Our A4 has hit the 115k curse. Just replaced the radiator, rear wheel bearings, and now the cat? I live in california and can only purchase a new cat from the dealer at a cost of over $800. not including labor. Is it worth the cost of trying to replace the post 02 senor first and pray that will fix the problem?

 

is you r CEL on? have you tried clearing the code and then seeing if it comes back?

usually you would expect the O2 sensor to throw a code if the problem was specific to the sensor

Cat efficencly below threshold is usually an exhaust leak by the post O2 sensor, and it's usually at the clamp that connects the down pipe to the rest of the exhaust. If there is even a small leak, then under deacceleration there is a pressure gradient that is created. And, then the air outside is then "sucked" back in to the exhaust system for a split second.

something to check at least before deciding to bite the bullet on cat

or you could always find a "friend" out of state to get you a non-California cat

 

CEL is on, reset and it came back on. Will check the clamp on the down pipe. Won't have to smog the vehicle for another year. Is there a specific code if the post 02 sensor is faulty? Thanks for the advise.