P0420 OBD Trouble Code

Its a 98 Transport with 138K miles on it. I've replaced the O2 sensors and cleared the P0420 code but it returns after about 500 miles.
I have an "Autotap" to read the engines #'s but not sure what else I can check out. Any suggestions would be great. Thx


Found this:
A code P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for - Not possible
An oxygen sensor is not reading (functioning) properly - Both new
The engine coolant temperature sensor is not working properly - Maybe
Damaged exhaust manifold / catalytic converter / exhaust pipe - No damage
Retarded spark timing - ECM controlled

P0420 is basically set when the value at the downstream O2 sensor is below a threshold compared to the one upstream. The downstream O2 sensor is just there to measure the efficiency of the catalytic converter. If it is starting to fail, then it doesn't coverter as well as it should and the O2 sensor picks up on it. The downstream converter doesn't enter into the engine management equation in any way, so it's more of a "oh by the way, you might want to fix this sometime" type of code. It's possible though that it could have secondary effects such as restricting the flow of the exhaust due to a partially blocked converter.

The catalytic converter can fail in a number of ways. If your was having other problems, the converter can get plugged up or "poisoned" decreasing it's efficiency. I had a similiar problem earlier this year when my head gaskets were just starting to go. Coolant was leaking into the cumbustion chamber, burning off, and eventually screwing up with the O2 sensor and converter. The converter also does have a limited life but I don't think that is the case here.

When I had my problem, I just lived with it as I had other priorities such as the intake manifold gasket and head gaskets I was battling with. It eventually went away and hasn't come back so I'm not worried about it.

Don't think the CC is my problem from what I've read. A clogged CC would have performance issues and a if it was falling apart internally it would rattle. :confused:

Think I'll check into the engine a bit further maybe other clues will surface.