Code P 0420?

[Broyer]

I just had the codes read on my "00 OBW. It came up with two instances of P 0420 "Bank 1 Catalyst Efficiency Below Threshold". The guy at Autozone said it has something to do with the ignition at cylinder one, either a spark plug, or plug wire. Does this sound right to you guys? I had the codes reset and the CEL light has been off for about 150 miles. He also said the CEL would probably come back on before I left the lot.

 

Some history: I have had a CEL for the last 80K miles or so. It was a different code in the past. P 0500, I think. I have recently seafoamed it, and then about 3 months later put techron in the tank. After the techron, the CEL light has gone off a few times for about 100 miles and then comes back on.

 

Any ideas? Thanks

[johnceggleston]

i think po420 is a cat converter code. in my 97 obw i remember it as a secondary o2 sensor circuit. i changed the o2 sensor and it seems ok. i don't have many miles on it though.

 

try searching po420 and/or p0420 and you should have plenty to read.

 

 

I just had the codes read on my "00 OBW. It came up with two instances of P 0420 "Bank 1 Catalyst Efficiency Below Threshold". The guy at Autozone said it has something to do with the ignition at cylinder one, either a spark plug, or plug wire. Does this sound right to you guys? I had the codes reset and the CEL light has been off for about 150 miles. He also said the CEL would probably come back on before I left the lot.

 

Some history: I have had a CEL for the last 80K miles or so. It was a different code in the past. P 0500, I think. I have recently seafoamed it, and then about 3 months later put techron in the tank. After the techron, the CEL light has gone off a few times for about 100 miles and then comes back on.

 

Any ideas? Thanks

[njdrsubaru]

P0420 happened to me last week when was more than 105 degrees outside, i reset the OBD system and i havent had it on since then. I think it has something to do when the weather gets too hot and the Cat converters temp raises too much and they dont work as efficiently as they are supposed to, that way the second o2 sensor reads as a bad exhaust.

[OB99W]

I just had the codes read on my "00 OBW. It came up with two instances of P 0420 "Bank 1 Catalyst Efficiency Below Threshold". The guy at Autozone said it has something to do with the ignition at cylinder one, either a spark plug, or plug wire. Does this sound right to you guys? I had the codes reset and the CEL light has been off for about 150 miles. He also said the CEL would probably come back on before I left the lot. [...]

 

As johnceggleston said, P0420 is a code related to the catalytic converter; that code doesn't indicate ignition (misfire) problems at cylinder #1 (although a misfire problem can damage the catalyst).

 

Your Subaru has two oxygen (O2) sensors, one before and one after the cat; the engine computer (ECU) uses the output of the first primarily to adjust air/fuel ratio, and the second in determining cat efficiency. Once the engine is sufficiently warm (running "closed loop"), the ECU expects to see the O2 sensor preceding the cat to have a varying output voltage, but the second one to remain fairly steady if the catalyst is functioning correctly. If the ECU doesn't "like" the O2 sensor output, it can set the P0420 code.

 

So, there are several possible causes of the P0420 code. A simple one is a leak in the exhaust system up to and including the cat. Sometimes, even a "bad" tank of gas can cause the code to be set. A failure of either O2 sensor, or even a poor connection to one, can certainly cause P0420. Of course, the catalytic converter may itself be damaged. The catalyst can become "poisoned" from certain contaminants; coolant from a head gasket leak (internal) or excessive engine oil burning are possibilities. The catalyst can be destroyed (even melted) if enough raw fuel (due to cylinder misfire) reaches it. You didn't say if the exhaust system is still OEM; if not, certain changes can also cause the P0420 code.

 

Considering the mileage on the car (certainly over 80K), a scan to look at the O2 sensor output in real time (not just reading set codes) would probably be justified to help determine whether your problem is a sensor or the cat itself. (If the car had lower mileage, you might have qualified under the emissions warranty, but it seems you're beyond the limitation.) If the code returns, I'd check for exhaust leaks first, and then do a real-time scan. Should you decide to chance replacing O2 sensors without further diagnosis, be sure that at least the one before the cat is an OEM unit.

[1 Lucky Texan]

definitely clear and watch how quickly ANY code returns.

 

I don't recall you saying how many miles on the car - but I'd 'guess' you need a front O2 sensor.

 

(bank 1 is the reference for all 4 cylinder cars, as most are inline, they number them that way so it can also be used for one side of a V8)

 

 

Carl

[johnceggleston]

when i pulled o2 sensors out of a 96 leg wag 2.2L, they appeared to be the same part, bosch LHS24, but the rear had a 4 wire connector on it, the front just a 3 wire. but the rear only had 3 wires. i haven't done any research yet or seen oem sensors, but i put the rear one in my 97 obw and my CEL cleared up. it had been on for a good while.

 

by the way, i think, 95 leg sed 2.2L only has one o2 sensor. corrections?

