The dreaded P0420 code

[Togemachi]

Hi,

I'm brand new to this forum as a poster, but I've come to this site several times for advice. Now I have a somewhat unusual situation.

I own a 2001 SLE, which I've had for about seven years, but I was living/working outside the country for five years and recently returned. I'd come back every Christmas and drive it for about 2 or 3 weeks out of the year. Sadly, it didn't get driven much outside of that during the rest of the year. Now karma is catching up with me.

A few months back, it threw the P0420 code, saying the Catalytic converter had gone bad. This struck me as somewhat odd because I had it replaced with an aftermarket a couple of years before (components inside the original were loose and making a lot of noise). But since it has sat on a driveway a lot since then, the mechanic I took it to suggested I might have a much larger problem on my hands--maybe sediment in the fuel line. I have no proof of this at all outside of the code itself. The car had generally been running fine in spite of that code and the engine is free of any exterior leaks. (I also did an oil change last week--the oil condition was normal after 4k miles.) In any case, I poured fuel injector cleaner in on an empty tank and replaced the fuel filter (which was about as new as the replaced catalytic converter). They also told me to try and use Mid-grade gasoline instead of Regular to see if that would help (which I have been doing for about 6 full tanks of gas now).

Then I took it on a 2000 mi road trip. About 600 miles into the trip, the engine began to sputter and lost about 60-70% of its power. It threw another code--this time for the Crank Position Sensor. Since I was on the road, I didn't have much time to diagnose, so I got the part, located a garage, and quickly had it replaced. The problem was not fixed, however. I took it to another shop the next day and was told the coil packs weren't generating enough of a spark and was basically leading to misfires in the cylinders. The symptoms developed pretty suddenly, because I hadn't noticed anything since the P0420 code had kicked in--easily put on 1000 miles between that and the coil pack failure. Anyway, I had 2 of them replaced and 300 miles later, replaced the final one myself (could have saved myself hundreds of $$$ if I had known how easy it was). Over 1000 miles since then, the engine seems to be doing much better. The only thing I notice that's painfully odd is the transmission's shifting times. It will shift into fourth between 30 and 35 MPH. But after clearing the computer, the P0420 code remains.

So basically, I'm stumped. At the moment, it's very drive-able. I've been told my car's coil pack issue is one that affects a great number of 'granny cars' that don't get driven enough. So now I'm not sure what could be causing this code. Is this a gasoline line problem? Is it electrical? Could the spark plugs and/or the wires (both with about 12k miles on them) be causing this issue? Or is it just the catalytic converter after all? Also, should I be concerned about the early shifting of the transmission? (I had the transmission rebuilt and a new gasket kit installed on the top half of the engine about 5 years ago.)

Any ideas of where I should go from here?? Should I just bite the bullet and have another cat installed? I've already pumped about $1500 into this car in the last two months--I'm hoping I don't have to do much more before emissions inspection comes up this year!

[Ken_W]

I'm not certain here, but one possiblity is your rear (post-cat) O-2 sensor needing to replaced. How many miles are on the car, and have you ever replaced either the front or rear O-2 sensors?

Ken_W

[SSEiMan01]

Depending on the type of cat you replaced it with, it could be throwing off the sensor. I had my stocker replaced with a magnaflow hiflow, and it set this code. You can either get it tuned to not set the code ever again, or do what I did and buy an emulator from ZZP.
EDIT for link: http://zzperformance.com/grand_prix/pro ... php?id=967
The connector you plug it into is buried in a heat shield about 6 inches forward from the sensor, I just drilled out the two back rivets and pried the shield enough to connect the emulator and replaced the rivets with screws. Went the next day and passed emissions and have been light-free since