O2 sensor voltage swings?

I recently measured my OBD I Corolla’s (pre-cat) O2 sensor voltage swings. Seems to be a pretty good test, as it quickly tests a lot of the drivetrain computer’s functions. At idle, after the engine is warmed up, the voltage oscillates from 0.1 volt to about 0.75 volt and back in about 3 seconds. The shop manual however says it should oscillate from 0.1 volt to 1.0 volt… My readings seem to be a little on the low side. Does that mean my air/fuel mixture isn’t quite right? Or is it just that the voltages from O2 sensors vary, one to another? Or that I shouldn’t test at idle, but at a higher rpm?

Just from when I used to watch the o2 performance while driving, I was more concerned about how quickly the readings changed rather than the high and low. Don’t think at idle you will get accurate readings anyway.

Maybe the O2 sensor isn’t the optimum temperature at idle. I think it has to be at least 600 degrees, but forget if that’s C or F. In any event It seems like the drivetrain computer must think the o2 sensor output is accurate, b/c otherwise you’d think it would refuse to enter closed-loop operation.

That’s very slow

I suggest replacing the oxygen sensor


Are you measuring this with a multimeter? If so, a multimeter display cannot update quickly enough to reflect the number of O2 sensor cross counts. As @Tester’s article pointed out, a scope is needed or a scan tool that measures cross counts.

No, I used an o’scope. I did the measurement w/warm engine at idle (800 rpm), but it appears it should be done at 2000 rpm. That may be why the cycle time measures slower than it should. I’ll have to repeat that job it appears. According to the NGK article, the O2 sensor cycle time should be 0.6 seconds, vs what I measured, 3.0 seconds.

At the moment i have bigger fish to fry, need snacks, have to repair my popcorn maker … lol …

Just some more food for thought…I missed the fact that we’re talking about an OBD I vehicle. It’s been quite a few years but I used to work for OTC developing their OBD I scan tool in the early 90’s. I recall most O2 sensors back then being pretty lazy - i.e. it took a few seconds to make a swing from, say, 0.2 to 0.8 and back. It’s been a while since I used a scope on an OBD I O2 sensor though.

Thanks for the add’l info about potential OBD I differences in O2 sensor time responses. Maybe the cycle time I measured is what it’s supposed to be by design. I replaced that sensor maybe 5 years ago w/a Denso unit, so I expect it is probably ok. I expect the prior 25 year old sensor was good too, as replacing the sensor had absolutely no effect on anything. The Corolla’s factory service manual doesn’t spec the O2 sensor cycle time, just the voltage range, spec’d from 0.1 to 1.0 volt. The 0.1 volt downward swing tests ok, but the upper swing stops at a little less than 0.8 volt, a little lower than spec. Like I mentioned above, probably should repeat the test at 2K rpm.