Can O2 sensors be bad without setting off a MIL?

[TomCat]

Here's the deal. I've got a 5.0 stang that had a bad O2 sensor (MIL on) I replaced that sensor with a used one, and the light went off. Question is, can the O2 sensors be bad, at least bad enought to effect performance, without setting off a code??? It's not like I'm noticing any hesistations and what not, but I really want my car to be running the best it can. So would there be any point to just buying two new ones? BTW, the old ones don't look too bad, they're just a little brown and grimy... I don't know if that matters, though.

Thanks,

Brandon

 

[At1Maverick69]

well, it has a tremendous effect on your fuel milage, so, it may not be too bad to set off a MIL/CEL, remember our cars are OBD I, one time you might have it on, and off the next. a rule of thumb I try to follow is replace them every 2 years

 

[Insane3D]

I'm not sure what you mean by "MIL"...the the "Check Engine Light"? As for can they be bad and not throw a code/light, yes. O2's will slowly be less and less accurate as they get mileage on them. The usual lifespan for O2's is 60K - 80K, so if you have more than that on them, you might want to switch them out with a new pair.

They probably aren't bad enough that they will effect your performance a whole lot, but they will affect your gas mileage, and your air/fuel ratio might not be it's best if the O2's are on their way out.

You can pick up new Bosch O2's at your local Autozone for $39 ea. I put in two new ones when I put on new headers and X-Pipe a few months ago, and my mileage shot way up. I was getting 8-10mpg before, and now I get ~18mpg average around town, and as high as 24ish on the highway with cruise control...

Makes a big difference when you run 93 octane that is $2.55/gallon now.

 

[HISSIN50]

MIL = Malfunction Indicator Light = CEL.

Input from O2's are not used at WOT, so if an issue persists at WOT, that suggests to look elsewhere (this info does not pertain to SN95's).

Good luck.

 

[TomCat]

Thanks for the replies. I'm not sure of the mileage on either of them, so I might just swap them out. Oh and my gas mileage isn't too bad. I'd say I get around 17 the way I drive.

Thanks,
Brandon

 

[jrichker]

Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter.