P0420 code

I understand that replacing the cat doesn't always solve the problem. Can someone help me troubleshoot this?

http://www.obd-codes.com/p0420

Do you have any aftermarket header?

Nope. all stock

i had the same problem with my aftermarket header.

 

did you check your O2 sensors?

check your o2 sensor voltage, but after i replace my cat, it fixed the CEL

hi guys

 

I just bought a 96 Legacy with a 2.2 motor. The PO420 code has come up, been turned off and come up again. I looked in the previous owner's paperwork and it had come on in the past (he was told it was the front O2 sensor but chose not to replace it).

 

For completeness sake I did a tuneup (airfilter, plugs, wires). It runs very well but the light is on.

 

I am trying to gather what a reasonable plan of action is. Please comment.

 

1) check connections at the O2 sensor (I'm in New England - corrosion is our middle name) . Front sensor, to be specific.

 

2) check voltage at O2 sensor (can someone tell me what it is supposed to be?)

 

3) replace the O2 sensor (with an OEM)

 

4) last resort - replace the CAT.

 

What do you guys think? I have heard people mention restoring

or upgrading firmware but I am really vague on this.

 

John

Without replacing parts, or having a scan tool to watch the o2 voltage, usually want to see it on a line graph..., try dropping a gear and running the car above 3000 rpm for 5 or 10 minutes at a time. This will burn the buildup out the cat. Then clear the codes and try again.

You'd want to focus on the rear o2 sensor, not the front.

 

Rear oxygen sensor monitors the cat system. The pre-cat o2 sensor is used to determine a/f ratio from exhaust gases and is unrelated to the cat monitoring

 

It should be a steady voltage for the rear- i believe around .5V. Expected range are between 0.1 and 0.9V - anything outside of that range indicates a bad sensor.

You'd want to focus on the rear o2 sensor, not the front.

 

Rear oxygen sensor monitors the cat system. The pre-cat o2 sensor is used to determine a/f ratio from exhaust gases and is unrelated to the cat monitoring

this.

 

the rear o2 sensor can be generic,

but i recommend the ones that come with the plug pre-attached.

usually it will cure the problem.

 

but if not , then replace the front with an o2 from SUBARU,

or much better quality.

the computers are picky.

 

if that does not do it, look for intake or exhaust leaks,

either one can cause this code.

 

do not replace the cats,

they do not go bad very often unless you drive with a flashing CEL for a reallly long time.

(flashing CEL indicates a condition which could damage the cats. misfires are the most common.)

 

and last, google 'subaru p0420 anti fouler'.

you modify a spark plug anti fouler and use it to back the rear sensor out of the exhaust stream.

works every time.

rumor says it will work even if you have no cats.

i had this code forever i fixed it by replacing both sensors w oem ones and new gasket kit for my uel header. gaskets helped w my cat below threshold code i had at the time too, cuz i had a tiny pinhole somewhere. it whistled like it had a turbo

I am still at this. Just got the car 2 weeks ago and P0420 has been

on since I got it.

 

I gave it a "tuneup" = new plugs, wires, air filter. Old plugs looked

good.

 

Took it to an independent garage as it needed some brakework.

 

He tested the O2 sensors and said they were OK.

 

Said one cat was junk and the other completely missing. Straight pipe welded in place of it.

 

So the exhaust system got put in shape.

 

150 miles or so later, the CE came back on. As if none of the work had been done.

 

Car runs well; gas mileage a tad on the low side.

 

Has 170K miles. This is a 2.2 for what it's worth.

 

Any thoughts?

Check your injectors... I had similar problems and injector cleaners like Marvel Mystery helped a lot.

 

Thanks

Hari

He tested the O2 sensors and said they were OK.

 

Said one cat was junk and the other completely missing. Straight pipe welded in place of it.

 

So the exhaust system got put in shape.

 

you need to do the anti-fouler fix.

buy the correct size spark plug anti-fouler,

drill out the bottom so it will thread on to an o2 sensor,

then install it on the rear o2 sensor or your car.

most times this will remove it from the flow enough to eliminate the p0420 code.

 

rumor has it, it will work even without cats.

not so great for the environment,

but good for you.

 

google it for more info.

Yea anti fouler too. I used a 90 degree o2 bung from eBay to get my rear o2 code off, and away from the ground. But thats w a catless downpipe from wrx for my 22t swapped legacy. Passed nys inspection

For sake of discussion, suppose I wanted to replace the O2 sensors. I gather from many sources that one should use Denso sensors and not Bosch or some other brand.

 

What I am not clear on is the precise part numbers for the correct Denso sensors. Is there a place I can look up Subaru part numbers like these? (or does someone know the #s ?)

 

Looking around Amazon, Denso sensors for the '96 2.2 do not seem very expensive (like $50 each) but there is more than one part number appearing for them, leaving me unclear about getting the right one.

 

At this point I am finding 234-3091 upstream sensor, 234-3088 downstream (both Denso)

 

As always, any advice and information most appreciated.

