Code P0420-Catalyst Efficiency Below Threshhold Bank 1

Had the same code on my 95 a few years back. Replacing both sensors has so far solved the problem. Sensors are not easy to get to/remove. Just unplugging them is a real PITA.

-- Message posted using

The Subaru procedure step by step:

1. Check if there are other codes (P0130, P0133, P0136,P0139, P0037, P0301,P0302, P0303, P0304,P1130, P1131, P0031,P0032 and P0038? If yes, investigate those and disregard P0420. If not:
2. Check exhaust system Check for gas leaks or air suction caused by loose or dislocated nuts and bolts, and open hole at exhaust pipes. NOTE: Check the following positions.

- Between cylinder head and front exhaust pipe - Between front exhaust pipe and front catalytic converter - Between front catalytic converter and rear catalytic converter If no problems:

1. CHECK REAR CATALYTIC CONVERTER. Separate rear catalytic converter from rear exhaust pipe. Any damage? If not:
2. CHECK FRONT CATALYTIC CONVERTER. Remove front catalytic converter. Any damage? If not:
3. Contact Subaru distributor :-)

So unless you have issue elsewhere, things point to a catalytic converter...

DK

Thanks. Will probably put this on hold unil closer to next June when emissions inspection is due again. I seem to recall catalytic converters are quite expensive but one online site has each of them for under $200. Does that not include the platinum stuff?

Jim

Amazingly, last week I got this same code on my 01 2.5L.

My cat is new as of about 10 months ago. O2 sensor is new too, I believe.

I reset the code with my odb-ii scanner, and drove 600 miles and it hasn't returned. I'm holding my breath and hoping at least.

-- Todd H.

Aftermarket cats are not a direct fit, they need to be welded to your pipes. Also they typically don't hold up very well. I would encourage you bite the bullet and get your cat(s) from the dealer.

Hi, Certain gas brand works better for certain engine. Here in Canada, I try to avoid Esso brand.

My son's car has the same code. I did a little research via Google and am going to replace the O2 sensors - they seem to be the culprit at least 50% of the time, according to what I've read on-line. Typically, when a cat 'goes', it breaks up inside and you would get (at least) some rattling and (at worst) a degredation in performance and/or gas mileage as the loose material blocks the flow through the cat. My son's car has neither symptom.

Dan D '99 Impreza 2.5 RS Central NJ USA

I have very same code on my 2000 Mazda Protege ES

having a hard tine figuring out what it is. Mechanics keeps saying the cat con..... but I'm weary abt that and suspect O2 sensors

The PO420 code sets when the correlation between the front and rear O2 sensors indicates the cat isn't doing it's job. If one of those two O2 sensors were actually bad instead, it would set a completely different code and not the PO420 code.

If your car has more than 120k miles, it wouldn't hurt to replace the O2 sensors. You're going to need them sooner than later anyway. But after clearing the codes you can bet the PO420 code will soon return leading you to replace the cat(s) too.

And yes, I am a mechanic. Not just playing one on the internet. ;-)

Hi, Some OBD II scan software can anlyze O2 sensor performance reading the output voltage. Around 2000, there was bad batch of O2 sensors covered by warranty. Believe there is a TSB for this. My kids' 2000 Impreza O2 sensors were replaced during scheduled visit to dealer.

Ok thanks so much Dave!

I didn't realize al this

Sounds like it is in fact the cat con!

Is it the cat con?

Or O2 sensors?

No its the cat con(s). O2 sensors are good.

Was first quoted $1400 plus, for parts, for OEM replacement, but now after checking around it's half that. Still a bit more than a grand installed. Does that sound reasonable?

Unfortunately, yes. They are about 500 buck a pop if you get a good deal. They aren't difficult to install; much like a muffler. But the beasts are expensive.

How many miles on the O2 sensors? Has someone 'scoped' them to see if they are slow switching? (lazy) Has anyone done a flow test through the cat con using the O2 bung? Or even done a sniffer test?

I dunno, I've just read of so many people replacing old sensors to get cat con codes to go away - of course, if your sensors are not old and/or you've ever had blown headgaskets or operated the car in a 'rich' condition - then most likely the converter IS bad. I hope whatever shop is doing the work has done good troubleshooting. If the code comes back in a coupla weeks and they want to change the sensor - are they willing to refund the converter charges? $80 or so is a lot cheaper than $1400

I'm just sayin'......

;^)

Carl

Ouch!

Damn them things are pricey!

Carl.. Thanks for your response. The CATS are original - 165,000+ miles. Ditto the 02 sensors. Never blown headgaskets or run the car rich (unless the computer chose to do so). Technician did say that one was just above the pass threshhold, which I assume means he measured something? Not sure if that addresses any of the suggestions in your first paragraph (not being a technician). I can run those by him.

Jim

Well, 165K is getting elderly for a lot of components - buthat would be a pretty extreme lifespan for the original sensors I think. many are getting lazy at 80-120K.

It comes down to this, it would be better to be wrong about the sensors than wrong about the converter. bya factor of 15-20! personally, I would want to try changing the front O2 sensor (it is exposed to more heat and somewhat more corrosive gasses than the rear one) and clear the codes and wait to see if any return. Start saving your money, and maybe research discount/online dealer prices for the converter (try

basically, these two sensors are monitoring the gasses entering and exiting the converter. If readings are outside some limit, they set the code. There is no 'direct' converter measurement so, if an O2 sensor is reading incorrectly - it could throw the very same code, erroneously 'blaming' the converter. Kinda like a person going blind and telling people the lights need replacing! Of course there is a 'heater' circuit that could also fail in the (front I think?) converter. But that doesn't seem to tbe the case here.

hey, I'm just some guy that reads a lot on the internet and occasionally tinkers with cars. If you feel the dealership is being up-front about everything and has done the proper testing - then you probably should proceed.

good luck

Carl

Try googling some O2 and cat con info. here's coupla sites I found quickly;

Carl