HVAC air quality sensor problem

[steamship]

Originally Posted by HPsauce

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

How about the following PDF, starting from page 6?

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

[HPsauce]

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 air quality sensor is a metal oxide sensor. This sensor is highly sensitive to various smells and pollutants that are typical of traffic.
The automatic air recirculation control sensor evaluates the concentration of carbon monoxide and nitrogen oxides in the intake fresh air.
The air quality sensor has reached its operating readiness after 30 seconds and responds to ambient air changes.
This sensor consists of two resistor elements and an electronic component.

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.

[MikkiJayne]

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!

[ainarssems]

Or the resistors are for heating it up to working temp