Hi all,

In the past two years, the following emission increases have been measured on my car (97 OBS):
HC: 1577 mg/km --> 2066 mg/km (ave for vehicle type is 1795 mg/km)
CO: 6.3476 g/km --> 9.3028 g/km (ave for vehicle type is 4.4284 g/km)

Coupled with these increases, fuel economy has been slowly dropping.

From what I can find out myself, all of these symptoms point to a defective O2 sensor. Is this a safe bet or is there something else to consider as well?

Final question: How easy is it to swap the O2 sensor? The mechanic I talked to started giving me horror stories of the sensor being seized in and threads breaking off etc. Does anyone have experience doing this?

Thx,

Chris P.

 

Easy to replace, with a 17 mm closed end wrench. Just becareful you don't break the O2 while removing it cause it will be tight. However, do you know your O2 counts and the difference between bank 1 and bank 2. Because if it doesn't set of your CEL, your O2's are still running within parameters. Don't replace it if is not broke yet, unless you really getting s***ty gas mileage, then proceed. Just make sure it's the O2 that causing it. Good luck.

 

Yeah, the Subaru shop I talked to said the main risk of doing in yourself is having the O2 sensor break during removal. The cost savings of doing it myself is worth that risk I think.

From what the shop told me, there's only one O2 sensor on the '97 OBS so I assume I only have one 'bank'? My mileage is quite crappy (>11 L/100 km) so I'm sure there's something that needs fixing. Another site (can't recall which one) said you can check the O2 sensor operability with a multimeter (check for 0-12V voltage toggling). I'll do that before I do anything else.

Do you know where the O2 sensor is located?

 

The O2 sensor is located with your catalytic covnerter(s) usually. The other thing I would check is your MAF sensor. Try cleaning it first, as well as your air filter- that can have surprising results. But, as old as your car is, I would plan to replace the O2 sensor. It is a wear item.

Your mechanic is giving you horror stories about your O2 sensor so you'll pay him to remove it. Even if it DOES break, you can still drive the car to him to remove the rest (which he would have had to do if you had let him do the job anyway).

Also, increased emissions is generally a sign of an overall poor state of tune. Do all your normal service bits- replace air and fuel filters, spark plugs, and possibly wires. This is always a good idea before emissions testing. Also, use the recommended octane rating fuel listed in the owner's manual- don't switch to premium expecting it to burn better, because it will actually be worse. However, that wouldn't cause the reduced milage, so I would expect it to be other issues.

Lastly, your car is getting older- I'm sure it's burning a bit more oil, which is a huge cause for increased emissions.

 

Air filter was cleaned about 30 k ago...hmmm, I should get on that. I change the oil frequently and all other scheduled service is up to date. The engine _is_ burning some oil as I'm not quite getting the 4 L out that I put in (no obvious leaks anywhere either).

I figured as much about the horror stories from the mechanic. Just wanted to make sure.

Thx.

 

BANS SUV... What other things would you suggest being replaced for bad fuel mileage. Ive been told to replace the PCV, Air Filter, O2 Sensor(cel said something bout Bank 1), Fuel Filter... Anything Else??? Im Getting under 200miles in the city on full tank

 

What car, how many miles, what driving conditions?

 

Any post-'96 car has two 02 sensors, one before the cat(s) and one after. (Well some have more but yours doesn't). The pre-cat one is the one that controls A/F, the second one is there just to monitor the condition of the cat(s).
Before replacing the 02, be sure that the required maintenance for the current mileage has been done. This may involve replacing filters as indicated above along with other items.
You may indeed have a bad 02, and if after doing the maintenance as needed the mileage is still poor by all means go ahead and replace it. You want to replace the front one.
It is also possible that the catalytic converter is on the way out, they do not last forever.

 

I also want to make sure he doesn't have like a 1998 Legacy Outback with 4EAT that has 362,000 miles, in which case 17-18 mpg isn't doing that bad.

 

Sorry it took so long to get back to you. Car is a 2000 Impreza 2.5 RS almost 99k i live in florida so its always hot and i do get on it sometimes pretty good but lately i have driving normal and still getting bad mileage.. how much mileage do you think an exhaust leak would lose???

 

Is that 5 speed or auto? Exhaust leaks wouldn't hurt fuel economy.

 

its a 5 speed are you sure that an exhaust leak wouldnt hurt fuel mileage?? Cuz i always figured with an exhaust leak its running more rich which would hurt lots of things... maybe i just learned something

 

The engine doesn't care where the exhaust goes once it's ast the valves, unless its a turbo.

What actual fuel mileage are you getting? "200 per tank" is not precise enough. Next time you go to the pump, fill it all the way and record your mileage. You should be around 23 mpg minimum unless you make lots of very short trips, like to drive throughs and stuff.

 

well i work about 20 minutes from home so about 16 miles round trip and im not getting 23mpg