the infamous P0420 code

[jeff18]

Hey, this is my first post - so my apologies if I'm not descriptive enough.

I have read many posts on the P0420 code and there seems to be varied answers (which doesn't surprise me).

I have a 2004 3.8L grand prix GT non turbo

The code from auto zone reads catalyst efficiency low-bank 1

[It then says cat defective possibly due to:
1-engine misfire or running condition
2-large vacuum leak
3-engine oil leakage into exhaust-valve guide seals, piston rings]
note: no mention of 02 sensor???

I have read things from checking the heat before & after the cat with an infared thermo scan to changing & not changing the rear o2 sensor as well as you must/don't have to change the cat

Can anyone "with actual experience on this particular matter" advise?

Thanks in advance

Jeff

[tblake]

Although, probably not a bad place to start...

1-engine misfire or running condition
2-large vacuum leak
3-engine oil leakage into exhaust-valve guide seals, piston rings]
note: no mention of 02 sensor???


If P0420 was the only stored code, I doubt any of these will be your problem as each of these will have their own code (ie: p03000 missfire, P0171 lean). Check for exhaust leaks.

The best thing to do would be to monitor live data on a scanner of the rear O2 sensor. Make sure it fluctuates between 0-1V. Then cause a vacuum leak and see if the rear O2 indicates lean, and dump some propane in the vacuum leak to see if the sensor responds with rich. If the Rear O2 is operating correctly, chances are your Cat is shot.

How many miles?

[Tech II]

I agree.....but you don't have to create a rich or lean condition......if you have a scan tool, plot the front and rear O2 sensors.....if they mirror each other, the cat is bad........the front O2 sensor should change rapidly, the rear should change slowly and and will be rich or lean, depending on if the cat is storing or utilizing the oxygen.....

From experience, I can tell you that if the cat is bad, the rear O2 is not far behind(so I recommend changing it)......this is usually what happens when a cat gets old.......

However, if the cat is damaged because of a misfire or other condition which could cause the cat to run overly rich or lean, in addition to replacing the cat, it's imperative that this condition for failure is remedied....

[gtpjay]

I bet it's the converter that's what fixed mine.

[Stealthee]

The cat doesn't have to be bad to get a P0420. It could be a bad o2 sensor. I got that code when I had an exhaust leak at the flange where the downpimp bolts to the crossover.

[danielsatur]

How many miles are on this car?
You might want to check the federal extended emissions warranty.
Also do a Google on '' H20 cheaters '' until a new cat.
OEM Cats www.catalyticConverterwarehouse.com