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Old 02-09-2013, 03:45 AM   #1
sisenor
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Default P0420 but running fine

So I got a CEL, had autozone run the code, p0420 was spit out...

No search nazis please, I've already searched and read about it.

I'm just wondering since my car is running fine despite the light coming on, should I try to replace the o2 sensor first, or should I just drive it? Will I hurt anything driving it?
Thanks
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Old 02-09-2013, 04:34 AM   #2
Zefy
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this code either means your cat isn't workign properly or the o2 sensor isn't working properly. The rear o2 sensor is mostly there just to make sure the cat is working. you might find a slight reduction in fuel economy...?

so it's fine to drive like that but you probably won't be able to pass any inspections or emission tests with it. Also, if your car actually develops a real problem, you won't really know about it because your CEL is already on.
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Old 02-14-2013, 09:34 AM   #3
SubaruTech84
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Unfortunately replacing the o2 sensor won't fix your problem, it will only just waste some $$. The car is just telling you that the catalytic converter no longer has the ability to convert unburnt fuel vapors into oxygen and carbon dioxide. It shouldn't effect your driveability at all and won't damage anything but the o-zone. Lol. You won't be able to pass emissions until you do something to remedy that though.
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Old 02-14-2013, 01:18 PM   #4
00redobs
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Quote:
Originally Posted by Zefy View Post
this code either means your cat isn't workign properly or the o2 sensor isn't working properly. The rear o2 sensor is mostly there just to make sure the cat is working.
this.

running rich damages the cat also, failing front o2 sensor (a/f ratio)
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Old 02-15-2013, 08:55 PM   #5
Pvt_Joker
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I'm seeing the P0420 code as well (all stock 2005 Subaru WRX STI). Here's my story... After nearly a dozen resets using a scanner tool, I observed that the CEL would only trigger after the car had warmed up and I was cruising at ~55 MPH. So I decided to investigate.

After jacking it all up, I noticed there was a crack in the midpipe where it attached to the back of the resonator and goes back to the muffler. Thinking this was the root of my problem, I took that pipe off and welded it back together. After putting it all back together I reset the CEL and cruised on home. CEL came back on after cruising for 15-20 mins at ~55MPH.

At this point I decided to do some more research. The P0420 code definition is: Catalyst System Efficiency Below Threshold. (http://www.obd-codes.com/p0420) It must be the catalytic converter right?

I purchased a used OEM downpipe and catalytic convert from a similar model year STI. Price was reasonable - $60. Unfortunately, I couldn't be certain if the catalytic converter was bad in this new pipe as well, but I figured I would give it a shot. This setup bolted right in. Reset CEL - Came back on at cruising speeds yet again.

Provided the new (used) catalytic converter I put on wasn't bad, perhaps it was one of the oxygen sensors? Purchased a new Denso (this is the OEM brand for STIs) rear O2 sensor (this is the one that screws in to the rear catalytic converter) from rockauto.com (very solid OEM parts store BTW). Installed it, reset CEL, cruised, CEL came back...

Purchased a new Denso front O2 sensor. This sensor bolts in to the front exhaust manifold and, from what I've heard, is the most sensitive sensor used for determining proper A/F for the car's computer. Installed, reset, cruised, CEL came back...

I'm somewhat perplexed by this scenario. I believe there may be two unresolved problems:
A) The used OEM cat and downpipe I installed were already bad.
B) This is complete speculation, but perhaps the previous owner tuned the ECU (Cobb ECU tuners are easy to use and widely available) because they had an aftermarket exhaust installed. Before they sold the car, the aftermarket parts were taken off and OEM put back on. They didn't put the factory map back on the ECU. Wouldn't I notice some strange drivability issues with an incorrect tune?

