A6 C7 Ultra diesel

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Car had done 55K with zero problems and has been Audi maintained but cam belt etc. looms....

Had #1 problem MAP sensor failure. Tested, analysed, replaced and fixed.

No access to VCDS at moment. Will have soon.

Days later, #2 DPF warning, regen'd by standard driving techniques. Came back days later and MIL now on permanently. Awaits fix so any advice welcome.

This week, #3 driving normally with DPF light on for 2/3 days awaiting analyis and fix. Driven 10 miles at 90 degs C, then rapid rise in temp and 'pull over' message. Did so and cautiously leapfrogged to destination whilst eyeing temp gauge to keep 90 - 95 degs.

This #3 is repeatable; 5-10 miles driving at 90 degs and then quick temp climb to a possible MAX warning. A workable old-school fix to allow limited mobility was to turn heaters on MAX and fans similarly and do not exceed 55 mph. If the car is stopped the cooling fan is whirring merrily away up front. Expansion tank remains at normal level throughout...

Analysis is limited but conclusions so far; coolant pump working as heating OK (?). Possible thermostat fail but why does engine climb quickly to 90 degs and then stabilise there for 5 miles before heading for the MAX ? Temp sensor working because cooling fan is trying its best? No leaks visible and expansion tank is normal.

Thoughts welcome...

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Does anyone have any information as to how to replace thermostat on A6 C7 Ultra CNHA diesel ?