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Testing O2 sensors with a multimeter...


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To test your O2 sensor is operating correctly you can use a multimeter connected to the sensor's wires while the sensor is still connected to the car, the car is warm and the motor is idling.

The voltage should swing up and down around the 450 millivolt figure. It shouldn't go above 900mV and it shouldn't go below 100mV. If everything is in good order it should swing between about 300 and 700mV and it should go from one end of it's range to the other end of it's range several times a minute. Sometimes at idle the fluctuations will be a little slow but if you bring the revs up to about 1400 revs it should fluctuate several times a minute... perhaps eight times.

If it doesn't fluctuate or it's at one extreem there is something wrong.

The easiest way to connect the multimeter is to get a bit of twin-flex cable and solder two pins to one end of the wires, one pin on each wire that is. Get underneath the bonnet or car and poke the pins either through the sensor wires(rough technique), or push them in the back of the sensors connector. Try not to damage the water tight seals on the connector and reseal the wires if you poke the pins through the insulation. If you don't reseal them you may find the check engine light coming on during heavy rain. Now pull the leads out of the gauge and poke the other ends of the wires in to the gauge's lead's holes(after stripping the wires of course).

(Edit).. It's just been pointed out to me that some digital multimeters have a slow "count", which is the rate the gauge refreshes at, or the number of times per second they read the voltage. If you have a cheap digital multimeter you may find that you don't read the voltage swing very well. In this case your reading should be the average of around 450 milivolts.

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Guest boostcut

when we road tuned rotaries at my last workshop we had a good known working o2 sensor and welded a strong clip to it and just bunged it in the tail pipe so thats also a good way, you just have to calculate the voltage drop over the length of wire used.

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Did you have to have a heating feed as well?

I would have thought it would have been too cold at the tail end of the exhaust even with the heating circuit rigged up, which is quite a complicated bit of circuitry in itself.

It sounds quite handy though if it does work properly.

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  • 2 weeks later...

alot of people that have am ecu's dont the heater hooked up, meaning depending on the o2 sensor location they will take longer to heat and then work properly. and the heater needs careful control as if overheated they are easy to damage. also using a narrow band stuck up an exhaust may end up with a false reading as it uses air from the back of the sensor to the differance in the pipe

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  • 4 months later...

so im guessing the test you have stated is for a 2wire O2 sensor?

and that the 3rd wire on a 3wire sensor is the heating circuit?

im trying to "chase" a reason for excessive fuel consumption and i can only put it down to the O2 sensor. plugs leads, oil, tire pressures/alignment are fine. only uses half a litre of oil every 8k so mechanically it seems fine. i should be able to go from hamilton to napier (300k) in less than half a tank. im using nearly 3/4 of a tank and have the usual traveling gear on board. ive done the trip over 30times in various cars in cluding my old rex and new one and the new rex uses the most

turbo was new 40k ago

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 jacsmiley said:

so im guessing the test you have stated is for a 2wire O2 sensor?

and that the 3rd wire on a 3wire sensor is the heating circuit?

im trying to "chase" a reason for excessive fuel consumption and i can only put it down to the O2 sensor. plugs leads, oil, tire pressures/alignment are fine. only uses half a litre of oil every 8k so mechanically it seems fine. i should be able to go from hamilton to napier (300k) in less than half a tank. im using nearly 3/4 of a tank and have the usual traveling gear on board. ive done the trip over 30times in various cars in cluding my old rex and new one and the new rex uses the most

turbo was new 40k ago

hey bro, your sams mate/bro arent you, noticed his plate on your car round hamz, and seen you parked outside my work once or twice, i work at Oil Changers :-)

um on a 3wire 02 sensor the 2 white wires are the heating curcuit, and the 3rd wire (should be black from memory) is the earth wire. thats from my experience fitting new universal 02's anyway. if your after a new one let me know, can do pretty good deals on them.

300kms on 3/4 of a tank sounds pretty bad but its not too bad, i know with mine before i took it off the road, id be lucky to get 450kms on open road driving, and thats not on boost all the time, just occasional passing.

02's are common to crap out though, they only last so long so may be worth replacing anyway, give me a pm if you want a price on a new one.

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