seanpooh said:
This does happen to me on occasion. If I don't run any lucas fuel injector cleaner or chevron techron for a good couple thousand miles, the CES come on. Same goes for my brother's Saturn Vue. I happen to drive with a scanner and it always states that the catalytic convertor is not up to threshold.
Run some injector cleaner in the tank and it goes away in less than 20 miles. I assume it happens with bad batches or long rides, something builds up and give a reading the computer doesn't like.
I think we are discussing separate things. I never had any code as you did for P0420 (catalytic converter efficiency).
Check for other codes as the P0420 is the end result of something wrong. In this case check fuel trims. They could be way off.
With the engine at operating temperature look at the front and rear O2 sensors on a scan tool with the rpm around 2200. If the rear O2 sensor is mimicking the front one, the converter will most likely need to be replaced. Usually if you are getting a P0420 then the catalytic converter is not converting HC's into H2O and CO2. The P0420 will not show if the PCM detects that the oxygen sensors are bad and throws a code for them. The converter efficiency test is done by the PCM and it depends on good O2 sensors.
A less scientific test for this condition is to stick your nose in the exhaust after the engine is fully warmed up and held at 2200 rpms and take a couple whiffs. If there is little smell then the converter is doing it's job.
Now, as to the reason for the P0420. It's no joke that too rich of a mixture will carbon foul a converter. That is why we are always on the lookout for any early symptoms of failed thermostat or coolant temp sensors. I would also be wary of excessive engine wear and oil consumption which could also foul the cat.
For a more detailed explanation see
The Misleading Nature of the PO42O Code - Eastern Manufacturing