Sort of at a loss on this one. Anybody have any thoughts?
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Old 02-16-2013, 11:49 AM   #6
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Sounds stressful


I have this code on my 01rs
But i did add uel headers so iam sure it due to that
I have a scan gauge so every gas tank it comes on i just clear the code

I was gonna try the 02 extender and see if that helps
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Old 02-20-2013, 08:03 AM   #7
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subaru22, also wrap your headers, when i did that it went away on my legacy when it was n/a.

the thin piping of UEL doesnt get warm fast enough so when the rear O2 sensor initiates to read the temp is too low and it throws a code, wrapping the headers will keep in the heat and prevent the code from popping

-B
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Old 02-20-2013, 06:56 PM   #8
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I get this code constantly and was told by two different Subaru service departments that the cats are "too sensitive," both recommended running higher octane gas. I put 2-3 gallons of 91 in every tank and the code goes away, but always comes back when I get down to 1/3-1/4 tank.
Also putting any type of generic fuel additive (octane booster or cleaner) into a tank will throw the code almost immediately, but name brand stuff doesn't seem to do it.
For reference, I almost never clear the code manually. I just drive until it goes away which suggests it has something to do with the diagnostics or the cats really are "too sensitive."
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Old 02-21-2013, 02:25 AM   #9
Dr. Octawakaoctapus
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If you ever have the ability monitoring the front AF sensor and rear O2 at cruise will tell you if cat. Or Sensor is the problem
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Old 02-21-2013, 06:01 AM   #10
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Quote:
Originally Posted by Pvt_Joker View Post
........................................
Sort of at a loss on this one. Anybody have any thoughts?
I'd say, check exhaust system for cracks again and make sure its properly connected. If not - replace cat by VIN with original one.

Quote:
Originally Posted by Dr. Octawakaoctapus
If you ever have the ability monitoring the front AF sensor and rear O2 at cruise will tell you if cat. Or Sensor is the problem
Does it mean I can check cat with the scan tool?
And what will proof cat problem? Or oxygen sensor problem? Or bad fuel problem? Or extra O2 coming from crack? Or improper fuel ratio?
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Old 03-03-2013, 10:40 PM   #11
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Two new thoughts:

1. As I mentioned earlier, I get this code all the time and just drive until it goes away. Yesterday I got my new catback installed, the 0420 cel was currently showing both before and after the install. After about 50 miles of driving the cel went away and has yet to return. Obviously it still too early to think it is gone for good, but I find it ironic and very coincidental that installing a new exhaust behind the cat would cause the code to go away. But, if indeed that is the case then the answer for n/a cars with this problem is simple...just get a different exhaust system from the cat back. For reference, the catback I installed is an oem from an '07 sti.

2. Could bad/worn out plugs contribute to the 0420 symptom? If the fuel isn't being properly ignited, then there is too much leaving through the exhaust. Which in turn could be overloading the cat with pollutants, causing it to work inefficiently or just telling the 2nd o2 sensor that the cat isn't working properly. Just a thought, and considering plugs are much cheaper than o2 sensors or new cats it seems like a good thing to try first.
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Old 03-04-2013, 01:31 AM   #12
Dr. Octawakaoctapus
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Quote:
Originally Posted by Automagrez View Post


Does it mean I can check cat with the scan tool?
And what will proof cat problem? Or oxygen sensor problem? Or bad fuel problem? Or extra O2 coming from crack? Or improper fuel ratio?
Depending on the scan tool you can view the readings of both the front AF sensor and the rear O2 sensor. Put car in park and rev the engine to 2000-2500, and if the front AF is moving to much its bad, and if your rear O2 voltage low its reading to much oxygen in the system so probably bad cat.
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Old 03-04-2013, 09:15 PM   #13
subaru222
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Quote:
Originally Posted by heresmymind View Post
subaru22, also wrap your headers, when i did that it went away on my legacy when it was n/a.

the thin piping of UEL doesnt get warm fast enough so when the rear O2 sensor initiates to read the temp is too low and it throws a code, wrapping the headers will keep in the heat and prevent the code from popping

-B
I think I am gonna try this next


You think the 90 degree defouler will work
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Old 03-05-2013, 09:28 AM   #14
ManualOverAuto
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Old 03-05-2013, 10:54 AM   #15
ADaughen
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I've done pretty much what Pvt_Joker did on my '00 n/a 2.5L ~160k miles.

New UEL header (ceramic coated)
New Stromung high-flow cat
Used Blobeye WRX cat-back
New front and rear O2 sensors
New gaskets front to rear.

