Month ago I got this P0420 code when I was getting on the interstate (Was getting on it pretty hard). Went to Advance and had the code cleared the next day. CEL description said it was dealing with catalyst insufficiency. Stopped by shell that day, filled up with 93 octane and never had the code come back.

 

Today on my way home from work I got the check engine light again. Two days ago I filled up with 87 octane ethanol blended fuel. I checked the maintenance history on my car and it shows the cat replaced before along with the recall for the front 02 sensor. I am running factory headers with an STi catback. Car has 112k. Could the crappy gas have something to do with it? Anything I can run in the car to maybe clear up some crap in the cat?

Kyle.. I have the same problem with my Legacy...havent tried running higher octane in it yet... but how is your gas mileage? Mine Sux and the car is a slug. and I dont feel like putting a new Cat on the thing. The 0420 is the rear O2..and I was reading somewhere about a way to fool it but forget where I saw it.

Kyle.. I have the same problem with my Legacy...havent tried running higher octane in it yet... but how is your gas mileage? Mine Sux and the car is a slug. and I dont feel like putting a new Cat on the thing. The 0420 is the rear O2..and I was reading somewhere about a way to fool it but forget where I saw it.

 

Connie, good to hear from you.

 

When people run unequal length headers on these things, they put a spacer on the rear 02 sensor to get it out of the stream so it fools the ECU into thinking its running within tolerance.

 

I get about 27-28 mpg with a mixture of both city, and interstate driving. The higher octane seemed to clear it up the other day. I have put about 1500-2000 miles on it since then. Don't know if it matters or not. The weather here has changed quite a bit lately, some days it sunny, other days really cool and drizzle. Another thing while I was thinking of it. The entire exhaust portion was off the car in July when I had a clutch put in. It just seems improbable that it all of a sudden decided to act up.

the rear o2 is just a monitor. usually the p0420 is a front o2 code. it can be the cat, but lots of times a new front "subaru" o2 sensor will correct it. aftermarket stuff will / may not correct it.

 

rarely is it the cat.

 

but it could be important to read the code description carefully, inefficient circuit might be different than bad cat.

Well that is some good news. There was a big recall back in the day where Subaru was replacing a ton of front oxygen sensors under the WXW80 campaign.

 

Found this on Nasioc archives:

 

http://forums.nasioc.com/forums/archive/index.php/t-470959.html

I am only getting maybe 20 mpg..out of mine..200 miles to a tank..its pitiful considering I am getting 460+ miles to a tank with my Imp..and its got the bigger motor. This is unacceptable considering I put 100 miles a day on a car

Plus my Lego feels like its towing an anchor...I hate the car because of this. I dont want to toss any money at this car on guess work.

 

I hope yours gets worked out easily Kyle.. I know mine drives me insane..have to clear the code once a week. ( I have a reader in the car and have been known to clear it on the fly)

Well I have a hand-held auto scanner that I bought in 2005 to clear codes on my old 97 Tacoma. I cannot seem to get it to read codes on my Impreza, it just errors out. I hate letting them guys at the auto parts store touch this thing. Luckily last time I was there, I just asked the guy to delete the code, he told me he didn't know how and just handed it to me.

 

Guess I am going to give my mechanic a call tomorrow.

Kyle.. I have the same problem with my Legacy...havent tried running higher octane in it yet... but how is your gas mileage? Mine Sux and the car is a slug. and I dont feel like putting a new Cat on the thing. The 0420 is the rear O2..and I was reading somewhere about a way to fool it but forget where I saw it.

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http://forums.evolutionm.net/evo-how-requests-questions-tips/114931-how-install-spark-plug-anti-foulers-remove-ses-cel-light.html

Cool link, thanks. Might just end up doing that.

 

I guess I will put it on the 02 sensor after the cat, correct?

Bump. Code cleared on Saturday, back today.

Edited October 21, 200916 yr by Durania

Cool link, thanks. Might just end up doing that.

 

I guess I will put it on the 02 sensor after the cat, correct?

 

Yes on the one after the cat. 9 out of 10 times it fixes the 420 code for me. I do a lot of them.

Thanks man. Looking like I will give this a try before I order some high dollar sensor.

Will one of them be sufficient or will I need to use two to space my o2 sensor back?

Will one of them be sufficient or will I need to use two to space my o2 sensor back?

 

I always have used two of them.

I should have adequate clearance underneath?