bamorgan7

i asked the guy at the parts store if they had o2 sensors but he said i have two in my jeep how do i know which one it is???

jpdocdave

are you going off a code? easiest way to describe them is one is before the cat (sensor 1, or upstream) and one is after the cat (sensor 2, or downstream)

bamorgan7

what are the codes?

HeepSpeed

do a search for oxygen sensor and you'll get a thread that lists the codes and the trick for being able to read them without a code reader.

bamorgan7

yea i can read it without cause i get 13 mpg all stock

jpdocdave

ok, so do you want to replace it for fuel mileage? the one to replace is sensor 1. but make sure you're tuned up, cap, rotor, plugs and wires, not just plugs. make sure fuel filter, air filter are good. oil is changed. fluids are mantained, and tire pressure is right also.

fuel system cleanings do good things also, injectors never go bad on jeeps, but the complete professional service works well, clean injectors, intake manifold, combustion chamers all that. and most important throttle body.

clean throttle body should be in the first list also. just a few things to remember. oxygen sensors aren't very cheap.

edit: i don't get a whole lot better than 13 either by the way.

bamorgan7

yea i told my dad that i need one and he said he goin to get a good tune up

BowtieGuy3

Tune up= New plugs, cap & rotor, check timing. Wires are usually optional for a fee. All things you can do at home for around $50.

If an O2 sensor is going bad, money is on the upstream one. I've rarely seen the downstream one go bad. (i.e.: I've replaced about 5 or 6 upstream, only 1 downstream on my '98)

Here's all the O2 sensor codes:
P0131, P0132, P0137, P0135, P0138, P0139, P0141, P0147, P0152, P0153, P0154, P0155, P0158, P0161 (Diagnostic Scan-tool)

or

21 (with a code reader, self-scan trick)

jpdocdave

^ya, the computer uses the upstream as part of the fuel management, downstream just monitors emissions. downstream will not effect gas mileage.

BowtieGuy3

I've done enough of them that now I can usually tell when mine is going bad just by paying attention to my gas mileage...
Just as a reference for ya...
'98: around 15mpg 2WD/12-13mpg 4WD (3" lift & 31's)
'92: around 19mpg 2WD/16-17mpg 4WD F/T/14-15mpg 4WD P/T (OEM)

jpdocdave

so you noticed a good difference by changing it? i might have to do mine, they usually don't get changed till the check engine light comes on.

BowtieGuy3

Yeah, that's how I usually keep dibs on it. And my CEL is out (along with my P/T light), and I don't feel like diving into the dash to change 2 lil bulbs. So when fuel mileage gets bad, I usually change it, then reset the codes.

HeepSpeed

hmmm, I'll have to reread the code I have for my CEL light, because I thought it was the rear sensor...but if you've replaced that many up front ( I also didn't buy new...I pulled the old ones from wrecked jeeps). So I'll recheck since I lost 5mpg highway over the past few months...

BowtieGuy3

I've always used the Bosch direct fit ones, I think they're around $60 at Autozone.

bamorgan7

where is the o2 sensor? the upstream one