CEL P0420

[vicdeng]

Hi, i know alot of ppl have been getting this code. I have a 96 LGT, with an EJ22 swapped.

The swap was done 4 weeks ago. The car drives fine, and later, it threw a code P0106, which it seems i mixed up the vacuum line, i switch them, and cleared the DTC and it didn't come back.

Yesterday, when i was driving back home, i went to Marathon for gas, and fill up the tank, i got 26.5 MPG, not bad for an old car and old engine. However, after 20 miles, i got a CEL P0420. Today, i have time to play with an expensive scan tool and read some O2 sensor voltage, the front one i believe, the voltage fluctuates quite a bit, between 0.01 to 0.8V, the rear one did not fluctuate that much, but still run between the 0.2-0.8V. I do not know if the front sensor is ok. However, the number didn't go 0 or super high volts, it's still fluctuating.

I am asking what i should do in the next step, i cleared the DTC, should i wait for them to come back up?

BTW, i saw someone here post a server which has all the MY's service manual. I cannot find it now, can someone find it for me? i am looking for a 95 Subaru legacy.

Thanks alot

[ShadowGT]

P0420 is a cat converter fault / not a normal efficiency. Some say it's fouled, some say it's at the end of it's lifespan.
Some say.. (just realized The Stig references)

Anyways.. in my own honest opinion... for what it's worth.. to be taken with grain of salt... results may vary..
If it keeps coming back, all the time.. you have a bad cat. Bad kitty.
If it's once in awhile. I wouldn't sweat it. You only have a cat converter to keep polar bears and seals from getting sunburns. I had it pop up once on my 98GT and I never saw it again after I cleared it.

[vicdeng]

Could it be bad gas? because the timing is alil odd, right after i fill up the tank, i know it also has to do with the air/fuel mixture.

[02_Legacy]

I had this code come up on my car and I cleared it and it hasen't come back in two years so I would start by just seeing if it comes back.

And here is the link to the FSM downloads:
viewtopic.php?f=14&t=20753&hilit=FSM (http://www.sl-i.net/FORUM/viewtopic.php?f=14&t=20753&hilit=FSM)

[vicdeng]

Thanks for your comments.

[ouch1011]

Its not from bad gas, the computer has to see the cat fail several times over a period of potentially weeks before it will set a P0420. What caused you to swap the engine to begin with?

The O2 sensor readings you saw are fairly normal. Normal narrow-band O2 voltage range is 0-1V. The front O2 sensor will switch as the computer adjusts the fuel mixture. Above .5V is rich, below that is lean. The computer tries to get an average air fuel ratio of 14.7:1 by switching it rapidly from leaner to richer than 14.7:1. The rear O2 is monitoring the effect the cats are having on the exhaust. Aside from when you are hard on the gas or coasting, it shouldn't really switch at all and should be pretty stable at around .7-.8V. A switching rear O2 is an indication that the cat is ineffective. The computer actually uses switching ratios between the front and rear O2s to determine cat efficiency and if the switch ratio gets too close to 1, the cat monitor will fail and set a cat efficiency code.

Clear the code 1st. If it comes back, then you definitely have a problem.

[fullCircling]

I have the same code... and it keeps popping up. The car hasn't had much done in the way of maintenance, so I've picked up:

-both manaflow cats
-Denso plugs
-ngk wires
-Denso o2 sensor
-Subaru coil
-seafoam
-new battery terminals/ground wiring

if that doesn't work I'm all out of ideas to get rid of the code... well, aside from putting some weed in the throttle body haha

[Dead91silvia]

A little warning with the seafoam... It cleans hella good, but it plugs lots of stuff! You might wanna use it before installing all the new parts. Your gonna need a fuel filter after that also... I used some in my sled (snowmobile) last winter and it cleaned everything alright! I have to have my carbs re-jetted from all the crap in them! haha

[fullCircling]

yea I love seafoam! I'd put it in my coffee if it wouldn't kill me. I used a bit in the intake mani a couple weeks ago, and while it did definitely help, I'm pretty sure I fouled out my old plugs. I'll start throwing parts at it tomorrow, can't wait to finally get rid of the code!