P0420 Code Thrown [Archive] - GrandAmGT.com Forum

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Sebgonz
03-18-2009, 06:36 PM
What do you guys think is the greatest possibility of being wrong when getting this code?

I had it read at AutoZone, and here's what their printout said.

P0420

Catalyst system efficiency below threshold (Bank 1)

Explanation:
The oxygen sensors monitor the Catalytic Converters ability to store oxygen.


Probable cause:

1. Catalytic converter defective (failure possibly due to # 2, 3 or 4)

2. Engine misfire or running condition.

3. Large vacuum leak.

4. Engine oil leakage into exhaust-valve guide seals, piston rings.

I'm almost positive it isn't number 2, since I can tell when the engine is misfiring.

In your experience, what's the greatest possibility? Where should I start?

DoCDooM
02-22-2010, 07:42 PM
#3 If the engine has any kind of significant vacuum leak the engine will hesitate VERY badly from idle to wide open throttle, get yourself about 2 feet of quarter inch vacuum hose, stick one end in your ear and the other gets stuck in all the nooks and crannies under the hood while the engine is running (be careful you'll or prove Darwin right yet again). The telltale "Hiss" will present itself eventually. FYI: This only works if you have a nice quiet exhaust system, if you have something rude and noisy that would be the first place to look for problems.
#4 Id oil is leaking into your valve guides then your exhaust gas will contain a LOT of blue smoke on startup which disappears after the engine warms up a bit (say 30 seconds). There is NO way for the car to hide this symptom regardless of emissions controls, the smell is also a telltale, if you don't have oil smoke, your guides are fine.
#2 My 2002 GAGT had a bad misfire after I made the mistake of paying a so called "professional technician" (Come on people MECHANIC!) to change the plugs, I told them to change the wires if it was needed and they didn't, the misfire wasn't really all that noticeable until I opened the hood on a wet day and saw the sparks flying, you might want to reconsider #2
#1 If the converter fails, take the pipe off the tail end and drive a tire iron through the honeycomb to open it up, solder a resistor on the wires for the O2 sensors to fool the computer, do your homework on the engine and drive free man, a properly tuned engine will run clean without them.

AaronGTR
02-22-2010, 07:49 PM
#1 If the converter fails, take the pipe off the tail end and drive a tire iron through the honeycomb to open it up, solder a resistor on the wires for the O2 sensors to fool the computer, do your homework on the engine and drive free man, a properly tuned engine will run clean without them.


That's absolutely not true. Yes, a properly running engine will put out less emissions than one that is worn out or has other issues causing it to run poorly, such as dirty injectors or a faulty sensor... but no matter what you do to it, it will put out less emissions out the tail pipe with a functioning cat than without one. This is not theory or debate. It's fact. All you have to do to prove it is stick an emissions tester up the tail pipe. Also it's a little more complicated fooling the rear O2 sensors on these cars than just soldering a resistor between a couple wires. :rolleyes:


To the OP, there is one other explanation that isn't on that list, and that is the rear O2 sensor itself could be bad. They do wear out eventually and have to be replaced. You might be able to try swapping the front and rear sensor and see if the problem persists. I think the rear one has a longer wire lead on it though so not sure if that will work. Either way, if you have to change either of the sensors, use some PB blaster or liquid wrench on them. They are usually stuck pretty good.

DoCDooM
02-22-2010, 07:50 PM
I tried it dude, works like a charm and the car passes emissions testing every year.

[ChaosweaveR]
02-22-2010, 07:52 PM
Replace the cat. N-bodies are known for clogged cats. Autozone sells the direct fit model, made by Magnaflow. They sell for $160.

AaronGTR
02-22-2010, 07:56 PM
I tried it dude, works like a charm and the car passes emissions testing every year.


Bull****. What kind of emissions test do they have up there? Do they just do a computer check for codes? Or do they actually test the exhaust emissions?

Because in many states in the U.S now, especially NY and CA, you won't pass without a cat. Plus they do a visual inspection of the car, and not only will you not pass without a cat, you'll get a fine. They are required by U.S. law nation wide.

stillriza
02-22-2010, 10:10 PM
To the OP, there is one other explanation that isn't on that list, and that is the rear O2 sensor itself could be bad. They do wear out eventually and have to be replaced. You might be able to try swapping the front and rear sensor and see if the problem persists. I think the rear one has a longer wire lead on it though so not sure if that will work. Either way, if you have to change either of the sensors, use some PB blaster or liquid wrench on them. They are usually stuck pretty good.

lol ...03-18-2009 is the post date of the OP.

that's ok cuz your adding valuable info. I'm just being an ass.