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DIY: Measuring air/fuel sensor resistance

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Old 10-15-09, 02:12 PM   #1
81corolla
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Default DIY: Measuring air/fuel sensor resistance

For what it is worth here is how to measure the resistance of the air/fuel sensor to see if it's good or not.

Here's my writeup on replacing the sensor.

http://www.clublexus.com/forums/rx-f...el-sensor.html

One can measure the resistance of the front sensor, Bank 2 Sensor 1 without taking it out as it's right there in the front of the engine compartment. The back sensor, Bank 1 Sensor 1 is more difficult, but could also be measured without taking it out of car.

Do this when the engine is cold. Unplug the sensor from its socket. There's a small wire-tie you can undo to create more slack in the sensor wire to move it around.
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Old 10-15-09, 02:13 PM   #2
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The resistance of the air/fuel sensor should be between 0.8 and 1.4 ohms when cold.
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Old 10-15-09, 02:14 PM   #3
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There are two ways to measure this.

First is to simply use an ohmmeter and measure the resistance directly from the air/fuel sensor. The two leads to test are the two at the top. The top is indicated by the little nub that holds the sensor in place. Those two wires are black.
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Old 10-15-09, 02:16 PM   #4
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Although this method works ok to see if the sensor is bad, it doesn't take into the account the resistance of the wires you are testing with. And since the air/fuel sensor has such low readings, the small resistance the wires create can make a difference in an accurate reading or not.

Another method is called the Kelvin method. It cancels out the resistance of the test leads to get a more accurate reading.

Testing in car.
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Old 10-15-09, 02:17 PM   #5
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The gory details.

Devices needed:
  • Two voltmeters with leads (black/red wires in diag.)
  • Two extra wires (yellow/green in diag.) Ones with small alligator clips at end are helpful
  • One AA battery (1.5 V)
The voltmeter should be on the 2 volt scale or 2000 millivolts.

The amp meter should be on the 10 amp scale.

Also note that each meter's red wire is plugged in differently.
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Last edited by 81corolla; 10-15-09 at 02:32 PM.. Reason: Tighter image
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Old 10-15-09, 02:18 PM   #6
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Setup the wires as indicated in the diagram. For the extra wires (yellow/green), put each alligator clip the pins the black sensor wires.

Measure the amps and volts and write the numbers down. The numbers will fluctuate a little as the power is slowly drained from the battery. Do your best to get a simultaneous reading of both meters. Write the numbers down.

The photos below are from the house floor with the sensor out of the car. It's easier to trace the wires this way for illustration purposes.

Please note the meters in these photos are swapped; the ampmeter is on the right not on the left as in the wiring diagram.
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Old 10-15-09, 02:18 PM   #7
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Then repeat the testing without the sensor in the circuit. Connect the green and yellow wires together with the sensor. This will measure the resistance of the leads. Write these numbers down too.
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Old 10-15-09, 02:19 PM   #8
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Code:
Now that you have your data, you can perform the calculations:

Volts
----- = resistance (ohms)
Amps

Sample numbers from photos testing sensor in car:

1017 mV      1.017 V      
--------- =  --------- =  1.094 ohms
0.93 Amps    0.93 Amps

I don't have photos of testing the leads but here are the numbers used:

224.0 mV     0.224 V      
--------- =  --------- =  0.085 ohms
2.63 Amps    2.63 Amps

Finally, subtract the lead value from the sensor value to get the final measurement:

1.094 - 0.085 = 1.009 ohms which falls in range - good sensor!
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Old 10-15-09, 02:20 PM   #9
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There you have it. Salim and others will probably have something to add. This may be a red herring, as you can probably just get away with testing the sensor straight up and you'll know if it's bad or not. Anyway, I learned something.
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Old 10-15-09, 06:04 PM   #10
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Don't forget to put a link to this in the sticky DIY forum at the top of our RX300 forum.
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Old 11-01-09, 10:49 AM   #11
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I am bit lost reading the circuit [I under stand the purpose so read on ..]. My concern is short circuit current which can impact the voltage & current and the sensor is not protected against max current. As long as the voltage is low and the current capability is low things should be fine. Since 81corolla, has performed the tests, it seems the sensor is capable of taking that much current ... but use small size battery.

VOMs provide 'zeroing' capability but not all are equal. Get one which does a good job on low scale. They typically have a bridge circuit and protect the device under test.

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Old 08-01-10, 05:08 AM   #12
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Of course, you could touch the ohmeter leads together, record the reading (0.4 ohms for example), then do the real reading, move the test leads around for the best contact, record it and subtract the earlier reading from the test leads alone. that would get you pretty close to the real number.
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Old 08-01-10, 05:38 AM   #13
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BTW, in post #2, the picture showing the pins to connect to for ohmeter measurement were not correct for my sensor (bank 1 sensor 1, California emissions). (OBDII error codes 1133 and 1135). Maybe they are correct for other sensor locations. The diagram would be correct if the outside plastic spine facing downmost was a single, instead of a double, spine.

Also, when you go to the parts store, they will ask you "is your car California emissions?". You need to know, some are some aren't. Mine was (even though I live in MA). Look at the firewall, and see if you have a red placard saying something like "this car complies with California emissions".
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Old 08-01-10, 05:38 AM
 
 
 
 
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1999, af, air, diy, es300, fuel, kelvin, lexus, ratio, readings, replacing, resistance, sensor, start, test

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