NOX (oxygen) Sensor!!


Our Nov 2002 1.6 FSI had the engine management light come on in April this year. They ended up replacing the NOX sensor (part 8Z0906261)- cost £300.


Engine management light

Similar problems traced to low battery volts during winter period of short runs.
Same problem wiyh Skoda and this traced at last to faulty battery (one cell down).
Even with the massive battery fitted winter usage and age take their collective toll.
Mike A


Well, just a brief follow up.... I've finally had my car returned to me. Been without the A2 for about 5 weeks this time, apparently the new ECU was fitted and tested two weeks into it. It's then taken them an additional 3 weeks to arrange collection of the courtesy car.

I requested that if I was to pay for a new ECU I would get a copy of all invoices and job cards. This hasn't happened, only invoices. They haven't included ANY paper work when it went in the second time for repairs.

The engine management light first came on last year (Roadside Assistance diagnosed what they thought was the problem). So it first went into the dealer back in February and now 6 months later, 4 courtesy cars, bill for a new ECU, I've got my car back. How long for I don't know?? What a nightmare!!!!

Any update on this, has it been cured? My missus FSi has thown up the NOx fault code on the VAG-COM and it looks as though it could be costly!




My engine management light is finaly off!! The last resort was to take it to Audi with the condition that they had one hour to fix it and then we wanted a call to confirm any further work. They worked on the car all day and came to the conclusion that it needed a new ECU (£700!!). They only charged us for an hours labour though as promised!

As the light was off we thought we would drive it until it came back on and then book it in for the ECU. That was over a month ago and so far no light!!


nox sensor

i think im just really lucky with the local dealer in cov, they look after my a4 so always look after dads car too they also understand that ill probably do all the work myself so always give me part numbers and stuff so i can get parts

the eml has come on dads a2 and we took it to audi cause we thought it would be an ignition coil (AGAIN) and a free MOT (Passed straight through!!)

they have diagnosed the eml as the nox sensor and want £488 to do the work which im thinking of doing myself

i read your posts went to GSF & Europarts today but i just became more confused.

the part they are describing is the lamba probe (euro parts price £65) am i buying the right part? on the quote sheet audi have given me is 8z0906261.

i have to admit i was also expecting to have to pay 1 hrs labour for diagnosis as the problem wasn't the warranty covered coils but they didn't charge me for this either.



My engine warning light came on the other day. As it happened, I had just ordered an OBDII dongle thing for other reasons. It arrived yesterday and I finally managed to get it to work today (problems with the software only working up to COM8 but Windows 7 giving it COM17!!!). It appears I have only one fault code logged and it is P2201. A search on the forum returned no mention at all. A google search translates it as:
P2201: NOx Sensor Circuit Range/Performance Bank 1​

First question is, should I just try clearing it and see if it comes back again? Or is it a one time only, your sensor is toast, do not drive anywhere because your engine will explode kind of error?

There was a comment above about dead batteries causing an out of range reading from the sensor and causing the ECU to think it is toast when in fact it all fine. I checked my battery voltage this morning and it was 12V (the OBDII scanner reported 10.9V!) with the engine off. So I'm guessing my battery could well be dead. Not hugely surprising I guess as the car has done about 10,000 miles in the last 4 years according to the MOT history. That kind of usage will mean it has sat around getting flat for long periods at a time I would guess.

Has anyone seen code P2201 before? How critical is a NOx sensor? Is it safe to drive around with a potentially dodgy one? It sounds like an expensive thing to replace if it might not even be the fault.



Good luck to you all as in my experience solving engine sensor issues (especially those that monitor emissions) is a near impossible job to conclude. Stupid engine lights. It’s all a money making scam I’m sure. I mean how can you need a new ECU when the car drives alright & passes it's MOT????

crazy stealers !!:mad:


A2OC Donor
the car may drive alright as it seems and will probably pass MOT. But when my EML was on and the fault code pointed to the pre-cat lambda probe, I had very noticeable drop in fuel economy as I suspect the ECU falls back to a less optimal mapping. I could clear the fault, but it would come right back after I started the engine.

As usual (for my A2 anyway), a tank of Shell V-power cleared the light eventually and normal MPG returned. I've seen NOX related fault code on mine too, but cleared it and it never came back. The lambda faults happened 3 times, each time cleared after some good Shell juice.



But you're more or less proving my point, you fixed it with no more than additives.

On one of my old cars the EML came on and the ECU codes were traced to emissions sensors. So i fork out £115 for one fitted, turn on the engine and the EML light was lite as bright as the sun. So they hook up their computer again and now it’s reading Pre-Cat sensors, not the sensor in the cat, PRE-CAT SENSOR.

Ok then a few house and £145 later I turn the key and blam, EML light, and their computer is telling them now “well actually it’s the sensor in the CAT, defo the Cat as we're not getting any reading from that what so ever” neatly failing to acknowledge that the sensors in question is new out the pack.

I just despise spending the best part of £300 to turn an engine light off, then driving away with it well and truly lit 6 hours later. And I have yet hear a story where a simular issue was resolved 1st time with minimal fuss, it always the same old story, cash heads out while frustration piles in. It's a set up i'm sure.
Me too...

