Oxyen Sensor values???

[rayclark]

I have a 97 Outback 2.5. Can any one tell me what the values should read for the front and rear O2 sensors. I have a OBD scan tool that tells me the voltage of the sensors but I do not know what they should read.

 

Any info woud be appreciated.

 

 

Thanks

 

Ray Clark

[99obw]

I would concentrate on the front sensor, the rear only verifies the catalytic converter.

 

My understanding is that the voltage should oscillate slightly and be centered on 0.45V.

 

A lot of times a lazy sensor will degrade fuel economy but not trigger a MIL. If in doubt replace it.

 

Has your fuel economy decreased? Have you had a MIL?

[frag]

I think the front one should oscillate between 0.2 and 1 volt. The rear one should roughly stay put at the middle of the range.

[rayclark]

I had an error code p0420. I thought maybe one of the O2 sensors may be going bad. With that code which relates to to Cat. I would think maybe it would be the rear sensor. Would you agree with that? It does seem to be flucuating bwt 0 and .750v. Maybe it was just a random thing.

 

Ray

[Commuter]

I also have a 97 OB. Now with 368k km on it.

 

Summer of 2002, my E-test did not look very good at all. Still passed, but well up in the range.

 

Earlier this year, I started to struggle with "carbon" issues again. My usual tricks to keep that issue in check were no longer working.

 

With various posts on here at the time about O2 sensors (positive results from changing), and simply recognizing the age and distance on the car, I wanted to change it.

 

The dealer checked it. I didn't know, but I guess these things (the logic that is) operate in an "on-off" type mode. The ECU looks for a certain value and either starts to add gas or back off the gas. Constant servo action. The mechanic told me that they look for 15 to 17 cycles per minute. Mine was generating 13. So, a bit slow, but not outside of expectations. (I don't know what the voltage reading were. We didn't discuss that.) They usually don't touch the O2 sensor unless a CEL is tripped. On the other hand, Toyota apparently calls for it to be changed every 80k km or something like that I was told.

 

The next time I was in, I had it changed. Subaru OEM unit.

 

My gas mileage went back up about 1 mpg.

 

My carbon problem improved significantly over the next few weeks. It has finally gone away it seems, but it took several months in total. Frag is right about the exhaust tip (other posts). Mine was looking black. Seems to be a bit more brownish now.

 

So in short... Change it. If it hasn't been done at all, or it's been several years, I'd change it. The way the sensors age, it will cause the mixture to become richer. These cars already run on the rich side, so it really can't tolerate more.

 

I'll be curious to see how my E-test looks the next time around.

 

Commuter

[99obw]

My DTC Lookup utility says "Catalyst Efficiency Below Threshold (Bank 1)"

 

I would start by clearing the code. If the code returns then I would replace the rear sensor.

 

Perhaps the cat is actually bad, but the sensor is a much cheaper place to start.

 

AFAIK the replacement interval for Subaru O2 sensors is 100,000 miles.

[frag]

Commuter wrote:

My carbon problem improved significantly over the next few weeks.

Commuter, by what means do you evaluate the carbon condition of your engine?

Merry Chritmas!

[Legacy777]

O2 sensor values should swing from 0-1 volt, 0.1-0.9 are prob the most common ranges you'll see.

 

The only time you should really see it at a fixed value or relatively fixed is if you are accelerating, or if the sensor is not warmed up.

 

Under full throttle acceleration I get around 0.7-0.8 volts. Which does indicate I'm running rich, but that's a typical subaru trait, especially at full throttle.

[lmdew]

0.45 to 0.49 is considered optimum for most cars. If you can drive the car with a recording multimeter hooked up you can then get the average. The Fluke 87 meter will do this, they run 130 or so used on ebay. Not cheap, but its a great tool.