 

john

 

definitely clear and watch how quickly ANY code returns.

 

I don't recall you saying how many miles on the car - but I'd 'guess' you need a front O2 sensor.

 

(bank 1 is the reference for all 4 cylinder cars, as most are inline, they number them that way so it can also be used for one side of a V8)

 

 

Carl

[1 Lucky Texan]

when i pulled o2 sensors out of a 96 leg wag 2.2L, they appeared to be the same part, bosch LHS24, but the rear had a 4 wire connector on it, the front just a 3 wire. but the rear only had 3 wires. i haven't done any research yet or seen oem sensors, but i put the rear one in my 97 obw and my CEL cleared up. it had been on for a good while.

 

by the way, i think, 95 leg sed 2.2L only has one o2 sensor. corrections?

 

john

 

not sure, but a '95 soob is probably still OBDII and if so is gonna 'bracket' the cat with sensors to measure its function. That why some people mistakenly change converters when actually the O2 (lambda) sensore is getting lazy or has gone bad. The front one gets the most heat and corrosive gases. A 4 wire MAY have an internal heater - not certain on that. so the rear may need it and the front heats up quite well from exhaust temps I guess - lol!

 

Carl

[2X2KOB]

There's lots of accounts of this code being caused by a bad front oxygen sensor. Try that before you spring for a cat convertor. In fact that front O2 sensor was the subject of a recall on that car, wasn't it? Might check it out, call a dealer with your VIN, they can tell you, possibly even for free.

[OB99W]

[...]In fact that front O2 sensor was the subject of a recall on that car, wasn't it?[...]

 

From NHTSA:

Technical Service Bulletins Summary

Make: SUBARU

Model: OUTBACK

Year: 2000

Type: PASSENGER CAR

Service Bulletin Number: WXW80

NHTSA Item Number: 618424

Summary Description:

SOME VEHICLES MAY EXPERIENCE FRONT OXYGEN AIR / FUEL (A/F) SENSOR ELEMENT CRACKING. *TT

 

From EPA (see top of page 6):

http://www.epa.gov/OMS/cert/recall/420b02003.pdf

[gbrand]

OB99W youre right on....

 

P0420 a pet peeve with me(on all vehiicles), see notes on thei thread with advice that saved one USMB member 800 bucks.....

 

http://www.ultimatesubaru.org/forum/showthread.php?t=52917&highlight=0420

[1 Lucky Texan]

From NHTSA:

Technical Service Bulletins Summary

Make: SUBARU

Model: OUTBACK

Year: 2000

Type: PASSENGER CAR

Service Bulletin Number: WXW80

NHTSA Item Number: 618424

Summary Description:

SOME VEHICLES MAY EXPERIENCE FRONT OXYGEN AIR / FUEL (A/F) SENSOR ELEMENT CRACKING. *TT

 

From EPA (see top of page 6):

http://www.epa.gov/OMS/cert/recall/420b02003.pdf

 

good post !

 

 

Carl

[johnl848]

I have a 2001 Subaru Outback with a PO420 code.The cat has been replaced(aftermarket)and front O2(aftermarket)She has spent $1200 on this light,to her fault,she has not gone after the shop for accountability.Something we are losing at a slow and steady pace!So she put it in my hands,I was a GM tech for 12yrs,left the grind of the dealership for a nice job on the railroad(good move)I have been going through post after post to get an answer to her problem.I called a Subaru tech of mine and he said it was a cheap cat that was installed in the car.He said it was a common problem when they hit 120,000.Thats my advise,put a Subaru cat in it.The reading on the rear O2 range from .2 to .8 pretty fast(with a fluke meter)now this tells me the cat is not doing much.I think if the cat was doing it's job it should be around .4 to .6 on average.Now,if you drove the car with a ignition problem for a while it will foul the O2 sensor(************ rolls down hill) A simple inspection of the O2 can tell you alot,Front o2 Black-problem is not the cat.White with alot of deposites,not the cat's fault.With all these problems I think it's a software problem,too sensitive,like women!

[J A Blazer]

2000 OBW, 128K. I get a P420 about once a year, and a P328 (knock sensor) code about once a month, even though that sensor is only 3 years old. Is there any correlation between the two? In other words, if my knock sensor is failing (again) might it be causing the P420?

[J A Blazer]

bump

[bsoelman]

In my experience aftermarket cats generaly contain 50-60% less precious metals in them, to bring down the price. AKA they dont work as well as the ECU would like, and here in Colorado with IM240 testing they rarely pass the test. Did the light come on before or after you Seafoamed it? That alone could put enough contaminates in the exhaust to foul the o2 sensor. I would suggest a scope test of both o2 sensors, likely the front is failing, followed by using a "Top tier fuel" and no additives. http://www.toptiergas.com/retailers.html.