Engine Code P0420

[rtobrian]

I cannot get rid of this code. I've reset it multiple times. I replaced the Catalytic Converter, the Oxygen Sensors, coolant temp sensor, ignition coils, module, plugs, wires, and checked/replaced all vacuum and breather tubes. The engine isn't misfiring and the car idles smooth. Could it be something with the timing being retarded and if so should I look at replacing the camshaft and crankshaft sensors? I really don't want to go to the dealership for a dummy light. It's an 02 with the 3.4. Thanks

[andretti]

So this is the only code it's generating? Then recheck the downstream O2 sensor and harness. Also make sure the exhaust system is leak free with no restrictions.

If all is good, buy a better catalyst.

[rtobrian]

That is the only code it's generating. I replaced the oxygen sensors with Bosch then a few months later with Denso's so I don't think it's a sensor issue. I went back to the muffler shop twice then to a different one compaining and they both said the convertor was functioning fine and that it was the correct one for the car. I don't see any exhaust leaks and as far as a blockage goes, don't know... How do you check for that, see if the exhaust is glowing?

[andretti]

I think when the GM service procedure says to check for restrictions, they're talking about a gross restriction like a crushed or folded pipe.

You've got no performance issues but the ongoing code shows the catalyst is not efficient. Now that you've added that the O2's have been replaced several times then the cat has to be suspect.

Does Michigan have I.M. emissions testing? I'm sure your pipe emissions would be over limits with this issue.

A more complex way to check the cats efficiency involves actual gas sample readings before and after the cat. This is difficult to do in the real world and not worth the trouble. Just have it replaced and stay away from the universal and no-name parts. I've had good luck with Walker and Bosal.