bmessina

2003 Allroad 2.7t. Failed my first go at smog testing this year - replaced the O2 sensor, failed again. Added alcohol to the gas, passed by the skin of my teeth. (literally by 1 point)

Car is now consistently throwing the catalytic warmup threshold code for both sides. So it's either ridiculous prices for the replacement convertors (I'm in CA) or PITA rear O2 sensor replacement. Is there any way to further isolate the issue? I have the ross-tech cable, etc.

indoor

O2 spacers for rear O2's would be the cheapest quickest option.
You might have a leak in the flex section of the exhaust pipe(s) between the precats and cats...or vacuum leak(engine). Is the car running rich? Log measuring blocks 32 and 33 I think for lambda. Check, clean your maf with maf cleaner.

bmessina

Would the spacers help it pass smog? That's going to be an issue.

Vacuum leak is a possibility. A few weeks after the car started throwing the cat codes it developed some kind of intake air issue. The car stopped running while my wife was driving it and wouldn't start. I had a previous issue with the MAF connector, one of the wires came out, and I had done a quick fix at the time so I thought it finally just came loose again. I did the real fix this time and the issue persisted - after a hard reset the car would eventually (after miles of driving) stop running and wouldn't re-start. Unless I unplugged the MAF, then it runs fine. I've cleaned the MAF, and tried a spare known-good unit.

I still need to do a visual on the turbos and their hoses, but everything I can get to from the top looks and tests fine.

indoor

Didn't try spacers, but everyone swears they pass emissions with them.
I am going the hard route: replace the cat on passenger side. It's the only one throwing the code. PO replaced the driver's side.

But before all that I fixed a small vacuum leak, replaced most of the vacuum hoses; replaced a clogged suction pump which helped with the brake vacuum assist, and put on a new(to me) maf sensor. After all that I did readiness with VCDS and passed all tests. BUT about 50 miles later the code came back. Which tells me that the cat is lazy(old). Under the right circumstances and temperatures(readiness with VCDS) it passes, but when driving it in traffic(it doesn't).

I replaced the rear O2 as well as the front. They're both tricky to replace, but the rear wire is clipped to the top of tranny together with the other side rear O2 sensor. I had a hard time uncliping the top clip(there are about three: one by the engine, one in the middle of the tranny at the top, and one by rear O2 sensor). I also used a tie wire to 'fish' the new O2 and route it on top of the tranny.

Puting on a spacers would be an easy job. Just need a good O2 wrench(I used a claw one with a long braker bar and a 6inch extension to pass the exhaust pipe) to loosen the rear O2.