 

John

http://opposedforces.com/parts

 

i think the 95 has 2 of the same part, one front and one rear.later by 97 the o2 sensors are the same item, but the connectors are different front and rear,

so they would have different part numbers.

(if you look closely the number stamped in the metal by the manufacturer is the same.)

 

these cars are picky,

i recommend getting the front sensor front a dealer.

it will cost more but it WILL work right.

it measures the exhaust before the cat, and adjusts the fuel trim.

you want this one working perfectly.

 

the rear sensor can be generic.

(but not one that needs splicing.)

it is much less critical.

all it does is measure the exhaust to see if the cats are working.

interesting . . .

 

so you are essentially telling me that if I walk into the local Subaru dealer and buy the front sensor, what I walk out with will not be a Denso 234-3091 ?

 

That is the purpose of my post and I appreciate your input.

 

Subarupartsforyou.com shows a front sensor for the 96 Legacy 2.2 I have as part # 22690AA220 for $120 kind of money.

The link is http://www.subarupartsforyou.com/sensor-switch/sensor-oxygen/22690aa220-5.html Is this the same item I would walk out of the Subaru dealer with?

 

(since my post, I did some online reading and find that the life expectancy of an O2 sensor varies but seems to have an upper limit of 100K miles, which my car has long exceded).

so you are essentially telling me that if I walk into the local Subaru dealer and buy the front sensor, what I walk out with will not be a Denso 234-3091 ?

i do not know.

what i do know is if you buy a suabru part you will have no problems.

some who do not buy from suabru do have problems.

and i know these cars are picky about some parts,

fornt o2

spark plugs

plug wires

i also will only buy suabru oil seals.

why save a few bucks on cam and crank seals if they only last 40k miles.?

 

if you have to buy the part twice, to get it right,

there is no savings.

but you do what you want.

Subarupartsforyou.com shows a front sensor for the 96 Legacy 2.2 I have as part # 22690AA220 for $120 kind of money.

The link is http://www.subarupartsforyou.com/sen...90aa220-5.html Is this the same item I would walk out of the Subaru dealer with?

yes,

''suabru parts for you'' is a DEALER.

there is also ''genuinesubaruparts.com''

and others .

so I went to Subarupartsforyou to get an O2 sensor. Was not 100% sure which one was the correct one. Emailed them and they used my VIN to nail it down (it was the one in my last post). This was $119 from them, or about twice what various people on Amazon are selling Denso sensors for that are supposed to fit that car. This tells me, as the previous person said, that what Subaru dealers are selling is different than what others are selling.

 

I find nothing that says O2 sensors are generally good for 100K miles, and my heap has 170K, so this is a good piece of maintenance.

 

After I have installed it and run the car for a bit, I will report back on code (hopefully gone!) and gas mileage (hopefully a bit better!)

April 3

 

I got the new front sensor from SubaruPartsForYou. Quick service, kinda high shipping cost ($20 to the next state)

 

Opened the package up. Box says Subaru of America Parts, 22690AA290.

 

Looked carefully at the sensor itself:

 

Bosch LSH 24

0 258 003 702

2041833 12v

982

 

was a bit surprised as I have read many people saying you have to go with Subaru parts and you have to get Denso.

 

Moving forward . . . will install tomorrow, clear the code and see how things go.

 

Happy Easter!

So I got the car up to operating temp and got a 22 mm wrench on the old one. Came out reasonably well.

 

Put some antisieze on the new one and installed it. Doing well. Went to hook it up and discovered that while both old and new sensor have 3 wires, the Subaru harness wants a rectangular 4 wire plastic connector while the new sensor has a triangular 3 wire connector. In other words they don't connect. Damn!

 

Both old and new sensors have 2 wires of the same color and a 3rd of a different color. IF I were to splice the old connector on to the new sensor (I don't mind soldering and heat shrink tubing), does anyone know if the 2 matching color wires are interchangeable?? (like maybe they are both ground wires)?

 

Any advice appreciated. Was hoping to drive this thing today.

 

John

you bought the wrong sensor, front vs. rear.

the sensors are the same.

which one are you replacing? rear?

 

 

regarding the splice,

check the other sensor, the one you are not replacing.

see what the same colors connect to on the harness.

 

my guess is they both voltagge wires,

one for the heater and one for the signal.

the heater probably usues the exhaust as a ground.

I am replacing the front sensor (see earlier posts on this thread).

 

Here is the original email I sent to them, which resulted in their asking for the VIN:

 

I need the FRONT 02 sensor for my 1996 Subaru Legacy Outback with 2.2 motor and manual transmission.

 

This car has two catalytic converters. The front cat has 2 sensors, the rear none. I am looking to buy the front sensor.

 

I cannot be sure from your website which O2 sensor to buy. I get down to two of them and then cannot be 100% sure.

 

you are correct in that the connector on the new sensor is identical with the connector on my current rear sensor. The front connector is a rectangular connector while the rear connector is triangular.

well this would be an expensive solution but it would work.

use this one on the rear and buy the right one.

yes you will spend $250 on o2 sensors,

but they will last you 100k miles at least.