Check engine light code P0420

[cat_woman88]

Help! I just bought a used 2001 Outback several months ago and all of a sudden on a very cold day, my check engine light came on. The auto parts store read it as a P0420 code - catalyst, low efficiency.

 

I don't want to spend a fortune at the dealership getting it fixed if there is something I can do at home. Any suggestions? I have already checked my gas cap and changed the PCV valve.

[f15xxx]

you should take this car to any sube dealer and they will install new catalytic converters for free under the fed 6-yr/80k mile emission warranty. don't let them bs you about other items, it's the cats. my service ticket would have been $1200. thanks sube. i had the identical code. i have speculated with friends of why the cats on these cars wear out and the 3 theories are: 1) short trips which dump rich mixture through cold cats, or 2) sustained high-speed driving, or 3) a bad batch of cats. we suspect it's short trips.

 

99 leg l sedan 2.2l 5-sp 30th anniv burgundy

[BillAileo]

Although it can be caused by bad converters, I believe code P0420 is often the result of degraded O2 sensor(s). Replacing the sensors it not a major undertaking (although doing both will run about $250 for parts, and ? for labor). Our local dealership claims the sensors are not covered under the emissions warranty.

 

Bill

[RodA2003]

I get this code ever few months on my 97 OB (225,000 miles). I just clear it and it goes away for months. You can clear the code display manually by disconnecting the battery for 10 minutes or more.

[cookie]

nearly a month ago. I cleared it and it has not been back yet. I do plan to change the ox sensors as they are due on my car by mileage. I think it came on because the rain cooled the cat so much it lost its efficiency.

[jlawlor]

Water could also be shorting the plug connection for the rear O2 sensor. Try some silicon grease in the plug. I was having similar problems but now it seems as though I have a fouled cat. I can now only go 200 to 500 miles before trouble code P0420 is set.

nearly a month ago. I cleared it and it has not been back yet. I do plan to change the ox sensors as they are due on my car by mileage. I think it came on because the rain cooled the cat so much it lost its efficiency.

[cookie]

I did get the PO 420 code again 7 months later on a dry day. I still have not gotten around to changing the ox sensors, but I will use a silcon grease as reccomended. Thanks.

[jlawlor]

You are probably also losing your cat. My problems came on slowly and seemed to start on wetter days but not always.

I did get the PO 420 code again 7 months later on a dry day. I still have not gotten around to changing the ox sensors, but I will use a silcon grease as reccomended. Thanks.

[cookie]

I imagine my cats are tired too. I was thinking of changing just one, but I have also been thinking of changing the entire car.

 

At 120,000 miles I have put a lot of money into it, maybe six grand in the last couple of years.

That piston slap continues on and I will be doing the cat work in the future I am sure. Then of course it will need rear brakes soon and tires after that. This is up to the point where I have to consider whether to put the money into this car that will bring it up to the standard I like, or is it cheaper to trade?

[gbrand]

Need to check-some of the cats and/or O2 sensors were covered under a recall, cant recall the years or find the post-perhaps lost when the board was migrated.. ALso there is a 8 yr/80000 mile warennty ofn the cat by federal requirements

[gbrand]

here is the text from the post..I had copied it in an email

 

 

mtsmiths

 

Registered User

Posts: 1

(12/6/01 9:29:06 am)

Reply

Persistant 'Check Engine' light The check engine light came on in the '00 Lagacy I just bought for the Pretty One. Just after it rolled 100,000 mi the check engine light came on, she read the manual and found the she had failed to tighten the gas cap, which will send a fault signal. We disconnected the batterey overnight and the light went out. It came back a day or so later, so we took it to the local Subaru dealer and had the fault read. They say it's a faulty catalytic converter signal, but can't find anything wrong with the cat. Dealer says that even tho the cat seems fine, if the signal comes back the only way to get rid of it is to replace the cat to the tune of $600+. It's come ba-aaaack.

 

 

Since I know the cat is OK I really don't want to pay $600 to turn off an idiot light. On the other hand - Now we won't know if anything else goes wrong that we *should* be paying attention to, since the light is always on. I'll hit the Subaru board, but anyone here got any ideas?

