I went to Auto Zone this morning and the correct code was not 220 but is Po 420 Catalyst beyond threshold. The reasons that it gave is that either I have a major vacum leak, leaking oil into the o-rings, have a bad convertor or have a mis-fire. Could the mis-fire be caused by bad plug wires. I just had them replaced two months ago. Could the car be running fine but still have a mis-fire.

mis fire. The extra gas is killing your catalytic converter. Next time you have the car running slow look under and see if its glowing.

Any mods? How many miles?

Like fallen angel said, sounds like your cat is bad. Maybe your rear O2 sensor.

Iirc this is his third cat this year. Check your coil packs and ensure they aren't corroded. Ensure your plug wires are pressed all the way on the plugs. Have you changed your front O2?

damn.. feels good to have an idea whats wrong for a change. Ive been here all before. Only difference was the cruddy location of the ignition modual on the cavy. I'd check the wires first, sounds like one might not be on all the way. These engines its hard to make sure.

Both o2 sensors relaced two weeks ago before this code came up. I've had trouble about six months ago that the garage said might have been caused by a bad plug. All plugs were burned down to nothing, plug wires were burned out and lots more.Message to framos242, no mods yet except to the stereo system ( amps, subs and cd player and new speakers). What mod's can be done on my car, 2002 Grand Prix se 3.1L

Wow more info. What do you mean by plugs burned down to nothing and wires burned out?

If it's what I'm picturing there is something more wrong with your car then we think.

plugs burnt to nothing.. someone else chime in here but I would think thats a case of running very lean. I'm going to have to take a second to think about this one.

After everything was fixed, I asked the mechanic what had caused all the problems. He told that it could have been something as simple as one bad plug that dominoed the rest of the system. They had to run my car through a de-carbonizer? for and hour and a half to get all the carbon out of the system. But the car was fine until about two months ago when the first cat went out. I think this was from the problem above and just didn't go out until two months ago. The mechanic told me that most likely what happened was that when the cat was plugged, it affected the #1 o2 sensor because it was so hot between the cat and the engine. I had the cat and both o2 sensors replaced about two week ago and the code (po420) came up about 4 days later. I'm just hoping that the cat doesn't go out again. It's not giving the same code that it was giving before the cat burned out two weeks ago.

Or busted up because of knock?

Are you using any kind of fuel additive?

There are so many variables. Could you possibly do a write up starting from way back when your original cat went out and give us all details. I really would like more info on those "burned" plugs.

About six months, my car started making a loud knocking noise that I thought was a thrown rod and the check engine light came on. I had it towed to a garage and about 800.00 dollars later I picked it up. They thought I had a bad rocker arm but after pulling the valve covers they found that one of my engine mounts had come loose because the car was running so badly that it had shaken it loose. The car was running so badly because my plugs were burned down to no ponits at all. The wires were burned out. My oil was full of gas and they put it on a de-carbonizer for an hour and a half to clean all the built up carbon out of the system. I asked what had caused this and they told me that they didn't know. They guessed that it could have been a bad plug or wire and it just made the other plugs or wires work harder and then burn out.I didn't notice any trouble with the car and no engine lights came on until it broke down. About two months ago it started to really struggle in order to get up to speed. After a couple of day's it got to the point that I had to floor it in order to get through an inter section. After taking it to a garage and having all sorts of test run they decided that it was the cat convertor. I got that replaced and then about four days later my service engine soon light came on. I took it to auto zone and they said the code was for my cat convertor and the choices were either leaking oil into my o rings, a bad oxygen sensor, a major vacum leak or a bad convertor. I took it back to the garage and they said that they had missed a weld on the convertor (vacum leak). They welded it and cleared the codes. About a week later the engine light came back on. I took it back to the garage several times and they told me that they were not positive but that my #1 oxygen sensor was a little slow. Due to finicial issues I did not replace the sensor at that time, hoping that it could wait a couple of months. After only about a month and a half the converter burned out again. I had the garage replace the converter again and told them to replace the sensor. They then told me that I had to replace both sensor's at the same time to do it right. About four days after this a code came up again and I went to auto zone. The code was po420 Catlyist beyond threshold. The choices given in the computer for this was leaking oil into the o-rings, a major vacum leak, bad converter or a engine mis-fire. I went to the dealership and the asst service manager didn't have a clue and told me that they would have to look at it to see. It sort of sounds to me like the converter is too small and can't handle the amount of exhaust coming through it. When I burned up my two converters could I also have clogged the muffler a little bit and this is causing the code. When the first converter burned out I was not showing any codes before the garage replaced the converter.

The main reason for cats to clog is raw fuel getting in there. At any point did you notice a rotten egg smell coming from your exhaust? Since we are leaning towards unburned fuel you need to ensure all your plugs are good all your wires are good and properly seated. Look at your coils and ensure the posts are free of corrosion. Battery connections are tight. I'm not an expert but my guess is you have weak coils.

The burnt plugs really worry me.

What I can't get is the shop can't figuere out whats going on. I say a new shop is in order. But definately some kind of fuel/ignition problem.