UK Car Repair Questions? Ask a Mechanic for Answers ASAP
it sounds like the temperature adjustment door is not adjusting properly, to be able to test it the scan tool is best as it will run a self-test, but you can access the area behind the trim panels on the left side, center of the dash area, and see if the motor adjusts as you adjust the heat controls for that side. More than likely the temp flap motor is sticking.
That is odd to be that specific of a problem, occurring at the same temperature each time. I can't think of a good reason why this would happen. There are temperature sensors inside the ducts for the vents, so maybe if one is not reading right it will think the temperature is too hot and turn it down on that side- but to only happen once it reaches a certain temperature is strange.
Have you checked your pollen filter (cabin air filter) recently? I have seen some issues with air flow due to this being plugged
are you saying that the temp gauge is rising above the middle mark? The gauge is not an analog gauge, it should not dip and rise as the thermostat opens and closes- it should warm up to operating temp, and stay there, like it is locked on. As long as the temp sensor sees it in a "range" of operating temperature, it sets the needle right at the midpoint, to avoid the driver having to watch it constantly.
If your temp gauge is rising above the midpoint, then you have a cooling system issue. Either your thermostat never opens, and the engine is overheating, you have a leak somewhere, or your coolant pump is not operating correctly
The only reason I can think of that you would have heat on one side and not the other is the flap motors- I can't explain what triggers it to drop when it reaches a certain temp though.