 

THANX

gbrand

Registered User

Posts: 177

(12/6/01 10:14:01 am)

Reply | Edit Re: Persistant 'Check Engine' light From what I understand the way the ECu sees the cat bad is that the second Ox sensor , behind the cat, mointors thecat, not used for controlling engine I am told. So, Oxygen content in air past cat should be less than at upstream sensor(in manifold?) as CAt should be using Oxygen to burn up partially burned CO and HC. O2 sensors put out a voltage that gets higher as the oxygen concentration gets less, so typically read 0 Vols in air, and 100-200 millivots in normal operation. My truck downstreamsensor was reading well over 300 millivolts, saw at emission test there was 0% oxygen in exhaust. SO, if measure output of sensor and compare to up stream side, if same the cat ain't working, if higherthe cat is. Or downstream sensor is bad. Check after engine has warmed up, but don't replace blindly. The three or 4 wires are heater voltage, heater gnd(will be to case if 3 wire sensor), signal outut, signal gnd. So measure between signal output and signal gnd and see what you get. Of course diconnecting and measuring with engine running will set code for O2 sensor, so clear it later or someone will sell you an O2 sensor you don't need mtsmiths

Registered User

Posts: 9

(12/11/01 3:15:47 pm)

Reply Check engine Light Follow-up Thanks for all the comments, especially to gbrand. I got the car registered with Subaru national, who promptly informed me that the car had not responded to recall WXW80 error reporting by front 02 sensor - resulting in *you guessed it* check engine light.

 

I called the dealer that had diagnosed a bad cat and asked, "Don't you diagnose a bad cat by comparing 02 sensor read-outs?" he said "Yes". "If the car had a bad front sensor could that lead you to deduce a bad cat?" he said "Yes". "Did you verify recall WXW80?" ... slight pause he says "Is that the front 02 sensor recall notice?" *I* said "Yes", he said "No"; and scheduled the recall replacement.

 

The moral of the story is ... the Subaru dealer was fully prepared to sell me an unneeded $800.00 cat converter, without checking a known recall, and of course during that repair they would have replaced both sensors, so I would never have known.

 

I don't know if they are crooks or idiots, but I sure don't trust them either way. I sure wish my indy mechanic that keeps our '87 alive could work on this NewGen Soobie.

[mtsmiths]

MAN, that's brushing the cobwebs outta the memory banks!

 

BTW, that same '00 Leg wagon is now at 165,000 and running just fine thankyewverymuch. I did twist a Maryland dealers arm into replacing the front O2 sensor again on warranty after we got repeated CEL codes on a cross country trip.

 

Still throws the occassional bogus CEL, so I just either battery dance it, or usually just ignore it until it goes away ... which it eventually always does.

 

Our only problem we ever have with the car is the mystery miss that pops up now and then for a few miles, then goes away - rain or shine, wet or dry, cold or hot, just started or running along after two hundred miles ... still haven't figured THAT one out.

[lorimansell]

2002 Forester here. Check engine light on and off all the time. Have performed all recommended maintenance. Had O2 sensor replaced, it was faulty, dealer put in another. Now stuck with same problem. On and off, on and off. I am so tired of this! Go to auto repair shops and they say possibly need new gas cap, fuel injector, O2 censor, knock sensor, possible vapor leak, catalytic converter. My car has 132,000 miles so I can't get emissions stuff done under warranty. What the hell is going on with check engine lights in Foresters? I can't get my vehicle inspection approved with the check engine light on. I have MS & support a family so like most of us don't have tons of money to spend on a car I have taken very good care of. Can the dealer reset the light long enough for me to get an inspection sticker until I have enough money for repairs? And how do I know for sure exactly what repairs are necessary or not? I'm no mechanic, I'm a nurse.

[gbrand]

If the dealer9or anyoneelse) turns the light off it also resets the "monitors" to off("not ready"), the indicators that tell the computer the tests have been run. In most areas most or all of the "monitors" have to be on ("set") indicating the car has been driven long enough for all the self tests to run. In my part of Texas you can have up to 2 monitors not ready and as long as the light is not lit you pass.

