DrRuckus13

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 ?

GaryDoug

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.

Injuneer

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.

DrRuckus13

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....