P0420 Code Help

Lola09

New member
I have that wonderful P0420 code on my 2009 WRX. We have been told it could be one of two things, either the rear O2 sensor or the cats. We replaced rear O2 sensor cleared the code, then one day later the code popped up again. So my assumption is I need new cats. However there is something that had us a little stumped. I have taken notice the last few weeks on when the light turns on and off. The light will shut off with a full tank of gas, the minute I have less than 3/4 of a tank the light turns back on. This has us stumped. I would think the light would just stay on until we fixed the issue.

We are not strangers to the wonders of Subaru codes by any means. This is however our first turbo charged Subaru and I know they can be rather fussy. So has any one else experienced this or is my car just special?

 

inski

New member
Generally the p0420 means you need cats.  Subaru (as well as most manufacturers) has have a very long warranty on emission related items which covers Cats, O2 sensors, EVAP stuff, EGR stuff, A.I.R.stuff.  If you call your dealer and give them your VIN and miles they can tell you whether or not its covered, they will need to verify it is an emissions issue by looking at the code themselves.  Now the tricky part.  If your car is over by miles the dealer might try to deny a warranty (free) repair.  You might have to call Subaru directly and get them involved, and 80% of the time they will tell the dealer to fix your car under warranty.  

The dealer will need to do a cat efficiency test.  Its just a short to mid-range test drive with the computer hooked up.  Only then can they verify if a cat has failed.  If you've done a lot of work at your dealer before your chances of getting covered are greater.  'Goodwill repairs' (free, beyond warranty repairs)  are very with Subaru dealers.

Your fuel level probably has nothing to do with the Cat code, but some EVAP system issues depend on fuel level.  You might have a another code now.  

The onboard computer is constantly testing your Cat efficiency but it can't test it until certain parameters have been met.  Those parameters depend on how you drive.

Replacing the rear O2 sensor probably was a waste of money.  The 09 can differentiate between bad cats and bad O2 sensors.

Now for the good news.  If you do not live in Cumberland County you do not have to get it fixed.  If you do live in Cumberland County you'll have to get it fixed before your next sticker is due.

 

Lola09

New member
Thanks for the response!

I will have to call Evergreen Subaru in Auburn and see what they can do. We have been dealing with them since my first Subaru 9yrs ago. It was a 1990 Legacy L FWD. I beat the crap out of it and it loved it! Lol

If the cats do need to get replaced and Subaru cant do anything, we are just going to put an after market down pipe on it. Only thing that sucks about doing that is it will void our powertrain warrantee. The OEM one is a ridiculous price.

 

IBratmanI

Subaru Ambassador- AdidaSubarus
Thanks for the response!

I will have to call Evergreen Subaru in Auburn and see what they can do. We have been dealing with them since my first Subaru 9yrs ago. It was a 1990 Legacy L FWD. I beat the crap out of it and it loved it! Lol

If the cats do need to get replaced and Subaru cant do anything, we are just going to put an after market down pipe on it. Only thing that sucks about doing that is it will void our powertrain warrantee. The OEM one is a ridiculous price.
Just make sure if you do replace it with an aftermarket downpipe, that you get a tune on it, so it doesn't hurt your engine!
 

Lola09

New member
How much are you looking to get for it?

If we put an after market downpipe on it we are definitely getting it tuned! However pretty sure I need new coil packs first. She sputters pretty hard under acceleration. Already replaced the spark plugs (no we didn't gap them) and it made no difference.

Having a blonde moment. I keep trying to quote you guys but it doesn't seem to be working. Am I doing something wrong?

 
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IBratmanI

Subaru Ambassador- AdidaSubarus
I use the full version of the forum on my phone, and you just click the quite button all the way to the bottom right of the message you're quoting.

Subarude is good to deal with, so a good used downpipe might be worth trying! And definitely get coil packs replaced as soon as you can too, if you have misfires, it's not good for the engine!

 

Lola09

New member
I use the full version of the forum on my phone, and you just click the quite button all the way to the bottom right of the message you're quoting.

Subarude is good to deal with, so a good used downpipe might be worth trying! And definitely get coil packs replaced as soon as you can too, if you have misfires, it's not good for the engine!
I think I got it! For some reason quoting works on my phone but not my computer! Weird.

Coil packs are top of the list for sure!

Thanks for the help!

 

inski

New member
+1 for Subarude, nice stuff for a wicked nice price.

+1 for Evergreen, they'll help if theres any way they can.  Tell them inski sent you.

 
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