P0420 check engine light: read if you've had this

[Kristi]

Originally posted by tdomino2
Both O2 sensors have been replaced. The rear was done in October and the front was done in November or December. I suppose they could be bad; any chance they are still under warranty?

Sounds more like the cat is no good. front o2 is used by ecm to monitor exhaust before the cat to control air fuel ratios. when front goes bad ussually the engine will run lean causing lack of pick up and a stumble off idle. the rear is ONLY used to monitor the condition and efficency of the catalytic converter and has no input or influence on engine operation. that being said parts and labor if done at the dealer are warrentied for 12m/12thousand miles

[oclvframe]

Originally posted by Kristi

Sounds more like the cat is no good. front o2 is used by ecm to monitor exhaust before the cat to control air fuel ratios. when front goes bad ussually the engine will run lean causing lack of pick up and a stumble off idle. the rear is ONLY used to monitor the condition and efficency of the catalytic converter and has no input or influence on engine operation. that being said parts and labor if done at the dealer are warrentied for 12m/12thousand miles

Kristi,

Now, I too am experiencing this infamous p0420 code. The dealer ran the vehicle for 10min or so and came back claiming its the catalytic converter...specifically the driver's side. I have an 01 LLBean so I have the H6. I recently have noticed a slight stumble off idle and some slight hesitations when getting going....Sounds like first attempt should be to replace the front o2 sensor...but how do I know which one needs replacing?

-r

[porcupine73]

Actually on my '00obw I've lately been getting P0420 every few trips. I keep the code reader plugged in so I can clear it when it pops up. I do have a bad exhaust leak in the rear pipe right now though. I will probalby replace front o2 coming up soon with lots of extra Subaru bucks on hand. Interestingly in the last couple of weeks this soob has been running really really good, like better than it ever has, which made me think something is wrong and then P0420 now often hahaha.

[ron917]

The following link has the best info on the P0420 and similar codes that I have seen:

http://www.troublecodes.net/articles/catfailure/

A PC based scan tool and/or a dual channel oscilloscope are the right tools to do the diagnostics.

[garthpro]

I had heard this code was extremely prevalent, but this is insane! I have been chasing my tail on this as well. Changed the rear. Came back. Changed the front, code was back in about 300 miles.
WTF!?
'00 OBW 240k, 2.5
Head gasket replace at 190k

I have heard all the urban legends, gas cap, bla bla. Shouldn't Subaru deal with this?
My mileage is down to about 25 on the highway, from a high of over 27mpg.
I am forever working on this piece, and I long for my 89 jetta, which I never touched! and it lasted 17 years.

[porcupine73]

Do you have any exhaust leaks? Even little pinhole leaks you can't detect can cause P0420. What is Subaru supposed to deal with? The system works fine when everything is in spec and throws the code when it is not which is part of federal emissions regulations.

[garthpro]

How about dealing with the fact that people are spending huge dollars on chasing their tails through repair shops and still getting the codes! no other cars seem to have this perpetual problem.
It is reminiscent of the head gasket melt downs, or do you think those were ok too?

[ron917]

Read the link I posted and do a google search. Lots of other cars get the P0420 codes and have trouble resolving them.

At 240K, it is entirely possible that your catalytic converters(s) need replacing. They do not last forever.

But the only way to know is to do a thorough diagnostic as outlined in the link I posted. If you don't have the tools or the knowledge to do that, be prepared to pay someone who does.

[porcupine73]

Right on, troubleshooting is a must. Yes the cats may have reached the end of their useful life with that much mileage. People who get things replaced at a shop and then still have issues I don't see how that is Subaru's fault, to me that is the shop's fault for not fixing what was causing the problem.

[MDossat]

This thread has made for some interesting reading since my 01 OBW is reading this same code indicating the catalytic converter is bad. The problem is this is the second one in a matter of about three years. I am the original owner and I have roughly 93K on my OBW right now and I believe the first replacement was around 50K. The cost of the repair will be huge. I've got a few other thoughts and issues but I'm not sure this is the thread to post them on so I'll keep looking.