P0420, O2 Sensor?
 

Need some help. Stock 2000 Z28 A4. Randomly getting P0420 for the last 2 years.

-2 years ago i got my first p0420.
-Found a cracked vacuum hose at PCV. Fixed, code went away.
-Code came back 6 months later.
-Found Cracked Vacuum hose on the Bank 2 valve cover to throttle body. (right next to PCV)
-Code is back now 1 year later.

In Scanning, i see the rear o2 moving correctly at the .7 /.8 range, then at idle, only at idle, right when i take my foot off the gas to press brake (like at a stop light), it drops to .1 or .2 then slowly climbs back to .7.

1 out of 10 stops is when it take a drop from the .7 to the .1. The fuel trims are also spot on.

Does this look like a bad sensor or is this actually a bad cat this time ?

Re: P0420, O2 Sensor?
 

Bad cat probably. The code can come and go with the seasons if the problem is marginally failing. Mine has been most active in the winter months for the last 3 years. The code sets when the throttle and vehicle speed are relatively steady for a while and not at idle. It sets when the post cat sensor is or is almost as active as the pre-cat sensor, signifying the O2 level is unchanged after going through the cat.

Re: P0420, O2 Sensor?
 

Swap the after cat sensors side-to-side. If the code follows the sensor, it's a sensor problem.

If by taking foot off the gas, you mean decel from speed, the PCM shuts off the injectors, at least down to a certain RPM. That might explain the 0.100 volt reading.

Re: P0420, O2 Sensor?
 

Thank you for responses. I will likely wait a tick before actually changing the Cat, but its nice to see they are < $150 for the 49 state model.

So to have some fun tonight... i put seafoam in through the Bank 1 header port?? (maybe its the AIR system on the stock headers?) anyways its just 2 10MM bolts on the tube and i could directly pour it into the exhaust. I'm gonna have it sit tonight and then burn it off tomorrow! Let the science experiment begin....