I'm still fooling around with the CEL. How many exhaust O2 sensors are on a 2000 1.8 quattro? I've got an exhaust leak in the flex piece on the down pipe, which I will get fixed this week, and I'm wondering if that can throw a code. I was thinking that the computer compares the O2 readings before and after the cat, thus two o2 sensors. It was P1128 before I started changing vacuum lines last week, I had it cleared, and it came back. Now I'll get the code read again after I fix the pipe.

 

You have two O2 sensors, and I've had a tear in my flex pipe for about four months now and I have never once thrown a (related) code.

 

The ECM does compare the readings of both sensors to determine if there is a problem with the cat's efficiency. Unmetered air, whether it's in the intake or exhaust, can generate a DTC. However, if your leak is generally a good bit downstream of your rear 02 sensor, it won't have a bearing on the readings.

 

Just curious. When you had your codes read/cleared earlier. Did you reset the fuel trims?

 

No, I don't have access to VAG so didn't check the fuel trim data. Just had Advance Auto guy clear it.

I still don't know where the second oxygen sensor is. certainly there is one on the cat, which I think is the heated o2 sensor. Is this sensor "before" or "after" the cat? Where is the other one? Is it in line somewhere or just measuring atmospheric o2 (but that never changes from 20.9, i think)

I got the exhaust flex pipe replaced today for $200 complete (and a hanger rewelded). I went across the river into Easton PA and saved about $50-100 vs what the Jersey guys wanted. The down pipe with flex is $155 on line and I didn't want to have to break those bolts loose on the cat, so let these guys cut and weld without needing to break bolts at cat. Autozone guy in PA wouldn't clear the code said they were "banned" from doing that. Damn lawyers are into everything.

I'll drive it awhile, get code cleared, then I may have to change that 4th vac line. Did a propane leak test last night and didn't hear/find anything, but with the belly pan on I can't get to the far end of the vac line from manifold to the mech recirc valve down below the turbo.

Any ideas out there?

Thanks

 

Locate your turbo oil feed line. Then follow your exhaust piping going toward the rear of the car. Your first (pre-cat) 02 sensor is about 6" behind your oil feed line. Then your exhaust swells in diameter (that's your cat). At the rear of your cat, where it tapers down, you'll find your second (post-cat) 02 sensor. I've seen quite a few oxygen sensors get replaced when they weren't the problem. I hope that helps.