Oxygen Sensor Need Replacing?

[plarsen111]

I have a 1999 Forester.

 

Check Engine Light is on. I had it check out, and the guy said the Oxygen Sensor needs to be replaced. I inquired about what would happen if I didn't have it replaced, and he said it could ruin the catalytic converter.

 

Is this true? Do I really need to have it done to the tune of $400?

 

Thanks,

 

Phil

[davebugs]

You need to buy yourself a 7/8 (IIR) open/box end wrench, some loosener of your choice (PB Blaster, Liquid Wrench, Kroil, whatever) and an O2 sensor.

 

Jack it up and get under it and look for about the least convenient place to get at on the exhaust system. You'll find the O2 sensor sticking up there by the front cat. If you unplug it the box end of wrench will fit over the plug. A little hammer action and you should be good. I anti-seize the new one.

 

This is from memory so it may not be correct but I'll bet I'm close!

[nipper]

Get one from OE.

 

It could hurt your cat, it could foul your plugs, it could cause your mpg to suffer, and it could make the planet a littly more browner.

 

But what it does is cause the CEL to be on, so you will fail an inspection, and it can hide a CEL for a more important failure.

 

 

nipper

[edrach]

You have two O2 sensors; the front one controls the fuel mixture and when it goes the car will run crappy (and if it's too rich will do in your cat). The rear one just monitors the exhaust and when it goes, it just tells the system that emissions are out of spec or the sensor is bad (or the wiring/connector are bad) and this one will not harm your cat. But as stated before, it can hide another CEL.

 

I'm currently running with a P0136 code (O2 sensor on bank 2) with my '97 Impreza. I think I have a loose wire at the connector since I repaired that once before. I'll take care of that when I can crawl under the car again. Car is running fine and the CEL is on but not flashing.

Edited July 16, 2009 by edrach

[Olnick]

You need to buy yourself a 7/8 (IIR) open/box end wrench . . . .

 

I think it's actually 22mm--but 7/8 should work.

[etc]

On 98 Legacy L, with 2.2L, where is the upstream one located?

 

I found one O2 sensor but I am not sure if it's the upstream or the downstream one.

 

I hear that you don't need to have a code set to have a bad one, or a "lazy" one - they slowly degrade with age. I assume mine is the original 11 years old with 150K miles, well within the replacement territory.

 

Secondly, where is a good place to get them? They seem to be over 100 tokens each even at places like AAP or Autozone and presumably more expensive at Subaru.

 

I got my favorite PB Blaster penetrating oil.

Edited July 22, 2009 by etc

[davebugs]

The O2 sensors move around a bit.

 

But the front one around your year range is always by the first cat (after the Y pipe) typically in the cat itself. Atleast some 99's seem to have 2 O2's in the front cat. Typically the first one is the hardest to access pointing upwards/frontwards and is at the start of the foist cat. The second one can vary greatly(I'm sure by year it makes some sense - but they vary widely).

 

If nothing else follow the single(?) wire on the passenger side that goes down to the O2 sensor. It's tied to the end/trans lower right bolt by a kind of twist tie. It's little connector is very roughly near the 3 large connectors under the black plastic intake box where the trans meets the engine. This small connector is in the vacinity. If you look in there towards the axle where it meets the transaxle you should see it. Just follow that.

 

Heat shields can be sharp so watch your hands, find some gloves, etc.

[etc]

OK. I assume you access the upstream O2 sensor from the bottom?

 

I cannot see how you can reach it from top but I thought I would ask just in case.

[davebugs]

Yep from the bottom.