Greetings All,

I had something strange happen on my '02 V6, and am wondering if anybody else has had this happen. Yesterday, I got a CEL and P0420 code, for the #1 bank catalytic converter low efficiency. Now I've known for some time the #1 cat is marginal, after this incident last November:

https://www.jaguarforums.com/forum/s...al-cat-143685/

Until yesterday, I've not had any fault codes, so had been pretty happy to have saved the expense of a replacement cat. But now the code was being set (pending) on every cycle, so I was very surprised it had failed so suddenly. I was expecting a gradual failure, with the occasional code appearing more frequently. Instead it was like bam, off the edge of a cliff.

This morning I had a little time to play with my scanner. I had left early for work and stopped at IHOP (25 miles away). I ordered the Swedish crepes and two eggs sunny side up, if that matters. Coffee, black in a dirty cup, the way it should be. I had the code on the drive to IHOP, cleared it, and then it returned yet again on the short hop to work. I compared the 4 oxygen sensors, both upstream and downstream. Both upstreams looked good, with normal rapid switching. Downstream #2 looked great, with very slow switching to indicate the good converter doing its job and cleaning up the exhaust.

Meanwhile, downstream #1 was switching rapidly at the same rate as upstream. This indicates a dead cat, doing practically no cleaning of the exhaust. That would certainly explain the P0420, as expected.

Here's where it gets weird. On the ride home, I watched the scanner. Downstream #1 is now behaving perfectly, with a very slow switching rate to indicate the cat is working again. No codes whatsoever. I made one stop on the way home, with the same good results on each leg.

What gives? How can a cat be dead as a doornail and then come back to life as if nothing happened? Most of the driving was on the freeway at 60-70mph, so I imagine the suspect cat must have reached normal operating temperature.

Any thoughts? I am baffled. Since the downstream #1 sensor had been tracking the upstream sensor perfectly, I don't think I'm looking at a sensor failure. The car has been running perfectly this whole time, too.

Call me confused.

 

The later PCM (2002.5MY-on), and maybe also the earlier one, tries to get a cat / O2 sensor to respond by deliberately odd fuelling. Maybe that's "worked" at least for now.

 

Maybe that's it, I don't know. The cat rapidly went from normal to complete failure for a few cycles and then back to normal just as quickly. No codes at all today, like it never happened. Fingers crossed it stays this way.