Was receiving a code for Catalyst Efficiency Below Defined Threshold on both Cat's. Audi dealer quoted close to $3500 to replace both. Took the car to a great muffler shop in Dallas who replaced both Catalysts with Audi parts. MIL light cleared and everything was good. The car was immediately shipped here to Washington so I didn't have a chance to drive it. Today after driving about 70 miles, I got a P0133 code reading O2 sensor Circ Slow Response Bank1 Sensor1. Is having the O2 sensors replaced a big deal? In fact, should they be replaced? I'm reading on the forum about checking the MAF sensor, vacuum lines or O2. I just moved to Washington from Dallas and need to have my registration done but can't until having an emissions test done. Any advise would be greatly appreciated!

 

Sounds like O2 sensor to me. Bank 1 is the passenger side. You just need a O2 sensor removal socket, really long extension for the socket wrench and a replacement O2 sensor. You can buy universal 4 wire sensor or buy the Audi OEM, which you don't have to mess with the wiring.

 

I had a bad spark plug wire to cyl 3 which triggered that code and others- Replaced all wires andso farno codes in 9 months (99 2.8 200K)-

 

Very interesting. A couple of months ago I got a Cylinder 3 misfire warning, but that went away on it's own and never came back. I'm taking the car to the dealer in the morning to have this emissions issue fixed once and for all but will mention that. Thanks for reminding me.