Great I will see if he will be able to get the car scanned and give me the code that pops up.

One of the reasons I was buying the car was because of reliabililty. I still own a 93 S4. I have had it for 80,000 miles and it barely costs anything to maintain. The only real money I had to spend was to get the brake lines installed. Right now it has 197,000 miles on it and there is nothing wrong with it (knock on wood). The two Aud's I owned before also were very reliable. Am I just getting lucky? If I am i hope my streak continues when I get a B5 S4.

If it turns out the code is for the Oxygen sensor, is there a way to get rid of the code? How would you know if something else is wrong with the engine if the check engine light is always on?

Yes, just clear the code. However if the sensor is bad, the code will come back. Do what the code is telling you. If it's a bad O2, then you have to replace the O2.

If the Oxygen sensors are sensing a lot more Oxygen/Air because of the bigger downpipes wouldn't replacing the sensor be pointless as a new one would recognize the same problem?

Bigger DP's do not throw a code. The size of the DP does not change the A/F ratio at all, it just affects flow. What sets the CEL off is the when people make "Piggie" pipes
& remove the pre cats. If that is what you have, then the secondary O2's need to be moved after the cat.

you could weld an extension node/pipe on the pipe which takes the secondary O2 sensor out of range thus keeping the reading from both O2 sensors out of range which is how they work

I don' t actually have the car yet. I am just trying to figure out if this guy is feeding me a line of BS before I buy the car. This is his current exhaust set up.

Exhaust System:
- MTM Catalyst
- MTM Cat back exhaust
- MTM Down pipes

I called the Audi Service technician who works at the Audi Dealership where the car I am looking at buying was serviced. He says that when they put in the MTM downpipes the code was tripped (Exhaust put on 2 years ago). He says he would reset it on the ECU but that it would come back on again. He says the reason the sensor keeps tripping is because of the bigger downpipes the A/F is vastly increased, meaning that it blows by the sensors a lot faster than normal. The pre cats are still there. He also said at the end that he had no idea how to get the CEL to not show up for the 02 sensors and that he usually does not do a ton of custom work.

So what do you guys think? Does his story check out? Other than the CEL he said the car is in great mechanical condition. This is also a one owner car.

DP's DO NOT CHANGE THE A/F!!!

MTM DP's DO NOT HAVE THE PRE CATS


I would look at the car from a lift & make sure what's on there. My bet is the car doesn't have MTM DP's
MTM stuff for the S4

Aftermarket DP's don't throw the CEL, there made & designed to NOT throw the CEL. Think about it, no one would want them, you can't pass any SMOG test with the
CEL on. To my knowledge, only PIGGY pipes (gutted stock ones) throw a CEL, due to the location of the secondary O2's.

2k S4 - You help is very appreciated. I called the guys at MTM and they confirmed what you were saying all along that the downpipes won't trigger the CEL unless the o2 sensors were not hooked up or damaged during install (which I believe you also told me). Sorry for beating this issue to death but I do want to clear this up to save myself money on a plane ticket if possible.

I will ask the guy if he has pictures of the MTM DP's either before or after install or if he at least has the receipt showing purchase or installation of the DP's.

Is replacing the o2 sensor something that you can do on your own (w/ limited mechanical ability). Or should this be done by a technician?