Can the Oxygen Sensor be bad even if the Check Engine light isn't lit?

[zeronero]

I think mine might be bad since my mileage is starting to drop, but the check engine light isn't on.

Originally posted by zeronero
I think mine might be bad since my mileage is starting to drop, but the check engine light isn't on.

My '91 300E has only one sensor and that's ahead of the catalytic converter. Later models added sensors after the cat so that the effectiveness of the first sensor could be checked. W/o the additional sensor, there isn't anything to use to verify the operation of the primary sensor. Hence, there may not be a check engine indication of a sensor going bad on your car.

[Cigar Havana]

On my 1989 420 SEL, my check engine light was not on, but the sensor was bad. On a cold start, the engine would stall. I could keep it going by giving it more gas.

[Duke2.6]

There are some on-board diagnostics for single (or two for V-type engines) O2 sensors on most pre OBDII models, but they are not all inclusive.

Dual sensors - before and after the catalyst - are primarily intended to monitor catalyst performance.

The O2 sensor has nothing to do with starting since it doesn't generate a usable signal until it heats up to about 300 degrees C.

The O2 sensor on KE-equipped models may be indirectly checked by evaluating the EHA duty cycle, or the actual O2 sensor voltage signal pattern may be checked directly with a scope.

Duke

[sjsfiji]

"The O2 sensor on KE-equipped models may be indirectly checked by evaluating the EHA duty cycle, or the actual O2 sensor voltage signal pattern may be checked directly with a scope."

Duke,

Checking the duty cycle of the EHA is via pin 2 and 3 at X11 right? If not, where if you don't mind.... I have yet to figure out where the EHA is...

thanks,

[Duke2.6]

Yes, I recall that pin 2 and 3 are used to check duty cycle, but I can't remember the correct polarity for the VOM. If it doesn't give the correct reading one way, just swap the probes and see if that yields a reading that makes sense.
Duty cycle checking tutorial is at:

www.landiss.com/mixture.htm

The EHA is on the back side of the fuel distributor. It's a little plastic box with a harness and connector attached.

Duke

[sjsfiji]

Thanks... Does this apply for EHA current as well or is the harness needed for that?

thanks,

[Duke2.6]

No, EHA current is a different test.

If you suspect the O2 sensor is bad, connect a scope in parallel to the signal lead and look at the O2 sensor output directly. It should cycle back and forth between about 0.2 and 0.8 volt at a rate of about 0.5 - 2.0 Hz.

Duke

[sjsfiji]

got it... thanks...


cheers,