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Easiest method to test O2 sensor?

Early 90’s Corolla, only has a single non-heated O2 sensor. Have easy access to the O2 signal directly from the sensor itself, at a test (check) connector under the hood, so that’s no issue. When I read the shop manual, it says to verify the level goes between 0 and 1 volt, and that it changes rapidly. And it says it has to be measured with a high impedance volt meter. The only high impedance volt meter I have is digital. If it was analog, with a needle on a scale, I could see the value go up and down as fast as 5 times per second probably. I have a small analog volt meter, but it isn’t high impedance. And with the digital meter, it doesn’t respond fast to voltage changes, just flashes a bunch of meaningless numbers until it settles down after a second or two. If the voltage from the sensor was bouncing between 0 and 1 volt as quickly as 5 times or more per second, about the only thing I could tell using my digital DVM is whether or not the level was between 0 and 1 v. So what’s the easiest way for a driveway DIY’er to check a non-heated O2 sensor? Do I need to purchase a special meter for doing this?


Can your DMM record?

Does your DMM have min, max and average?

Remove the O2 sensor and place it in a vise.

With the DVOM set on the 2 VDC scale, connect the red lead to the signal out wire on the O2 sensor and the black lead on the O2 sensor body or vise.

Take a propane torch and apply the flame to the tip of the O2 sensor. As the oxygen is removed the voltage reading should climb steadily to up to 1 VDC. When the flame is removed, the voltage should should steadily drop to .1 VDC. If the voltage reading is erratic while increasing or decreasing the O2 sensor is defective.


@db4690 … no, mine isn’t that fancy. It can’t record, neither can it display max/min/avg.

@Tester … yes, I’ve seen that done on a U2 vdo. Seems like it would prove the O2 sensor is working or not. Still, I prefer to see how it works on the car rather than on the bench, but your method may be my best option sans purchasing a better volt meter.

Actually, you need a scope/meter to read voltage at that frequency.

Here’s the one I use.


Yeah, I have a Fluke 88 DMM

Personally, I think Fluke is a great company, in part because they really stand behind their product

My Fluke 88 is kind of old already. Twice it gave me problems and wasn’t giving accurate values (I verified this by comparing the values from other DMMs.)

Anyways, each time Fluke recalibrated it, at no charge to me. Try that with your Blue Point, Radio Shack, etc. I’m not exactly sure how much work was needed, but each time the problem was fixed.
The turnaround time was a few days, and I just used my cheapo backup DMM in the meantime.

The bench test with a propane torch will tell you if it’s working, but for the cost of a single wire O2 sensor, if you’re taking it out just put a new one in. That way you’ll know it’s good and you can move on.

There’s no way an analog meter could read a Toyota O2 sensor with any meaningful results, and your DMM might help. But like Tester said, a graphing meter or labscope is what you need. Fluke makes some good ones, I prefer Snap-on. But for the DIY crowd, I think just replacing it is the quickest and easiest step.

If it has over 100K miles on it, just replace it…

Back in the day, in the world of radio electronics and sensitive circuits, we used analog VTVM’s vacuum tube volt meters…VERY high impedance…but by the 1970’s these were replaced by solid state (transistor) VTVM’s which I’m sure could measure a oxygen sensor’s output…So could any oscilloscope…

I still have my Eico VTVM (kit) from 1973.
My Fluke 23 has a fast responding bar graph below the digits; newer inexpensive DMM’s I’ve seen have this too.

You don’t need a high Z voltmeter to tell if the voltage is sweeping at the required rate. The only issue will be that the meter will not read the voltage accurately. Generally if the O2 sensor is sweeping at the required rate, the sensor is working.


“The only issue will be that that the meter will not read the voltage accurately.”

Care to explain that one?


How fast should I expect the O2 sensor reading change between the high and low values at idle? Is it on the order of 10 times per second? Or more like 10 times per minute?

The voltage change occurs in milliseconds. This is why it requires a scope/meter to capture the voltage changes.

Here’s a sample of what the voltage looks like from an O2 sensor using a oscilloscope or a scope/meter.


About once a second for those old sensors.

Tester, the impedance of a meter is placed in parallel with the load and therefore affects the total impedance of the load. The lower the impedance, the greater the effect.

The source and the load form a Kirchoff’s loop with the source impedance in series with the load impedance. This forms a voltage divider. If the load impedance goes down, then the source impedance drops more of the supply voltage, so the meter will read a lower voltage than the true voltage. It “swamps” the supply voltage.

DVMs are more accurate because they have an almost infinite impedance, therefore they have little to no effect on the load impedance. A good VOM will usually have an impedance of 20kohms/volt DC and 5kohms/volt AC. Some lower cost meters will be 5k and 1k respectively and some super cheap small meters will be 1k and 200 ohms/volt.

You see the same effect when putting a volt meter across the battery terminals and hit the starter. With the load out of the circuit (the starter), you measure 12.6 volts. In this case it doesn’t matter much whether the meter is a DVM or a cheap 1kohm/volt analog meter because the batteries internal impedance is on the order of a fraction of an ohm.

For example, a battery with a rating of 630 CCA would have an internal impedance of about 0.02 ohms. 1k, 20k or 1M ohms would not make a measurable difference. Any way, you hit the starter and the starter motor has an impedance of less than an ohm, the batteries internal impedance will be significant and will drop a couple of volts.

In these high current applications, again a 1k, 20k or 1M ohm meter won’t make a measurable difference. but in very low current, high impedance circuits like the O2 sensor, the impedance of the meter can have a measurable difference. The cheap meter may only be off by a tenth of a volt or even less, but if an absolutely accurate value is important to you, you need the higher impedance.

I don’t think that kind of accuracy is needed in this case and I tried to imply that in my response. If the voltages are in range and sweeping at the correct rate, the sensor is good to go. Who really cares if the voltage reading shows 0.66 volts when the book says “about” 0.7 volts. I do get a little anal myself now and then, but not that anal.

I do have an old x/y crt monitor I could dig out of my junkpile. It would respond fast enough, and is properly calibrated, so I could see the voltage limits of the excursions anyway. That may have to be the plan unless I break down and buy a used o’scope or plotting dvm.

Edit: The 90’s unheated sensors transaction only once per second @keith? hmm … well, I could use my little vom maybe, and just build a little high impedance battery operated interface. I think the needle would respond fast enough for one second transistion, at least fast enough for gov’t work.

Curious, I was expecting there would be something economical available – but less robust than a pro tool of course – available for the DIY’er to test this O2 function, like from a place like Harbor Freight. Maybe this is an opportunity for an inventor.