I've searched a number of threads and have a number of options but whats the most likely. My 2000 dhse is blowing hot and cold out of different vents. I have completed 'o' ring work recently and also sprayed each blend motor with ptfe lubricant. The flaps seem to open the correct vents, its just the temp difference. One other thing to mention is that the temp reading at the base of the hevac fluctuates quite a lot. Sometimes reads -12 then jumps to 15c. Could this be an issue?

Differential Temps is almost 99% a Blend motor issue...(there are 3 of them)

As for the reading that is from the Ambient temp sensor behind the grill, could be a loose connection!

That is outside air temp. Sensor is mounted behind grille to right, looking from front of car. Sounds like it's faulty if it is fluctuating like that. But it will only change from last temp indicated when car stopped, when car moves off and reaches 25 mph. The heater needs to know incoming air temp to control mixture of hot air to reach selected interior temp. So yes it may affect the heater if it is fluctuating to that extent. You need someone with diagnostics such as Nanocom to run the blend motors and check them for correct function. They can be motored to various % positions or force calibrated.

Thanks both. There's an independant not too far away. I'll pay him a visit and get a diagnostic.