So... I've had this problem since I bought my S4. The previous owner said he'd replaced the sensor with a genuine part just before selling it but if I'm honest I have my doubts. The sensor looks brand new but genuine is debatable, cheap eBay jobby looks more realistic.

I only have a basic fault reader (not VCDS) and this shows an issue with the driver side pre-cat/upstream sensor. I'll scan the code again and add an update, as I can't remember exactly what it was.

What I'm wondering is whether this issue could be caused by the Milltek sports cats? I know that sports cats can cause problems with lambda sensors, but I would have expected it to be post-cat/downstream sensors.

If anyone has had a similar problem I'd appreciate some input. Audi want around £180 for a new sensor and I really don't know if this will solve the problem. The wiring all looks good so I don't think that's the issue.

My next step is to swap the two pre-cat sensors, and see if the fault changes to the other bank. If so, faulty sensor. If not, it's a whole different problem. For anyone that's changed the pre-cat sensors on a B7 S4 I'm sure you'll appreciate that this isn't a 5 minute job. 

I should also note that I've had a few misfire codes on random cylinders. I've never felt a misfire but it is a little rough on idle sometimes. I thought the lambda code may have been related to the misfire initially so changed out the coils and plugs. The coil packs and spark plugs are now brand new (both NGK) so this shouldn't be the issue. I suspect it's linked to the lambda fault, but I could of course be wrong. 

Am I missing something obvious? Is it the sports cats? Is the misfire likely to be related? Any input would be greatly appreciated.

Hello Ian,

Welcome to the forum. It certainly seems you have your head around what you are doing, and I too would suspect that the recent lambda sensor fitted by the previous owner might well have been the much cheaper generic type rather than a genuine VAG one. Understandable I guess, if he knew he was selling the car. Swopping them is an option as you say, or seeking more specialist advice/assessing equipment via. an independent. A small investment there might save you in the long run. 

Generic sensors can sometimes be identified by slightly different wiring colours and usually more 'cobbled" looking connectors.

If it turns out this sensor is the offender and you know they cost £180 from VAG, it may be worth shopping around to see what discounts you can get. I have bought quite a bit off one dealer with discounts and good service, and I am happy to share this with you should you need it. No doubt there will be other dealers offering such discounts. 

Good luck in getting it sorted.

Kind regards,

Gareth.