HVAC air quality sensor problem
 

Not a biggie obviously, but I do tend to run the HVAC in "auto-sensing" mode to keep out crud from exhausts in front.
VCDS threw up this: 01592 - Air Quality Sensor (G238) 57-10 - Electric Circuit Failure - Intermittent

Does anyone know where this sensor is and if it's easily accessible/replaceable?

Its attached to the electronics box cover. Remove the trim round the box and its sitting there clipped to the side +++

http://forum.a8parts.co.uk/attachmen...1&d=1578766579

Often wondered what that was.

Never even knew there was an air quality sensor! How does one run the HVAC in "auto-sensing" mode?

You press the relevant button on the HVAC controls Mark! RTFM. ;) It has a recirculating arrow with an A in it IIRC.
(It has an indicator light in it too and stays on between restarts so set and forget; I'll try to find a picture....)

Many thanks MJ. I just had a rummage on ELSA and it was talking about the "Plenum Cover" but apart from being under the bonnet it wasn't clear where.
I'll try replugging it and blowing out crud, mines bound to be covered in it!
Obvious place really, right under the cabin air filter. :o

It's the mirror button to the standard recirculation one but has an A on it as well as the looping arrow. Drivers side of the little grille (alarm sensor?).
Press once to engage and leave it on.
The system takes about 30 seconds to warm up from start before becoming active. When it activates by detecting pollution it has a time delay after going "clean" so won't constantly flip-flop between incoming air and recirculation.

Just to add that our sponsors seem to have used ones cheap enough should I need one; part number is 4B0907659B and common to a few other VAG models.

Sure its a 4B number and not 4D? The one in the A6 (4B/C5 chassis) is quite different to the D2 sensor and has a nasty habit of letting all the magic smoke out.

Well it looks the same and they are listed by A8Parts (and some other sellers) for a number of "Prestige" models. e.g. https://www.ebay.co.uk/itm/VW-PHAETO...9/272528915618
I'll take a look at mine when it's daylight, dry and not blowing a hoolie!

Curious. I just checked Etka and that is indeed the number, despite only being used in the D2!

I wonder if the part was developed for the C5, used on the D2 (but not the C5 for maybe cost reasons?) and then on a few other expensive models?
I can't recall if this function was there in PF models to be honest.
I'm not familiar with the C5 version (is it a simpler/cheaper item?) but various references online suggest the D2 part might even have ended up in Bentleys as well as the Phaeton referenced above!

Its weird - the D2 sensor was around for four years before the 4B C5 arrived but somehow has a 4B number despite only being used in a 4D chassis.

The C5 sensor (4B0907659A) turns up in 1998 and is fitted to everything until 2008 that isn't the D2, including the D3.

That is indeed very weird part numbering, especially with the A version following the B version by several years...
If only I could. This job has just gone to the bottom of the priority list. :-(
I need to take 4 screws out, which also hold the ECU box lid down.
The middle 2 are fine but the outer one (by the pipes) has been damaged so a normal screwdriver will not grip.
And the inboard one just turns (not easily) without lifting so I suspect the metal insert it screws into is rotating in the plastic moulding.

It's much more important to keep my ECU box sealed and dry so I'm staying out of there for now. :tuttut:

Thanks +++ I'll have a look for that when I go out in the car later today. Can't say I've ever noticed it, and yet I'm the sort of person (nerd) who will RTFM from front to back when I buy something new (although to be honest, I usually have a play first and then pick up the manual at the "I wonder what's that's for ..." point).

Is there a recommended method for dealing with these seized screws/bolts on the ECU cover?
For now I'm very tempted to just cut the trim so that I don't need to undo the recalcitrant screw, but in the longer run it ought to be undone and fixed "properly"

Did you find it OK? See attached picture:

Are they just spinning? If so, force the lid open while turning the screw and it will pull the threaded insert out of the lower box. The trim will have to be forced out unfortunately. A bit of 3-in-1 / WD40 under the washer can help it come out without destroying it, as long as the screw isn't too tight. If they spin, it can probably be recovered.

Depending on how badly corroded the screws are, sometimes they can be loosened using mole grips on the insert once the lid is off. Worst case drill the heads off. I replace the factory inserts with M5 D nuts like these and fit new screws.

https://media.screwfix.com/is/image/...odImageMedium$

I did. I don't know how I missed that button in nearly 10 years of owning the car!

I guess I just saw the recirculate symbol and never really noticed the 'A'.

I tried it briefly last night (on a 1 hour round-trip to see Sandi Toksvig at The Lowry, which was rather funny and enjoyable I must say :) )

As I set off, I noticed the button wouldn't stay lit/active. It seems that this was because I was using the windscreen demister at the time. Once I turned that off, the auto-recirculate remained active for the whole journey. So I assume it's working as it should, even though there seems to be no indication that it's actually doing anything!

Excellent advice MJ, many thanks. +++
Found them on Screwfix, though these are M6 not M5 if that matters: https://www.screwfix.com/p/insert-nu...-50-pack/59937

M6 are too big - there isn't enough plastic in the box to hold them. There are plenty of M5 versions on ebay. I drill the original holes out to 8mm before fitting them. If you're replacing the screws I'd go for something 5mm longer than the originals as the threads are recessed in to the D nuts slightly. The original screws will just catch, except where the HVAC sensor is - you definitely need a longer one there.

Wonderful advice, many thanks. You've been there, done that and are an invaluable fount of knowledge. +++

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?