Catalytic Converters

P0420?

Search this forum for p0420. Some suggestions in that that would have one think twice about aftermarket cat and o2 sensors.

I'm trying to solve this very issue on my vehicle as well.

-- Todd H.

2001 Legacy Outback Wagon, 2.5L H-4 Chicago, Illinois USA

I've had two pair on non-OEM replacement CATS put on (for about $1100) with better warranty (2 yr) than OEM (1 yr). Unfortunately the CEL/code P0420 keeps coming back. All other angles have been checked including replacing an 02 sensors. So as soon as the return details are worked out on the non-OEM, I'll be getting Subaru replacements front and back (for an additional $500+)

Clear the code. If only one comes back, definitely try changing the front O2 sensor. 78K would be early for a cat to go bad on a 'typical' car. But not too early for an O2 sensor. order of magnitude difference in price. Still, a GOOD shop should be able to scope the sensor and , if good, flow test or tailpipe test for a bad cat. I suspect your dealer would change both but 'say' the cat was bad. I 'might' be tempted to get a second opinion anyway. Many people have had (is it 420?) cat efficiency codes/w'ever and changed the sensor and been OK for another

80,000 miles. Do get the Subaru sensor though. Some folks have had problems with aftermarket units.

Carl

I agree. When my car was upgraded to a Stage 2, a high flow cat was part of the package (aftermarket). It required a software upgrade to recalibrate the cat efficiency and defeat the P0420 code. Many aftermarket cats and sensors will cause it to be thrown.

Get your gaskets checked first.

I blew through several O2 sensors and had all sorts of indication my cat was going bad before getting the final (bad) news that my head gasket was leaking coolant into the exhaust. Which will also screw up the sensors and throw errors.

ESPECIALLY on the 01 motors. So, look in the coolant tank while it's running for bubbles, check coolant level, and get the coolant checked for hydro carbons before throwing another 1k at a problem that might not be that.

The head gaskets were checked at the last major service, and I don't see any other indications that they are bad. The gas mileage hasn't change (I'm getting 26 mpg in mixed driving) and it runs OK. I say OK only because there is some rough running in low RPM in hot weather, but otherwise it runs like when I bought it.

Like I mentioned, the light comes on intermittently. I've notice if I drive from a big change in altitude, or if I fill up with a different brand of gas, I'll get the light to come on. I also noticed that after an air filter replacement the light stayed off for quite awhile. Based on some of the comments made on the O2 sensors, I think I'll replace those first and see what happens.

I'll relate my experience with a 2002 Forester, but I am not sure it will shed any light on this phenomena.

At about 120000 miles or so, it started throwing 420 CEL codes. I purchased a reader (BR-3:

and, because it was wintertimehere in Wisconsin, I simply re-set the code. My reader also allowed meto look at the raw sensorvoltage from the sensors, and at times, it seemed the rear sensor hadan intermittent connection. I checked the rear sensor connector, but all looked good, but I kept getting CELs (which I would clear)

Then, early this year, I purchased a new rear sensor, and replaced the old one with it. Feeling good about myself, I was sad to see the CEL return about 200 miles later, and started to think what my next fix would be. I re-set teh CEL, and kept driving. The car threw maybe

3 or 4 more more, sometime within 50 miles, sometimes after 250 or so, and then it stopped.

So far, it has been 5000 miles or so since my last CEL. I do not know if there is any 'burn-in' period for new O2 sensors placed into service, but something happened to make the code go away (for now).

I'm not sure there is a definite 'take-away' from my experience, but there it is.

Thanks for your time, Duane

Did you change gas brands? Possibly just the change from winter formula gas to summer formula stopped the codes.

Some cars are fussy about their fuel quality - I know 2 hondas that throw 420 codes whenever the vehicle is fueled with ESSO (Exxon) gas - and NEVER with Shell.

Might have something to do with Ethanol concentration? If so, changing the FRONT O2 sensor may be the fix. A rear sensor problem would be much less likely to cause the 420 code.

Could be, I suppose. I am not sure how much the gasoline mixes change here in southern Wisconsin for the winter (I do know the diesel sold does). BTW, here we run 10% ethanol pretty much year-round.

Then again, one can never proclaim complete victory on a repair, until one gets rid of the car....

This 2002 Forester hasn't shown any preference in fuels.

Perhaps. I based my diagnosis (replacing the rear sensor, not the front), on the sporadically erratic rear sensor reading, and the fact that driveability and fuel economy had not changed (thus making me think the front O2 sensor couldn't be too bad)

Duane

e:

I've been dealing with intermittent CEL with code p0420 for about five years on my '00 outback. Between the dealer and my other shop, I think I've been through 5 O2 sensors.

I have heard of other people with similar issues resolving it by replacing a leaky y-pipe or y-pipe gaskets.

Is there a definitive test of the catalytic converter?

ote:

A couple of weeks ago, my daughter's forester threw the P0420 code after I had replaced the front O2 sensor. It had thrown a code related to that sensor a couple of weeks before. Since it complained about "efficiency ratio", first replaced the rear O2 sensor, figuring that maybe the rear O2 sensor was reading something it shouldn't, but the 0420 code came back. Since she had to pass emissions before the 17th to re-register, we bit the bullet and changed the CAT two days ago. The light stayed off and it was ready for emission testing yesterday. It passed the test today.

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ion my cat was

the sensors and

Just to add this: The front O2 sensor was definitely bad. It ran very rich causing it to idle badly. After I replaced the front O2 sensor, the mileage was again what it should be. Her car clearly had two problems, with the cat being bad as well.

It may have had a leak or something, causing the P0420 code. It is hard to see with all the heat shields in place.

If it had been running rich for a 'significant' amount of time, that is very bad for converters. Add general age/condition issues and I would not be surprised if 2 or all 3 components were bad/compromised.

Carl

te:

That's what I figured as well. The O2 sensors were the easiest to replace so did them first.

If you have a leak ahead of the converter you WILL have troubles. Fix all leaks first.

A good indication of whether a cat is working is to check temperature at the front of the converter and at the back. The back should be hotter.

A bad O2 semsor causing the engine to run rich will often overheat and kill a cat. When the CEL comes on, find the problem and fix it. Usually a lot cheaper that way.

I had the 420 code, put in a new rear O2 sensor, and the CEL went away. My mileage also increased from about 26 mpg to about 28 mpg. I passed emissions a week later. Then about another week later the CEL came back. It's been on for two or three weeks now, but I am still getting improved mileage. I guess I need to go get the codes read again. Hopefully the fact that my mileage is still improved means I'm fine and the CEL is for another reason than a 420 code. Hopefully.

Sorry to burst your bubble, but a rear O2 sensor is a monitor only and can not affect your gas mileage - period. ANd as stated before, a 420 code will not be solved by replacing a rear sensor alone. The front sensor is more likely to be the culprit - or an actual bad cat. (which would generally not pass e-test)

went through a incredible class a few years ago concerning rear 02 sensors and fuel trim. Even tho the manufactured say it won't, or in some cases will say "minorly" it can major effect fuel trim/gas mileage. its the difference in what the tell you, and "us" the manufacture tech's and what really happens. Steve