This one could be tricky to solve.
First, a bit of deductive reasoning.
In order to overheat the catalyst to that extent, an excess of fuel must be entering the catalytic converter. Since there is no other path to the catalyst than through the engine, the excess fuel must be coming through the engine.
Since a mixture that is sufficiently rich as to overheat the catalyst must exist, it would follow that the O2 sensor should see this and cause a trouble code to be set in the computer. This in turn would light the check engine light.
Now to the diagnostics.
First verify that the check engine light is functioning correctly. It should ligjht up when you turn on the ignition, and stay lit until after the engine starts.
Second, check for trouble codes. It is possible for codes to exist without lighting the check engine light. It would be an unusual condition, but this is an unusual problem.
Some possible causes of an excessively rich mixture would be:
Mass air flow sensor.
Air leaks in the intake path.
Bad O2 sensor or sensors.
Charcoal cannister overfull.
Fuel tank vent restricted causing excess fuel into the engine.
Also, there is a possibility that your catalytic converter is internally restricted.
These are a few ideas that come to mind, and should help you with a starting point to figure out this problem and work towards solving it.
Saturday, March 7th, 2009 AT 6:14 AM