I lose ~2mpg from the problem. I just roll with the CEL on. :/

I have anti-foulers, but the weather has sucked too much to put them on.
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Old 03-05-2013, 07:36 PM   #16
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did you put the foulers on both front and back 02?
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Old 03-06-2013, 09:20 AM   #17
ADaughen
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did you put the foulers on both front and back 02?

I haven't done it yet, but it should be on the back only.
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Old 03-09-2013, 10:48 PM   #18
2004ImpressYa
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I just got thise code today on my '04 WRX. It can't be the cat because I literally had a brand new cat converter put in about 2 weeks ago. the mechanic did mention that he never replaced the o2 sensor. Just purchased a downstream Bosch sensor. Hopefully that resolves it. isn't there a cat on the up pipe? would that cat throw the code??
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Old 03-13-2013, 09:35 AM   #19
ADaughen
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I wanted to update - I cleared my code after my monthly check of the CEL. Hasn't been back on in the last 208 miles (used to come on within 60 miles of reset). It might have been this crappy winter mix finally being done with.
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Old 03-13-2013, 01:11 PM   #20
Zefy
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the cat efficiency check only happens after a steady state is reached of about 20 mins on the highway at a constant speed/rpm. If you don't do that, then the check will never be run and you will never get a code. I've seen people go for quite a while without codes because they never drive very far.

If you have a code reader, these are listed under the 'readiness monitors'.

if you have done this already, then cool. Your car has fixed itself.
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Old 03-13-2013, 02:21 PM   #21
ADaughen
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I know, I run 20+ miles highway twice a day to and from work.


I only have the ScanGaugeII to read and clear codes.
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Old 03-13-2013, 07:56 PM   #22
subaru222
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I drive about 100+miles aday lol
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Old 03-14-2013, 12:33 AM   #23
Zefy
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Quote:
Originally Posted by ADaughen View Post
I know, I run 20+ miles highway twice a day to and from work.

I only have the ScanGaugeII to read and clear codes.
scangauge 2 can check readiness monitors. It looks like a chore to do but it would be worth while.

No matter how far you drive, if you're not driving consistently for that period, it won't set. Slowing down for traffic and things stop it from working.

But by the sounds of it, it is working properly and your car is running well.
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Old 03-14-2013, 01:35 PM   #24
ManualOverAuto
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I had a bad cat for about 8 months before I bit the bullet and got it replaced with a brand new one.

It started off as the good 'ol p0420 code. At first when I cleared the code it would stay away for up to 2 months. Then it only stayed off for one month, then it started coming back sooner and sooner. It was finally at the point, to where I was clearing the code every week, maybe twice a week.

No rotten egg smell, and the car ran fine.

After the cat was replaced (Subaru oem, $$$$$!) I never saw the code again (knock on wood).
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Old 03-14-2013, 03:05 PM   #25
ADaughen
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Quote:
Originally Posted by Zefy View Post
scangauge 2 can check readiness monitors. It looks like a chore to do but it would be worth while.

No matter how far you drive, if you're not driving consistently for that period, it won't set. Slowing down for traffic and things stop it from working.

But by the sounds of it, it is working properly and your car is running well.

1. I have an early model '05 SGII . I'm not sure mine comes with the same features the latest ones do.
If it requires connection while driving that's almost a no-go. Some a**hat decided to disconnect the lower panel while the ODBII port was connected and shorted one of the wires. 1/2 the time I can't get ANY scanner to read. I have to stick a hand up under the dash and push the wires a little. A warm weather project... hopefully after I get my other car back on the road.

2. It has always popped ~2/3 the way home. I usually run at 65-70mph on the highway. Usually within ~300rpm variance. I'm trying to baby this thing now that the HG is leaking more than my daughter's diaper after a bottle binge.

3. Running well is a wishful thought. I just fixed one issue of the many I bought. Head gaskets and timing belt are next. Then bushings and more bushings. THEN maybe I can start to have a fun little wagon.




Back to the topic though. A lot of different things can cause the error. I tried some of the cheap stuff, ended up ~$800 in before it fixed itself. I would not suggest doing what the dealership did to "fix" it originally - remove the CEL bulb and leave the diagnostic plug plugged in.
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