P0420 -- Catalyst System Efficiency Below Threshold (Bank 1)
I would NOT replace the O2 Sensor(s) or the CAT at this time.
The code says that the stuff coming out of the CAT is not up to snuff, but the stuff going in is okay. This means the O2 Sensor(s) is(are) okay because they would throw their own code if they were having a problem. The CAT has no moving parts, and you are hard pressed anymore to find leaded fuel to put into the tank -- which is the primary means of destroying a CAT. All you have left is a false reading from the after-CAT sensor.
Of course, there is an outside chance that the wiring is hosed, but I'd be inclined to discount that possibility for now. I have a newly acquired '95 Bronco that was throwing a code tied to the O2 Sensors, I replaced the affected sensor and the code is gone. My point is, the wiring is in good shape, and my truck is 5 years older than your car.
I'd be at the parts counter requesting the after CAT sniffer before I spent more than a second or two on other possibilities. Replacing the sensor is an easy job, it's much like replacing a spark plug. The only difficulty I foresee is that it might be corroded in place.
I have P0420 faulty code issue for over 6 months, and now it is the
> time for PA state inspection. I talked to some guys at Firestone auto
> shop and was told that both O2 sensors need to be replaced along with
> catalytic converter in order to ensure catalytic converter warranty. I
> tried to back-probe the post O2 sensor for my 2000 Corolla with 150K
> miles on it, but failed(Followed the auto repair book, but cannot id
> anything, maybe it is too complicated for me). Firestone asked $100 for
> computer diagnose for the problem and they also said that if post O2
> sensor was bad, they would not know whether catalytic converter was
> good or not because of misleading signal given by post O2 sensor.
> Basically they cannot give positive conclusion on whether post O2 is
> good or not by running computer diagnosis. I have it diagnosed at
> Toyota dealer 6 months ago and when I tried to retrieve diagnose info
> from dealer, what they have is the suggestion to replace both O2 sensor
> and catalytic converter(at that time, I had no idea O2 sensor can cause
> P0420 faulty code) at $1500.
> Here are my questions:
> Is it necessary to replace both O2 sensors for bad catalytic converter? >
> It seems to me that I run out options but go ahead to have both O2
> sensors and catalytic converter replaced. Any other suggestions for > what I should do?
>
> Thx
>