I have a 6th gen (4 cylinder, automatic) with about 115k miles on it. The check engine light came on for the first time ever. So I took it to Autozone and had them check it out for free and came back with Code P0420.

The light only came on after my sister drove the car from NJ to NC and back. Never before. Car has no problems at all and still drives perfectly fine. I don't think it's any of the possible causes listed below because the car runs fine without any problems and if it was something like listed below, the car would not be running good. Sine the gas tank was filled a few times on the trip, is it possible to have gotten some bad gas at a station or the maybe someone didn't tighten the gas cap a minimum of 3 clicks and that's what setting off the CEL light?

I haven't reset the light yet because I would like to know what could possibly be setting it off. I think once I go to fill it up again and it still stays on, I'll reset it and see what happens.


I know what the code means...

P0420 67 Catalyst System Efficiency Below Threshold

On the "receipt", they have the definition and explanation of the code, as follows:

Definition - Catalyst Efficiency low-bank 1 (bank 1 identifies the location of cylinder #1, while bank 2 identifies the cylinders on the opposite bank).

Explanation - The oxygen sensors monitor the catalytic converters ability to store oxygen.

Probable Causes:

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

 

Has anyone else got this code as well, if so, how did you clear the code/remedy the problem? Thanks.

 

what year is the car? just want to know if its obd2 or obd1

anyways, u need a scanner to diagnose it.

u need to do a swing test on the o2 sensor and it will isolate the catalytic converter.

but usually, in this case, ur cat is degraded already.
if the post 02 sensor passes the swing test then u will need a new cat but do not change the post 02 sensor

 

The car is 1998.

 

change the cat keep the o2 sensor

 

Update:

The check engine light cleared itself!! Turned off yesterday as I was driving. I basically didn't do anything except that I put in a bottle of Lucas Fuel Injector Cleaner and let the gas tank run almost empty before filling up again. I didn't do any resets, didn't pull any breakers, didn't disconnect the battery, didn't replace the cat or the O2 sensors, etc. This time I made sure the cap was on there securely and heard a few clicks, because the person pumping the gas just threw it on without a single click. I drove it for about 50 miles before the CEL extinguished. I did step on the pedal like once or twice and had the throttle wide open for a few seconds while driving.

 

that doesnt make sense, if your gas cap was loose, you would get an evap code and not a cat efficiency code.

anyways, im still guessing that the code will come back again.

but for now, im glad that your light was off.