for the past few weeks/months my car would randomly throw a 420 code. It would stay on for a while then it would go away and come back a few hundred miles later, then go away again. (Repeat) They guys at Autozone said it was a misfire, after they did a free scan. I was due for new plugs anyway and wires so I went ahead and changed them (fun job ) Well that didnt help. I went to the local speed shop and talked to the guys there and they said that its either your cat or a bad O2. So they put the car on the Dyno to check if it was loosing any power due to a blocked Cat. It ran strong. right were I expected it to be, slightly better actually , (they did say it was running a little rich nothing to worry about though) They said its most likely the O2s then. So i went out and bought a set of new rear O2s (not cheap) and put them in. Well after about 150 miles the SES light went off, so i thoght the problem was finnally fixed. The light has since com on and off a few times since then. Now it seems to be constantly throwing the 420 code because the light hasnt gone off in well over 1000 miles. I had it checked and it is still throwing a 420 code I had them clear it to see how long before it came back on. It came back one within 25 miles of leaving the Autozone.

So do i bite the bullet and replace the Cats or is there something else I should check first. I am out of ideas but I hate to Dump all that money into new Cats. Inspection is comming up in less than 45 days so I must do something soon.

If I have to replace the Cats is there anyplace to get it done cheap? or if possible any way I could get the cats cheap and do it myself?

Thanks guys,
Sorry about the long post. But any help would be great!!

Ed

 

The code is intended to indicate that the cat is not "storing" the levels of oxygen required to clean the exhaust. It is set by comparing the pre-cat signal to the after-cat signal. When the after-cat sensor approaches a preset amplitude level of approx 90% of the pre-cat sensor, the light comes on. Its not an indication that the cat is "plugged". If you have ruled out the after-cat O2 sensor on the drivers side, the drivers side cat would be the next likely candidate for replacement.

 

and that thing is a easy to replace. I replaced one because of damage to it (long story) on a freinds car, luckily you can order the whole driverside pipe assembly from GM making the welding and fitting not an issue. Just make sure to soak the manifold flage bolts good and soak the pipe junction up good as well. It takes some patience and a lot of rubber mallet pounding for the junction, but its nothing that is too hard.