Ok first a little back story:

Car is a '98 9000. Last saturday (the first hot day of the year) I was pulling away from a toll booth and hit about 5k in 1st spun the tires and the car pretty much died on the spot going into 2nd. No other idiot lights came on, and I pulled off at the next exit to check the car. Coolant level/temp is fine no visable blown off hoses. The car still boosts a-ok. Brought the car to autozone and found out the car misfired 2 times (has 2 misfire codes), and a code for the idle air control valve, which my car doesnt even have.

Now I know the first suspect are the sparkplugs, so I replaced them with the correct ngk's gapped to .9mm. And reset my battery to clear out the code. Check engine light came back the same day, and then went away a few hours later by itself. Now today, about 3 days later the check engine light came back on. I havent had the code read, but I wasnt pushing the car at all. Didnt fully boost, dont go above 3.5k rpm and wasnt going much over 45 from the time I started the car up till the time the light came on. The weather was almost the exact opposite, 45 and raining vs 85ish and sunny.

So yesterday I swapped di cassettes with a known good one and went for a drive. Neither car had a check engine light. Swapped the di's back and within 20 minutes I had a cel again. Brought it to autozone to have the codes read. Came back as p0420 bank 1.

Last may I had the same problem, and replaced both o2 sensors and cat converter in order to get my car to pass inspection. Now a year later i have the same problem, very frustrating.

Anyone know what o2 sensor p0420 bank 1 refers to? Is it possible the misfires actually damaged the o2 sensors? Maybe removing them and cleaning them off with a torch would work?
thanks

Hey there, it's Newf9000 (TSL) I found this bit of info for you. Hope it helps!


P0420 OBD-II Trouble Code

Technical Description

Catalyst System Efficiency Below Threshold (Bank 1)

What does that mean?

Basically this means that the oxygen sensor downstream (after) the catalytic converter (http://www.obd-codes.com/trouble_codes/generic/p0420-catalyst-efficiency-below-threshold-bank-1.php#) on bank 1 is detecting that the converter is not working as efficiently as it should be (according to specs). It is part of the vehicle (http://www.obd-codes.com/trouble_codes/generic/p0420-catalyst-efficiency-below-threshold-bank-1.php#) emissions system.

Symptoms

You will likely not notice any drivability problems, although there may be symptoms.

Causes

A code P0420 may mean that one or more of the following has happened:


Leaded fuel was used where unleaded was called for
An oxygen sensor is not reading (functioning) properly
The engine coolant (http://www.obd-codes.com/trouble_codes/generic/p0420-catalyst-efficiency-below-threshold-bank-1.php#) temperature sensor is not working properly
Damaged exhaust manifold (http://www.obd-codes.com/trouble_codes/generic/p0420-catalyst-efficiency-below-threshold-bank-1.php#) / catalytic converter / exhaust pipe (http://www.obd-codes.com/trouble_codes/generic/p0420-catalyst-efficiency-below-threshold-bank-1.php#)
Retarded spark timing
Possible Solutions

The simplest thing to do is to reset the code and see if it comes back.

Next step is to measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter). The voltage should be low and steady (~200mV, depends on the vehicle). If the voltage varies (e.g. 100mV to 600mV) then it's time to replace the catalytic converter. In fact, it would be a good idea to test each oxygen O2 sensor (http://www.obd-codes.com/trouble_codes/generic/p0420-catalyst-efficiency-below-threshold-bank-1.php#) while you're at it.

One thing to note is that many vehicle manufacturers offer a longer warranty (http://www.obd-codes.com/trouble_codes/generic/p0420-catalyst-efficiency-below-threshold-bank-1.php#) on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.

hmm.. I was afraid it was the catalytic converter. Is there a seperate error code if the rear o2 sensor fails?

Why do these things always have a way of cropping up right before inspections. I guess a 55 dollar catalytic converter is cheeper then a 100+ dollar o2 sensor.

hmm.. I was afraid it was the catalytic converter. Is there a seperate error code if the rear o2 sensor fails?

Why do these things always have a way of cropping up right before inspections. I guess a 55 dollar catalytic converter is cheeper then a 100+ dollar o2 sensor.

Here's the site I got the info from.... http://www.obd-codes.com
So you don't have any exhaust leaks before the cat do you?
It also mentioned the coolant temp sensor as a possible cause. Have you replaced that? It's fairly inexpensive and generic so you should be able get to one easily.

Well it is possible where where the turbo and downpipe attach there is a leak. One of the studs broke 1/2 way in when I was replacing the downpipe to 3" so the downpipe is slightly ajar. But in my scientific hand test I didnt feel any gasses escapeing. I'll get some seafoam and see if there are any leaks. I'll look into the coolant temp sensor too. The gauge seems to be acting correctly. Slowly goes to about 8:30 and stays put.