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

[pitweasel]

Hi guys and gals,
I've had the P0420 code almost since buying this car used, and it's finally come time for inspection. I've been reading many, many different threads on the topic here, but I'm having a hard time keeping track of all of the different responses, solutions, and what worked and what didn't.

So: if you have had the wonderful error light come up that suggests a replacement catalytic converter, what did you do? Was it a leak somewhere? Was it a bad part (O2 sensor, etc)? What ended up fixing it?

I'm trying to consolidate everyone's experiences into one thread for my own benefit, and hopefully others who encounter this wonderful problem down the road can use this to help them out as well.

Oh, and for the record: when I took this in to get the code read last year, the shop acknowledged that it's often not actually the converter that needs replacing, and that even if it was, it's not really hurting the car except for halting an inspection pass. So that's their current stance on it...

Thanks so much for your help!

[blindrummer]

same problem happened to me last year 1996 OBW 2.5
front O2 sensor was the culprit
the most common problem for P0420 seems to be a bad O2 sensor
front O2 sensor needs to be OEM at about $100, rear O2 sensor can be parts store brand >$100
I also replaced my sticking EGR valve when i replace my front O2, dont know if that had anything to do with P0420
now it could also be a leak in the exhaust, replacing the gaskets seems to me an easy and cheap job
fuel mixture becomes rich when running the car with P0420
some say this can lead to catalyst failure, a really expensive replacement
well seems you have a good shop, most will try to make a thousand dollar sale on a converter
if you change the O2 sensor: pull neg. terminal off battery, replace O2 sensor (easy) and replace battery terminal after 20 min and that usually resets the computer

[TwoPointers]

Good deal - would love to see a "summary" of all the answers. I too have had the code since buying my car used - but it goes on and off, depending on weather. Subaru was NOT helpful and recommended the cat of course. My local garage is alarmed at the CEL going on and off - but I've stopped worrying at this point.

But would still love to see a meeting of the minds as to yes, you DO need to replace your O2 sensor or not.

[ETC]

for me, dirty pcv valve

mine was intermittent also, cleaned the pcv and it hasn't been back for 10k miles.

[fossilized]

Replacing the front 02 sensor worked in our 99 wagon. It took care of the code and sluggish performance. Have the same problem with my newer 06 outback. Have not purchased an 02 sensor yet. Much like you, I am reading other's posts to see what is the best move.
Curious to see if anyone has had this code pop up as early as mine. The 06 has only 58K miles!

[bheinz57]

My exhaust modifications gave me that CEL constantly. And then the rear O2 sensor finally went out, replaced with new, put stock cat section in, no more CEL...


Brian

[blindrummer]

Originally posted by ETC
for me, dirty pcv valve

mine was intermittent also, cleaned the pcv and it hasn't been back for 10k miles.

+1

[nipper]

O2 sensors for me (fixed one lasted a few months light came back then fixed the other).

O2 sensor is much much cheaper then a cat, and usually have to be replcaed with the cat anyway if the car has over 100k on it.

nipper

[f0rgiv3n]

I just got my sub about 3 weeks ago and it had the same thing on it. The previous owner just ignored it after his mechanic told him it was the O2 sensor. I worked on it this past weekend and reset the CEL and it hasn't come back yet. It still might but it'll be hard to tell exactly what fixed it :P . I did a lot of stuff this weekend. i pulled out the spark plugs (they were quite loose), I also replaced the air filter, and the main thing that I did was cleaned the throttle body. The throttle body was DANG dirty o.O . The one last thing I did was my main air box was missing 2 screwed so it was sucking dirty air straight through so I sealed that up. If it comes back I"ll probably replace the O2 sensor. But so far so good!

[pitweasel]

All right, now this is just weird...

The light had been checked and gave the P0420 code back in march or april of last year. It was intermittent at that point, but became solid over the summer and stayed on all of the time.

This weekend, it shut off - I figured it was just a dead bulb, or at best, a short-lived break. However, it was still off four days later when I went for inspection, which I ended up passing with nothing but a bulb out in one of my rear lights.

So, this begs the question...does anyone have any idea what could possibly have caused that light to read that code for so long, only to eventually go away on its own? I'm hoping it doesn't mean that something ended up breaking down so badly that it's not even registering a code anymore, but the shop knew about the problem before inspection, so it seems like they would've caught something like that. Any thoughts?