorrow. Let's see if this makes any difference.

Thank you all for your input, I will update here with my new findings (filter, manual control).

The FL2 HVAC system uses a sensor to detect the cabin temperature, this is behind a grill behind the steering wheel on the inboard side of the car (closest to the climate controls). The sensor is a simple NTC temperature probe, with an aspiration fan behind it.

The issue you describe is typical of a blocked/faulty/failed aspiration fan. The fan draws air over the sensor, and uses this to calculate the position of the temperature doors. When you first start the car from cold, the sensor is cold. As the cabin warms, the sensor warms, but because its cold and not aspirated its taking the air flow immediately in front of the grill, which is almost directly from the foot ducts as the hot air 'sticks' to the knee roll/interior trim. The HVAC controller sees the air getting too hot and drives the temperature lower.

Have a look into the grill with a bright torch and you should see the NTC sensor, you may be able to see the fan, especially if it is broken and not turning. I'm not sure of the orientation of the aspirator on the FL2. A short circuit or open circuit of the fan should raise a DTC, so i'd guess its full of crap.

Sorry for the lengthy reply, I suspect this extra information might help others in the future.

Vanny, thank you for your reply, I will check the fan on the weekend.