Looks like I am just starting on this same saga. Not great. I will continue to read this post with joy. I have started another one which I hope does not go the same way.


New Member
Right, after a sudden rush of blood to the head (see my PM Giles - you of course have the FSI, there aren't any NOX senders on 1.4 petrol cars, doh!), I've got to the bottom of the NOX issue - I think!

I've attached 3 photos, showing firstly the wiring between the sender, the control unit and the ECU and then 2 diagrams showing the locations of the parts involved (to orientate yourself, the NOX sender diagram shows the underside of the car and the exhaust pipe, the control unit diagram is showing the passenger footwell compartment).

As you will see, there is ONLY 1 NOX SENDER.

Also, the wiring all leads back to the ECU, so it could well have been a faulty ECU to start with. I know exactly what the dealers have done - they started by replacing the least expensive part (the sender), then when this didn't work, they moved on to a more expensive part, and now finally, they're going for the daddy, in cost terms - the ECU.

They did this with my power steering faults - firstly doing absolutely jack, then going for the fuse, then finally having to bite the bullet and replace the controller.

So, I would surmise that in fact your ECU was faulty from the start and that as the fault occurred within warranty, it should be repaced free of charge, even though the car is now technically out of warranty.

Just imagine - your gearbox was playing up and you booked it in to have it looked at only to be told that they couldn't fit you in for a fortnight, when your warranty has 10 days to run - would you expect to stump up for the parts? I wouldn't and I don't think you would either.

Say no to any 'goodwill' gesture, you're entitled to have this fault rectified free of charge - it's not your fault that the dealer couldn;t get to the root of the problem in time is it?

Hope this helps,

Hi would you have the three photo as I am having problem with post cat sensor I think thank you


New Member
Thanks Skipton01 really useful I guess the part number 3 is a dpf? as there are two number 20's. With the 1.6 fsi I guess only the two as I have not yet ventured under the car. WIth the scan I am getting NaN% reading for the bank 1 sensor 2, I did a temperature test of the cat seems to be working but has P0139 code cheers Andy


A2OC Donor
Thanks Skipton01 really useful I guess the part number 3 is a dpf? as there are two number 20's. With the 1.6 fsi I guess only the two as I have not yet ventured under the car. WIth the scan I am getting NaN% reading for the bank 1 sensor 2, I did a temperature test of the cat seems to be working but has P0139 code cheers Andy
Number 3 is the second CAT on the FSI. Its a spectial NOx storage catalytic converter which is unique to the FSI. When running in lean burn mode, the engine produces lots of NOx gasses and these are captured in the storage catalytic converter. When this is "full", the engine switches mode and the stored NOx is converted into nitrogen to be released out the exhaust. The NOx sensor is used to detect when the converter is "full" so is required for proper operation of the engine. If the NOx sensor isn't working then the engine will never go into lean burn mode and fuel consumption will be higher.


New Member
Many thanks for the info I guess that is why I am only doing about 30 mpg....... will replace the NOx sensor soon but guess I can test it by removing and heating it up taking a reading with multimeter. The sensor for NOx is number 21 also the cat unit 3 has number 20 is that a NOx sensor too rgds


A2OC Donor

items 20 and 21 as per the above diagrams are the same thing, there are actually two different diagrams in the earlier post with different numbers.

The top diagram shows the placement of the sensor on the exhaust, number 20. The bottom diagram is how the same sensor, number 21 here, connects to the ECU. The NOx sensor is under the car and you can see it easily. It is to one side of the pipe and angled slightly upwards. I have changed this sensor in my first FSI. The wires goes through a hole in the floor in the passenger footwell and to the emissions control unit. This is a separate box to the engine ECU and is bolted to the floor. The wire goes from the exhaust and is clipped above the heat shield and the place where it enters is covered with a plastic cover that protects where the engine wiring also enters the car. This will need to be removed by undoing the retaining nuts and bolts. The gromet is part of the cable and this just pops out which allows the plug from inside the car to go through the hole to the ouside.




The NOx sensor is actually a combined sensor and acts as both a lambda probe and NOx sensor. It therefore costs more than a regular lambda probe. The lambda probe part forms the usual post CAT lambda check, and mostly is used to verify the correct operation of the calaytic converter. The second lambda sensor acts in the same way as in the FSI as it would do in a "regular" petrol car. If the lambda part is broken then the emissions control engine warning light will be on as the car can't verify that the emissions are correct. In my car, the lambda sensor part was OK but the NOx part wasn't working so the warning light was off but a fault code was logged for NOx. Unfortunatelly for me, after changing my sensor, the car then tried to run in lean burn mode and instead just told me that my inlet flap actuator was broken. :-(




Well-Known Member
Pleased that headway is being made with this issue. I don't know what's happened to the original 3 photos - they're not on my old photo bucket account, and as I don't have an FSi, I can't take any detailed photos of the sensor installation, so it's good that other FSi owners are stepping into the breach.
Hopefully, this can be resolved now at minimum cost - it's such a shame that these otherwise great petrol A2s can become scrap when in need of a couple of sensors, from a financial point of view.