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

[nipper]

Originally posted by 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?

Its a flaky O2 sensor.


nipper

[blindrummer]

Originally posted by nipper


Its a flaky O2 sensor.


nipper

+1

[bikerboy]

I'm going to bump this thread up.

Just had perhaps a similar experience today. One of my roommate's just came back from NJ today; his parents own an '05 Outback 2.5i, 78k miles. Apparently the CEL had recently come on, along with the blinking Cruise Control light. Since I have a code reader, I plugged it in and brought up PO 420, which is obviously related to the catalytic converter. They already tried re tightening the gas cap, which hasn't turned off the light (then again, I don't know how long ago they tried it).

Also, last week the CEL came on in my 2000 Outback, for about 2 days, then went off. I didn't get a chance to read it, but I have a feeling it was probably the PO 420 code.

From everything that I have read, it just seems that Subaru's tend to have picky front O2 sensors?

[jlezlanc]

My Check Engine light has now been on permanently for 6 months, ~20,000 miles ('01 Outback Limited, 200,000+ miles, I am original owner). 2 mechanics said it needed a new catalytic converter (which is super expensive in CA because third party cat converter makers apparently don't make a CA version); in reading the posts on this it sounds like one of the sensors may be the culprit; where are the front and rear sensors located and what is involved in changing them?

[tdomino2]

I too have had ongoing problems with the CEL since we acquired the car a year ago. The light came on approximately 2 weeks after we bought the car. Dealer of course said cat. conv. We held off because of the expense, and it was intermittent through the winter. Came on and stayed on all summer. At the end of the summer, took it back to the dealer, was also giving a code for the front O2 sensor, so I asked them to only replace that. Light stayed on. Took it to an independent mechanic for the cat. conv. They pointed out that the rear sensor had been replaced not the front sensor. Had them do cat. conv. only. Car lurched out of the parking lot, began surging at the first traffic light, and stalled as I rolled into a parking lot across the street. Mechanic picked us up, replaced front O2 sensor, light went out, car ran great for 2 weeks. Exhaust began to get louder and louder until I felt like I was driving a motorcycle. Dealer looked at it, thought it was a cracked Y joint, thought it would cost $1000. Independent mechanic looked at it, replaced 2 gaskets to the tune of $125, problem solved. CEL came back on a week later, with the P0420 code, which mechanic told me means "low flow." I don't know what else I can do to the exhaust system, I feel like I've replaced everything there is to replace.

When the problem first started LAST January, we had the dealer reset the light a couple of times, and it always came back on as I was leaving the gas station. This year, the first time it came back on was right after getting gas, as well. I am pretty good about tightening the gas cap, though, and CEL has not turned off. ANY suggestions?

[keninn]

Originally posted by tdomino2
I too have had ongoing problems with the CEL since we acquired the car a year ago. The light came on approximately 2 weeks after we bought the car. Dealer of course said cat. conv. We held off because of the expense, and it was intermittent through the winter. Came on and stayed on all summer. At the end of the summer, took it back to the dealer, was also giving a code for the front O2 sensor, so I asked them to only replace that. Light stayed on. Took it to an independent mechanic for the cat. conv. They pointed out that the rear sensor had been replaced not the front sensor. Had them do cat. conv. only. Car lurched out of the parking lot, began surging at the first traffic light, and stalled as I rolled into a parking lot across the street. Mechanic picked us up, replaced front O2 sensor, light went out, car ran great for 2 weeks. Exhaust began to get louder and louder until I felt like I was driving a motorcycle. Dealer looked at it, thought it was a cracked Y joint, thought it would cost $1000. Independent mechanic looked at it, replaced 2 gaskets to the tune of $125, problem solved. CEL came back on a week later, with the P0420 code, which mechanic told me means "low flow." I don't know what else I can do to the exhaust system, I feel like I've replaced everything there is to replace.

When the problem first started LAST January, we had the dealer reset the light a couple of times, and it always came back on as I was leaving the gas station. This year, the first time it came back on was right after getting gas, as well. I am pretty good about tightening the gas cap, though, and CEL has not turned off. ANY suggestions?

you need to take it back to the place that did the cat. federal law kicks in here, iirc, for replacement cats, factory or aftermarket, MUST be warranteed for 5yr/50k, it might be higher. must perform like original cat. make them fix it.

[cptnjosh]

My 0420 code was the gas cap was loose. tighten it all the way till it clicks. if it is not clicking replace it. could be as simple as that, hopefully.

[Kristi]

As of this december subaru of north america has issued a specific test for catalitic converter po420 inefficency codes. The test is performed over an hour by driving the vehicle and monitoring the front and rear oxygen sensors to determin if the catalist is faulty or the rear o2 sensor is not rwesponding acuratly. I have perfomed about 10 tests since christmas and mor than half of the instances are the rear sensor .

[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?

[Kristi]

Originally posted by cptnjosh
My 0420 code was the gas cap was loose. tighten it all the way till it clicks. if it is not clicking replace it. could be as simple as that, hopefully.

Unfortunatly who ever told you that didnt know what they are talking about. a p0420 is a code used to monitor catayst efficence or the ability of that catalytic coverter in the exhaust sytem to clean the exiting exhaust gas orf the engine, it is uneffected by the evaprotive emissions system that the fuel tank, cap, lines, and charchole canister are apart of. Loose gas cap would result in p0457,p0456,p0440
the person reading the code probly didnt know or didnt want to get involved in the test and blamed it on the gas cap. I have seen some questionable acts like this where i work when someone is busy or dosent want to perform the test (these so called mechanics give the rest of us a bad name) hope you didnt pay for that diagnosis.