Jim Grant's Tech Tips
'00 Pontiac Grand Prix GT, Inefficient Converter Code?
Q:
I drive
a ‘00 Pontiac Grand Prix GT (3,8L Engine) and the "check engine
soon" light came on today. I went to a local parts store for their
free diagnostic machine and it reported that the problem was a “PO
420” The PCM has determined that the catalyst system efficiency for
Bank 1 is below threshold for the current engine operating conditions.
What does that mean? Oxygen sensor? I checked the gas cap and it was on
tight. Could a bad air filter cause the problem?
-
John T.
A:
What
does it mean? Each time you start the engine to your vehicle the computer
runs
tests of the catalytic converter. If the converter fails the test the
computer turns on the “service engine soon” light and will report
the diagnostic code P0420. What this all means is the converter is not
working right. A replacement converter may be needed but I would suggest
a few diagnostic checks first. The vehicle’s computer uses information
from 2 different oxygen sensors to compare the oxygen entering the converter
to the oxygen exiting the converter. The sensor at the exit of the converter
will read a different value than the sensor at the front of the converter.
That’s because the converter uses up oxygen in the final phase of
operation. If a sensor is not reporting to the computer correctly (lying)
the computer could falsely fail the converter. Also, be aware that the
exhaust cannot have any leaks. Exhaust leaks near the oxygen sensors can
and will cause false readings. One last note, if your converter has failed
check with your dealer. There is a time and mileage warranty on the converter
and your vehicle maybe covered. Most of the time, all the dealer will
need is your vehicle identification number and the mileage when you call
to answer that question.
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