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Digital vs Analog voltmeter for O2 sensors

My daughter has an 03 Corolla that is giving a P0420 code. When I reset the code, it can be a week or more before it comes back on. I checked the O2 sensors using a analog voltmeter. The front sensor switched between low and high (.2-.9v) at a normal rate. The rear would measure a steady high state at idle, but drop to a low rate anytime it went off idle, but it did not switch when off idle. As soon as I returned to idle, it would switch to the high state.



I did a little internet searching and all sites recommended that the O2 sensor should be checked with a digital voltmeter, not an analog one. I do have a digital voltmeter, but its an old one. The last time I needed to check an oxygen sensor, the DVM was not recommended because the display tended to respond too slowly.



I know that most analog meters today do load circuits more than they used to. I see models with loading as low as 1000 ohms/volt. Mine is rated at 20,000 ohms/volt DC so I don’t think it would load the O2 circuit too much.



Should I repeat the test with the digital voltmeter. Do the newer DVMs have a faster response rate? I have never seen a response rate in the specs of a DVM but LCDs have certainly improved since I got this meter some 20+ years ago.

My text says the tests you can do with a DMM (does not even mention trying it with an analog meter) are limited. That is, you can’t make a conclusion if the switching rate was happening at the correct rate, just that it happened. To observe the rate of switching, prefered is a lab scope but valid conclusions about a lazy sensor can be made with a scanner.

One feature the DMM does need is min/max.

As you know, the O2 sensor is used not really for absolute voltage level but in a switched mode where the ECM changes the fuel/air mixture and checks the O2 sensor to gauge its reaction to the input change. The output voltage typically swings from one extreme to the other as the sensor reacts to changes in the oxygen content in the exhaust. Often, this is referred to as switch counts and is more of a frequency measurement than a voltage measurement. That’s not to say the voltage level is unimportant because the voltage must exceed certain trigger voltage levels within a set period of time for the ECM to count it as a min or max value and the transition to be valid.

The impedance of your measuring device meeds to be sufficiently high enough so that is does not present a parasitic load on the circuit. Cheap analog meters are the worst offenders as you have found out. Most older analog meters and almost all digital meters will have a high enough input impedance to be useful in measuring these circuits.

A lazy O2 sensor reacts slowly to actual changes in the oxygen contents, usually caused by combustion byproducts coating the element. This parameter is the change in voltage over the change in time (dV/dt) or slew rate and is an important factor to assess. Analog meters have trouble in this regard due to the mass of the indicating mechanism. Its rate of change affects the ability of the meter to hit the maximum and minimum voltage levels. The faster the input signal changes and the larger the change in voltage, the more this damping effect comes into play. It basically performs an electro-mechanical averaging of the pulsed input signal. The rate of change the analog meter is capable of achieving will affect your ability to gauge not only the absolute voltage levels but also the slope of the O2 sensor rate of change.

A digital meter does not have this limitation but it does have trouble with indicating to the user at switch rates that exceed the ability of the display to change and also the user’s ability to see the change. More expensive meters will have functions to display Min and Max values as noted by oldschool. As you move up the food chain in cost, you can get other features like frequency measurement and even oscilloscope like displays of the input signal.

Of course, the ultimate tool is an oscilloscope because it presents very little load on the circuit, is very fast response time, can display absolute voltage levels and the slope of the sensor’s rate of change in real time. You can save a bunch of money buying one of these used from equipment resellers.

As you have found, you can get by with simple meters if you understand how the circuit works, what’s important to the operation and the limitations of your measuring tools. Using both the DC and AC measuring functions of a simple meter can help find most issues with these sensors but not all of them…

Good info from the previous posters. In addition to what they stated you can get DVMs that have a fast response bar graph on the bottom of the display which responds faster than the numerical display. Meters from Fluke are pretty reasonable and it sounds like you should look into a new model since yours is fairly old though still good. You are most likely already aware that Fluke makes some of the best meters on the market so just about any model from them would be a nice tool asset to have, and Christmas is just around the corner.

Going from your voltage readings it sounds like the analog meter wasn’t causing much of a voltage error. The readings you got should be close to what a high impedence meter would indicate also. There are different kinds of meters available. At work I had one that was sort of a hybrid. It was an fast responding analog meter but it had a 10 megohm input impedence and was also auto ranging. I’m not sure if Simpson still makes that unit or not, but I though it was a great design when working with fast changing voltages.

I guess I wasn’t real clear. I have a DVM as well as an analog, both are older models that I bought in the 90’s. I had always used the analog for checking O2 sensors on vehicals from that era because DVMs were not known to react fast enough.

I think the analog meter is probably as good as you can get today as all the high quality meters have gone digital. I haven’t had an O2 sensor give me problems in quite awhile so when I did some reading to update myself, I found that everyone recommends using a DVM these days.

Might be time to get me a new DVM. Its close to Christmas as you pointed out.

I am a little concerned about the rate of switching of the down stream sensor. I expected it to switch at a much slower rate than the upstream one, but this one only switched when going from idle and back.

If the sensor has over 100K miles on it, just replace it!

Sorry I don’t understand the question, vtvm vacuum tube volt meter vs tvm, transistorised volt meter?

My check engine light concerning a lean bank came back on in my 2004 F-150. Seems like I will have to change an 02 sensor. First actual repair (not maintence) in the trucks 90K life.

From what I understand the rear sensor should run at a pretty steady rate (say between .6 and .8 volts) when things are up to normal operating temperatures. There may be a small air leak somewhere causing it to go low in voltage when the engine is reved.

Wow, you go waaaay back.

Yes I do, but the bright side is I get to meet brand new people every day!

You can’t measure the crosscounts of an O2 sensor with a digital or analog VOM with the engine running. These meters don’t have the resolution to measure the crosscounts that quickly. It requires a scanner that can monitor the crosscounts or an oscilloscope.

You can test an O2 sensor out of rhe vehicle. Place the sensor in a vice. Connect a DVOM meter, + lead to output signal and - to the sensor body, and set the meter to read 2 VDC. Take a propane torch and heat the sensor probe. This removes the oxygen from the sensor. The voltage should climb steadily to close to one volt DC. Now remove the flame. The voltage should drop steadily to near zero volts DC. If the voltage acts erratic while increasing or decreasing the sensor is bad.

Tester