what are rear O2 sensors for?

[idosubaru]

through my searches for O2 sensor info i've seen a number of comments about how the rear O2 isn't as important and the front affects gas mileage more, don't replace the rear unless you get a code...etc.

 

why is that? what does the rear do?

[wondercow2]

The rear checks the efficiency of the catalyst (and also double-checks the front sensor for air/fuel mixture, if it sees a lot of uncombusted O2 coming out of the catalyst).

[idosubaru]

so is this a good item to replace as preventative maintenance or not?

 

i'm replacing my front O2's on my legacy and impreza but i'll hold off on the rears if it doesn't do anything to fuel consumption.

[porcupine73]

I think it's mainly for monitoring catalyst efficiency. I forget the user's name but he is a subaru tech and he said some time back the ECU has a fuel trim #3 or something you can view on the Select monitor that is based on the rear o2 sensor output.

[schlit]

so is this a good item to replace as preventative maintenance or not?

 

i'm replacing my front O2's on my legacy and impreza but i'll hold off on the rears if it doesn't do anything to fuel consumption.

 

No, it is a waste of money to replace your rear O2 sensor as preventative maintence. What are you preventing? If it goes bad, you will see a cel, but will not leave you stranded.

 

IMHO, replacing front O2 is also a waste of money unless you have symptoms of a bad one. It's not the cheapest part to replace either.

[idosubaru]

No, it is a waste of money to replace your rear O2 sensor as preventative maintence. What are you preventing? If it goes bad, you will see a cel, but will not leave you stranded.

 

IMHO, replacing front O2 is also a waste of money unless you have symptoms of a bad one.

 

i'm not worried about being stranded, they'll run without the O2 even plugged in!

 

if it increases gas mileage 0.5 mpg it will pay for itself in 20,000 miles.

1 mpg pays for itself in 10,000 miles....(and that's if i buy the more expensive Subaru part)

 

replacing every 10 years or 100,000 miles makes it a $10 per year or one tenth of a penny per mile item. i can handle that.

[griffenrider]

i'm not worried about being stranded, they'll run without the O2 even plugged in!

 

if it increases gas mileage 0.5 mpg it will pay for itself in 20,000 miles.

1 mpg pays for itself in 10,000 miles....(and that's if i buy the more expensive Subaru part)

 

replacing every 10 years or 100,000 miles makes it a $10 per year or one tenth of a penny per mile item. i can handle that.

 

1) Grossgary makes a good point on the cost issue, My gas mileage jumped by about 25% after replacing the 02 sensor (18 to 24 mpg in the city) And brought my highway back up to 28 from 22. Definitely worth it.

 

2) Currently have the annoying P0420 popping up every couple hundred miles, which, as I understand it, can be caused by a bad rear 02. This shouldn't affect gas mileage, so if that's your concern, the rear 02 isn't, as far as I understand it, factored into the ECU fuel injection calculation. If you do end up with a CEL P0420 and you don't have access to all the fun diagnostic equipment, it might be worth changing it out before attacking a several-hundred-dollar cat.

 

3) Also, if price is an issue and you're willing to do the work yourself, you can get a 4-wire 02 sensor for about 50 bucks, just attach the extra ground to chassis ground (and you actually get a quieter signal to boot, which can get you an extra 0.5% mpg)

[nipper]

If you have over 100,000 miles on a car, and your in that frame of mind to replace the O2 sensor (front) go ahead, it can't hurt. Below 100,000 is a waste of money, unless you have poor gas mileage.

 

 

 

The second O2 sensor just cheks on cat performance. let that one go till you get a CEL.

 

nipper

[idosubaru]

The second O2 sensor just cheks on cat performance. let that one go till you get a CEL.

copy that. that's my plan. i got no codes.