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Old 07-23-2010, 12:56 PM   #1
Scooby Newbie
Member#: 192322
Join Date: Oct 2008
Location: Springfield, VT
2002 Impreza TS

Default O2 sensors

So Im replacing both sensors this weekend. Pretty much just looking for a second opinion on what to get . OEM or after market? Got a hold of an OBDII and threw out both codes for front and rear sensors. Im wonderign if the garage fried em when they welded my exhaust. never had a proble mfor two years the nbam shortly after I had my exhuast fixed engine light goes on. Also tossed an evap/emission leak code so that'll be fun tracing that down.
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Old 07-23-2010, 07:50 PM   #2
Scooby Newbie
Member#: 192322
Join Date: Oct 2008
Location: Springfield, VT
2002 Impreza TS


Well made a few phone calls and wow aftermarket FTW. $112 at the parts store or $193 at the dealer. Easy choice there.
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Old 07-24-2010, 01:00 AM   #3
Back Road Runner
Scooby Guru
Member#: 60082
Join Date: Apr 2004
Chapter/Region: MWSOC
Location: Minnesota
2004 Forester STI


www.Rockauto.com Cheapest place for most everything.

The front one, commonly referred to as an O2 sensor, is an air/fuel sensor that tells the ECU if the mixture is correct. If not, the ECU will adjust as needed. When this one goes bad, the engine starts running poorly. If this one goes bad, you have to replace it. It typically goes bad mainly due to contamination and that is commonly oil consumption and a lot of soot build up. Heat can damage them, but it pretty much requires a torch directly to it for a while. I mean they are designed to cope happily with exhaust gases that can get north of 1000F. Unless the tech melted wires, they're hard to damage just from exhaust work elsewhere.

The rear one, also commonly called an O2 sensor, is a cat efficiency sensor. It just makes sure the cat works. If not, the ECU will throw a code for cat efficiency. The rear one does absolutely nothing for how the car runs. It can even be removed and nothing will happen outside of the check engine light staying on. With a lot of the aftermarket headers, this sensor triggers quite easily. There are eliminators and things you can do to help it not trigger, but really it doesn't matter unless the check engine light bugs you. I'm not sure how much you did with your exhaust, so I don't know if this is avoidable with your setup. If you are running an aftermarket header and/or high flow cats, you may just have to expect this to always be there. You might look at an eliminator.
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