Cause of P0420 on 2002 Pathfinder

[dsurls]

I've had P0420 on and off the last couple of months, indicating an inefficient catalytic converter on Bank 1 (passenger side). So, I've been doing some research to determine the culprit of the problem, and I've run diagnostics using my OBDWiz software scanner.

If you look at the attached files, you can see a recent scan of the waveforms for the O2 sensors and the Short term Fuel % Trim values for both banks. Before running the scanner, I reset the code (only P0420 was showing) and ran drive cycles to get it ready for emissions testing. I ran the vehicle at idle at about 750 rpm for about 5 minutes, then at 2750 rpm for about 5 minutes, then back to 750 for about 2 minutes.







My initial take on these graphs is that the values look OK. I'm certainly not an expert, but the short term trim values seem reasonable on both banks. If anything, it looks like perhaps the downstream Bank 2 O2 sensor could be mirroring the upstream sensor a little too much, but it seems to settle down. But the Bank 1 values look pretty good, which is where I would expect to see mirroring with the P0420 code.

So, the question is, can anyone see anything wrong with these graphs that would signal a potential problem?

BTW, the vehicle is currently at about 175K miles, with a rebuilt engine at 160K. The catalytic converters are original.

Thanks for any suggestions you might have!

[Towncivilian]

Admittedly I'm clueless in terms of interpreting your waveforms. Very rarely, a rear oxygen sensor can malfunction and throw a P0420/P0430 code despite the catalytic converter operating within tolerances.

There is a diagnosis procedure for P0420 on page EC-340 of the factory service manual. Try to follow it as best as you can. I wouldn't go so far as to remove the injectors to see if they're dripping, though, heh.

Does your engine burn any oil? Why was the previous engine replaced/rebuilt?

What is the manufaturing date of your Pathfinder, found on the driver door jamb sticker? There is a TSB affecting 2001 model year Pathfinders with P0420/P0430 code(s) and the TSB calls for reprogramming of the ECM. Since yours is a 2002 Pathfinder, I doubt the TSB applies, but it wouldn't hurt to check.

[dsurls]

Yeah, I had seen the diagnostic procedure, but I was a little skeptical of it. I'll try to follow it and see what I find.

The engine does burn a little oil -- only because I notice when changing it that I have less emptied than I put back in it. The reason for the engine rebuild was because the timing chain jumped and bent the valves. Ughh.

The Pathfinder was manufactured in March, 2002, so I'm guessing the TSB does not cover it.

Thanks for the suggestions!

[dsurls]

Sorry to be late returning to this thread, but I've recently discovered something new related to this problem. I still haven't run thru the entire diagnostic procedure, but I have noticed if I run a bottle of Techron thru a tank of gas, or if I use Chevron gas with Techron in it, the CEL will usually turn off for a few days. It eventually comes back on, but if I repeat, the CEL will turn off again.

So, the question is, does this indicate a likely cause of the CEL coming on? Is it likely to be worn out O2 sensors, or indeed a bad catalytic converter, or something else?

Thanks again!

[Towncivilian]

Try driving at extended high RPMs on the highway for 5 or 10 minutes (shift into "2" at cruising speeds) to help blow out any carbon buildup; if you don't reach higher RPMs fairly frequently and drive like a grandma (not that that's a bad thing if you're trying to save gas of course), carbon buildup is likely. The downstream sensor also could be dirty and Techron could've helped clean that - have you run two tanks back-to-back with a bottle of Techron?

Does your Pathfinder consume oil? Do you know if the spark plugs are the OEM specified NGK PLFR5A-11 and are gapped to spec (0.043 in nominal)?