P0420. The nightmare will never end!!

[Flexthis2011EB]

Hello everyone yet again. The nnigtmare continues. Now i have a cel for p0420 that i get rid of. Most would think catalytic convertor replacement. Which im sure ill have to do but im wondering of one possibility. On live scan, the rear o2 sensor is fluctuating rapidly indicating a bad cat but my question is could an o2 sensor go bad causing false flucation readings or do o2s typical just get stuck or not report at all? Because if the cat was good but rear sensor was bad throwing false readings then i can see someone mistaking that for a bad cat.

[SHOdded]

http://www.obd-codes.com/p0420
A code P0420 may mean that one or more of the following has happened:
•Leaded fuel was used where unleaded was called for
•A damaged or failed oxygen sensor (HO2S)
•Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
•The engine coolant temperature sensor is not working properly
•Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
•Retarded spark timing
•The oxygen sensors in front and behind the converter are reporting too similar of readings
•Leaking fuel injector or high fuel pressure
•Cylinder misfire
•Oil contamination

I would suspect a damaged downstream O2 sensor/wiring, possibly from the trans work you just had done.  Replacing it might be all that's needed.

[ZSHO]

Depending on the current mileage you have would at least change both of the rear 02 sensors which are fairly cheap and have the same part # and hopefully the TRANS TECH did not screw anything up and best of luck to ya.  Z

[Ryan02Stang]

My Mustang threw this code.... Was fixed by replacing the front 02 sensors.

[AJP turbo]

Ive seen plenty of bad o2's in the older eco's...why not just change it like Z says before you look into cats...bad o2's can stick or occilate

[Flexthis2011EB]

I actually had the code before the trans rebuild. I thought it had to do with the shutter. But during the whole trans build i kinda blew off the code as i got the car back clear of cel. And then it popped back on. Thinking it may be damaged cat due to the bad engine that was replaced before the trans build. But imma change o2s and hope that works.

[SHOdded]

It does seem like a shot in the dark, the cats may be affected, but hopefully with good O2s in the mix, they will have a chance to rebound.  There are cat "cleaners" out there, I don't think they work too well though, and how they work with direct injected engines is another guessing game.