Ongoing P0420

2000 safari 170,000 miles.

1 year ago I had a cap and rotor fail and I pumped a lot of raw fuel into the exhaust trying to get the van started, replaced cap and rotor and van started right up. About a week later I got a P0420 code, figuring I had pumped to much fuel into the exhaust and burned out the CAT I went and had it replaced with an aftermarket CAT. In november got P0420 code several time after reset so had the CAT replaced under warrenty. Four weeks ago I got a multipule misfire code and P0420. This time I replace cap and rotor with delco parts and cleared codes. 2 weeks ago I got a P0402 EGR code, removed clened and replace with a screened gasket and cleared code. Last week got a P0420 code again, cleared code and as soon as MIL became "ready" code reset, this has happened 2 more times. 2 CATS in one year and I am getting a P0420 again........anyone have any ideas.

Thanks

P0420 is low efficiency of the TWC, (three way catalyst). The monitor that can set the P0420 relies heavily on the fuel mixture controls, (oxygen sensors, and fuel delivery components). Oxygen sensors have a reaction time that detects changes in fuel mixture, from lean to rich and from rich to lean. When the monitor runs its test, the PCM rapidly changes the fuel mixture from one extreme to the other and measures the time for the change to be detected at the post catalyst oxygen sensor. If the reaction time is slow on any of the oxygen sensors, the test results are not reliable. The PCM has no way of measuring the reaction time of each oxygen sensor, it's the responsibility of the technician to determine if they are within specs. At approximately 100,000 miles, the oxygen sensors reaction time has slowed enough to cause the P0420 to be set. This is only one of the reasons why it's recomended to replace them every 100,000 miles. You might want to check fuel pressure and leakdown rate also.

I will get the O2 sensors checked.

Thanks

More expensive to get O2 sensors checked then replacing them. Changed all three with Bosch direct replacment sensors, code returned first time out???????

Are there any other DTC's displayed along with the P0420? Are there any exhaust leaks?

No other codes and I checked for leaks after O2 install and all was good.

A little more to the story now that I have thought about it some more. After I cleared the latest code I went thru and checked all systems to see if they all showed "READY" typicly I find they will still show ready until after I shut the key off, after an "ERASE" then they will all reset to "NOT READY" I remember seeing during that check that the heated O2 sensor was not showing ready. Since the reset and my last email my wife has made a trip from southeast Texas to central Missouri and is now on the way back and the CEL has not come back on. I will check again when she gets home with the reader and see what I find.

Thanks for all your advice and help "old master" I will keep you updated.

Marty