 

You state you are not a mechanic, it sounds as if you need to find a good one, who knows diagnostics(not throwing parts at problems hoping to fix them), OBDII(the computers and codes) and hopefully has some experience on Subarus. The Parts stores that read codes are good to get teh codes, always make sure to get the actual code numbers(P01234, etc.) but I would not put too much faith in thier diagnostic ability. Did you get the code numbers and could you post them here? We may be able to help

 

I would be leery of a dealer unless you know it's service dept is good, too many dealers use thier service depts to pressure customers into buying a new car and thier overhead can be high. Thee are some board memebers in your area, can any of them point you to a good repair shop or perhaps a dealer that does good service work?

 

Let us know how it goes and good luck.

[Bmm001]

Is it still showing the PO420 code? My independent garage said the only time you actually have to worry about that code is when it's inspection time, if your state does emissions inspections.

 

All of our Subies have over 120K with the original O2 sensors and converters, and we just have the light reset whenever it periodically comes on.

 

Our guys say when they turn it off, so long as it doesn't come back on in a day or two there's nothing to be concerned about.

 

Last time mine came on was July, before that, December.

 

Interesting observation: I've had extended discussions about this and other similar problems with them. In their experience (over 20 years of just working on subarus), poor quality gasoline causes more problems with CELs and misses and generally poor performance than anything else on Subies.

 

They also see PO420 when the weather suddenly changes (usually colder), or when it's raining or snowy and cold (water/snow splashing up on the converter and cooling it off too much).

 

In any case, their first route if the code won't stay off would be to check the O2 sensor, then the converter.

 

Brian M.

[cookie]

When I would reset mine before I replaced the O2 sensor it would stay off for months.

See if a parts store will reset for you and take it in for inspection quickly.

Back east you have so much corrosion on wires and and the exhaust that a car that old can easily give you a PO 420. It can be anything from a leaky exhaust to the cat.

If you can get through inspection at least you have another year.

[jon38iowa]

I imagine my cats are tired too. I was thinking of changing just one, but I have also been thinking of changing the entire car.

 

At 120,000 miles I have put a lot of money into it, maybe six grand in the last couple of years.

That piston slap continues on and I will be doing the cat work in the future I am sure. Then of course it will need rear brakes soon and tires after that. This is up to the point where I have to consider whether to put the money into this car that will bring it up to the standard I like, or is it cheaper to trade?

It sounds like a minimum of $ 1500. not counting the engine. That would add another 1-5K depending on which method you opted for.

I know this first hand; as of now I am into my Forester for $10,000., just in repairs(not counting tires and upgrades). This, over the past three years. The car runs and looks great, but it is only worth maybe $3000.00, if I am lucky.

Looking back, It would have been wiser to trade it in on a newer Subaru and make a car payment.

[cookie]

It's a hard call isn't it? I just put a couple grand into axles and transmission. On the other hand a new car would be around $3,000 down, taxes, license, and maybe $4800 in payments for a year.

My car has had about 18,000 miles since I replaced the ox sensor with no codes and it did very well for a couple of years until the rare transfer case spacer problem.

It does have a leaking cam seal and I think I really should change the left axle one more time to a real Subaru one to get rid of the last little noise.

The battery is really timed out and I should change it, but that's not big bucks.

Brakes are fine since I did them a few years back, and I did struts then too.

I'd like to get a couple of more years out of the thing after all the recent money. Like you say it's only worth a few grand at this point so anything I get out of it is free. The thing about a mostly in CA body is that there is little rust and it does not look much worse than when I bought it several years back.

[jon38iowa]

The thing about a mostly in CA body is that there is little rust and it does not look much worse than when I bought it several years back.

I Lived in CA, and I remember. I drooled when the poster showed the used CA Subaru for $750.00. Around here, for that price, that look ready for the crusher!

[cookie]

The worst enemy this car has is me not rust. I've managed to get a few few tiny scratches and dents but at 30 feet and polished up it looks pretty good.

[Bmm001]

My girlfriend found out the hard way that resetting the CEL right before getting it inspected doesn't work. She didn't drive it long enough to return it to the "ready" mode, so they couldn't test it right away anyway!