View Full Version : Will a PO420 automatically lead to PO301/302?
JLav211
10-05-2009, 07:45 PM
Hey guys,
My car is going in to Dent Sport tomorrow to get the anti-fouler work done to try and shut down the CEL so I can pass inspection. I got the codes read tonight and it's also throwing the PO301/302 (misfire on cylinders 1&2). Would the PO420 cause the 301 and 302? I can't think of why on earth it's misfiring, especially since I don't notice anything "rough" about its operation. Just have my fingers crossed that the antifouler will clear up the other codes.
Thoughts?
Thanks - Jeff
RedEvilWgn
10-05-2009, 07:52 PM
No it won't.
Typically those codes are unrelated.
as far as your misfire issues, I would water test the Coil and sparkplug wires, as those seem to be the most common culprits we get here at the shop.
lozanoa11
10-05-2009, 08:26 PM
If you have a clogged cat it could cause a misfire or what seems to the computer as a misfire. I would check the usual things for a misfire but fix the p0420 and see if it helps.
edit: actualy the misfire could be causing the p0420. (That is the cat code right? i forget now)
JLav211
10-05-2009, 08:39 PM
I have the Stromung high-flow cat and UEL Borla replica headers, which is right when the CEL was initially triggered. Could either of those be culprits?
lozanoa11
10-05-2009, 09:06 PM
oh idk then. Usally p0420 is a bad cat but i didn't know you had a a/m cat. It should not be related. Check wires, plugs,coils first. Try spraying water lightly at the wires at night while running if you see sparks you have a problem.
Navi271
10-06-2009, 12:21 AM
The UEL borla replica headers are probably the reason it's throwing a p0420. Something about the higher flow makes the sensors freak out. There's a whole thread somewhere here about it.. Happened to my car and as soon as the sensor spacer was put in place it went away! But never had a problem with the other codes. So like everyone else says, check/replace spark plugs and wires, maybe ignition coil.
Huffer
10-06-2009, 10:40 AM
I'd clear the codes and check your sparkplug connections after your O2 spacer goes in.
If you get a misfire code again - pull the plugs and inspect them.
JLav211
10-07-2009, 06:11 AM
Well, I feel like I wasted everyone's time: spark plug gaps were WAY off. Seeing as the last person to touch them was a Subaru tech, all I have to say is "Thank you, Dent Sport Garage."
Car is running much smoother, idles like it's not even turned on - what a difference. And no more &*%$$#! CEL.
JLav211
10-07-2009, 08:21 AM
AARRRRRRRRRRRRRRRRRGH THE FING THING CAME BACK ON THIS MORNING
I hate this. I really, really do. I want this thing GONE.
150,000 miles...time to replace the coil??
LunchBox04V
10-07-2009, 08:44 AM
The misfire codes came back? That sucks about the bad gap on the plugs. Like they said check coil, and wires. Just a though, but how old are the plugs and wires... Maybe time to get a new set.... As to the P0420, I understand it is the front O2 runs the show, and the rear O2 basically checks that the cats are working. The extra flow is causing the ECU to think something is wrong with the cat system, due to the extra flow.... Anti-fowler the thing, that do it. Lol
Huffer
10-07-2009, 10:38 AM
AARRRRRRRRRRRRRRRRRGH THE FING THING CAME BACK ON THIS MORNING
I hate this. I really, really do. I want this thing GONE.
150,000 miles...time to replace the coil??
What codes? Don't get mad, figure it out.
JLav211
10-07-2009, 10:39 AM
I will get it re-scanned at lunch to see which ones it has thrown. I suspect the PO420 is gone as the anti-fouler was done and that usually takes care of it. The plugs were clean and in good shape so I don't think they are the problem.
The guys at Dent Sport said the coil would be the next logical place to look for repair/replacement...
JLav211
10-07-2009, 12:34 PM
The PO420 is back....fouler didn't work. Dent Sport says the Stromung cat will just continue to cause the problem, so....who knows. I may take the whole exhaust apart and pull out the bad apple or just deal with it until next year when my inspection comes due.
LunchBox04V
10-07-2009, 01:01 PM
I might be just blowing smoke, but I seem to remember my Subaru tech saying the the P0420 works both ways. That is too mich flow makes it mad, as well as to LITTLE flow. That is check for leaks. I know it sounds a little silly, but I thought I would pass it on. I get a random P0420 every now and then, and have done nothing except a cat back... Eh??..... Hope this can help some...
Huffer
10-07-2009, 01:36 PM
Do you have any pics of the anti-fouler Dent Sport installed? It needs to be about 2" high with a big hole drilled through both antifoulers.
At any rate, I would invest in a good code reader/scanner that can clear codes. It's very helpful and not just with your own car.
JLav211
10-07-2009, 02:37 PM
Roger - I will be able to get under it this weekend.
Stupid question time: is an anti-fouler the same thing as an O2 sensor extender? Didn't know if that would make any difference as there are still some of the extenders kicking around on eBay...
rougeben83
10-07-2009, 04:20 PM
Roger - I will be able to get under it this weekend.
Stupid question time: is an anti-fouler the same thing as an O2 sensor extender? Didn't know if that would make any difference as there are still some of the extenders kicking around on eBay...
yes. And even a cheap $30 Ebay scanner like I have will work in scanning and clearing codes. For occasional use for simple things like that, I don't see the point in buying one of those $100+ scanners shops use.
lozanoa11
10-07-2009, 06:59 PM
Roger - I will be able to get under it this weekend.
Stupid question time: is an anti-fouler the same thing as an O2 sensor extender? Didn't know if that would make any difference as there are still some of the extenders kicking around on eBay...
yes. And even a cheap $30 Ebay scanner like I have will work in scanning and clearing codes. For occasional use for simple things like that, I don't see the point in buying one of those $100+ scanners shops use.
"$100+"
more like $1000+
ouch1011
10-07-2009, 11:20 PM
P0420 is a cat efficiency code. It is caused by the HF cat. The P0420 sets when the comparison of the front and rear O2 sensors indicates that the cat is not reducing emissions as well as it should. HF cats don't work as effectively at reducing emissions as the OE one, and Subarus are notoriously picky about their cats. The shop I work for will only use new OE Subaru cats when they come in with a cat code because they have had so many come-backs from using aftermarket cats.
Huffer
10-08-2009, 10:35 AM
^^ yes we know this - but the reason the HF cat triggers the code is usually because the REAR O2 sensor is either too close or too far away from the exhaust stream.
How old is your rear O2 sensor? If it's not giving accurate readings it will also trigger the cat inefficiency code.
ouch1011
10-08-2009, 11:22 PM
^^ yes we know this - but the reason the HF cat triggers the code is usually because the REAR O2 sensor is either too close or too far away from the exhaust stream.
No, the reason the HF cat triggers the code is because it isn't reducing emissions as well as the PCM expects. Moving the rear O2 sensor out of the exhaust stream makes it less sensitive, so that it doesn't switch from rich to lean as rapidly as the front O2, which is one of the things the PCM watches for when determining cat efficiency. So, you are just tricking the PCM into thinking everything is ok.
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