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 pictures, 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!

The ECM monitors the switching frequency of HO2S1 (upstream) and HO2S2 (downstream of cat). A good catalytic converter will have a high oxygen storage content and the downstream O2 will switch; higher and lower than 450mV, much slower than the upstream sensor. A bad cat will have a low oxygen content, and the rear O2 sensor will switch at near the same rate as the front O2, and a Catalyst malfunction will be detected. Unfortunately your graphs are not very helpful. I can tell you that according to your fuel trims it is running fairly rich because it is running the trims negative trying to subtract fuel. Good numbers stay within 5% +/-. Anymore than 5 on the STFT either way indicates a problem. Easy way to diag the cat though is to warm it up by doing what you described, then while monitoring the B1S1 and B1S2 voltages over layed in the graph format, hold the engine at 2,500 Rpm and watch for the Rear O2 to start mirroring the Upstream. This indicates a faulty cat. The ECM's thresholds for appropriate switching frequency are very strict. But anyway... If you are certain your dont have issues like exhaust leaks, un-metered air leaks, leaking injector/regulator etc, and setting a P0420, rest assured you have a bad cat on Bank 1. Dont scratch your head too much with it.

Thanks for the reply, slopoke. I certainly understand what you're saying about the STFT values being possibly too negative. However, if I understand what you're saying about monitoring the sensor voltages on Bank 1, the 4th graph should be showing that between the 300 and 600 seconds timeframe when I was running the engine at about 2750rpm. Correct? And it doesn't look like the voltage on B1S2 was mirroring B1S1 during that timeframe, correct?

However, I still don't know for sure about any air or exhaust leaks or leaking injectors, etc. I haven't had the code show up in about a week of driving though, so I'm wondering if the fuel system treatment (Lucas) I added could have done anything to help?

The ECM setting a P0420/430 is an almost fool proof DTC. In my experience I have seen nothing but a bad cat set this code ever. Usually it's a bit more obvious than your particular case. It is possible that the cat just barely fails the ECM's test of switching frequency and not appear to the eyeball to be bad. Like I said don't scratch your head too much with this problem just put a pair of cats on it

I put a full flow K&N filter on mine and the P0420 quit setting off the check engine light but when it gets over 80 degrees outside it will come on again just not as often as before.What cat is throwing the code if it is the P0420 code? Those cats look like a pain to replace.

I found this write up that is supposed to cure the problem?
http://www.energeticforum.com/renewable ... ulers.html