Made some surprising progress today. +++
As it was fine and sunny I decided to have another look and potentially remove one of the bolts to check exact dimensions.

I removed both the middle 2 in the end and saw that under one of them was what looked like a partial washer. Realised quickly that was the sensor fixing and it was now loose! :o
Decided to prise up the clips on the front edge to peek underneath and the innermost fixing of the panel actually popped out undamaged! The jammed screw there was actually undone just enough to let the panel slip out.
With only the outer screw left holding it the panel is easily flexible enough (quite rubbery at the outer edge) to lift up and get at the sensor.

I took the sensor off and indoors to look at it. Saw debris under the little 2-hole cover and managed to blow it out. Then decided to see if the little cover was removable as it looked like it was. It just prised off easily and I found yet more debris inside, cleaned it all out with a clean dry artists brush and clipped it back on.
Finally I took my Dyson to the space under the panel, it was full of plant debris! And I also remembered to clean the underside of the panel itself as there was loads of stuff there too. Put everything back, that's quick and easy. +++
The two dodgy bolts can wait for another time; I currently have no need or desire to get into the ECU box. And if the sensor is still faulty I know I can get another and replace it easily enough now. :cool:

I don't know how the sensor works so apart from brushing away debris didn't touch any surfaces and definitely no solvents. :tuttut: Does anyone know what the principles of it are?

I guess not, and any amount of "googling didn't throw up an answer either.

But my cleaning efforts seem to have been rewarded so far as this error has not come back:
In fact today I had a totally clean VCDS scan, apart from the inevitable low alarm battery warning. +++

How about the following PDF, starting from page 6?

https://iopscience.iop.org/article/1...1/1/012033/pdf

Thanks for that Sean, well researched. +++
It doesn't say a huge amount (and appears to be based on BMW designs, probably very similar to Audi) but these are the key sections:

The 30 seconds "warm-up" time is consistent with the Audi system and I presume the resistors are the sensor and the electronic component converts that to a signal used by the HVAC system.

Funnily enough one of my neighbours specialises in developing sensors of this type, though mainly for use in the oil and gas industry, so I'll ask him what he knows when I see him next. :cool:

That sensor is identical to the one used in every Audi other than the D2. I think it basically works like an O2 sensor, but looking for oxides rather than oxygen. The two resistor elements are probably some exotic metal compounds which respond to the particular oxides.

I know they stop working when the magic smoke escapes!

Or the resistors are for heating it up to working temp

Out of interest, does anyone know if these two versions are compatible? e.g. could a D2 sensor be used in a C5?