1997 Pontiac Grand Am 2.4l. Had misfire problem
first, replacing the ignition parts fixed problem. But,
catalytic converter went bad. Replaced converter
and front and rear o2 sensors.
Then gas cap for
evap leak. Now have a p0420
code.

Is it the converter or rear o2 sensor? Or something
else? Mass airflow or map sensor? Thanks for your
help, if you could respond soon it would really help
me. Because I think the shop that replaced the
catalytic is trying to get out of warranty. Thanks

Answer:
Since you seem to be in a bind, i will help now. We
were all taking a break for a while, its been busy.
P0420 is cut and dry 'catalytic Converter
Inefficient' 99% of the time it IS the converter.

Have only had 1 bad post cat. O2 sensor cause
this in the last 10 years. If the post cat. O2 is bad,
it will set a code for 02 sensor, not usually catalytic
Inefficient.
Problem with my 2001/ Pontiac / Grand Am2.4 L.
Changed spark plugs.
The car intermittently misses under normal driving conditions
every once in a while. Changing the spark plugs makes it go
away but a day or so later it comes back.

Answer:
If changing the plugs helps, i would look closely at the plugs
that were removed. Make sure they all seem the same color. If
one is oil fouled, or looks a little greenish or orange
(depending on which coolant is in the system at this time) this
would indicate an internal engine problem. If one is totally
black, this would indicate a rich fuel condition which may be
caused by a faulty injector or is more than one is black, then i
would suspect a leaking fuel pressure regulator.
MORE...
P0420 On Pontiac Grand Am

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