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Old 03-12-2003, 07:09 AM   #1 (permalink)
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How to test a 4-wire oxygen sensor?

I have a code in my '93 XJ 4.0 that is oxygen sensor related. The actual code is 21 and here is the best description of what it means that I've found:

21** Neither rich nor lean condition detected from the oxygen sensor input.
or
Oxygen sensor input voltage maintained above the normal operating range.

My first thought is the O2 sensor is bad but I just replaced it four months ago with a new Bosch unit. Before I go out and spend another $50 on another one how can I test this one? I took it out last night and hooked it up to a multimeter while it was hot. I could only detect resistance across two of the terminals and I tried all the possible combinations between the four. I then re-installed it and tested all four terminals with the red lead while the balck lead was grounded. I never detected any voltage output. It seems to me the sensor is bad but did I check it right? Anyone out there know anything about this code or how to correctly check an oxygen sensor?
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Old 05-13-2003, 03:45 PM   #2 (permalink)
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Wes, did you figure it out? I'd like to know as well.
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Old 05-17-2003, 09:38 AM   #3 (permalink)
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Here you go...

Testing Sensors:

First, a note on multi-meters: Testing should be done with a good quality digital multi-meter (DMM). Analog meters impose too much impedance on the circuits and lead to false readings.

In order for the sensor to function, it must be above 600 degrees. For this reason, there is a heater built into most O2 sensors to speed up the heating process. That is why the sensor has four wires. Two are for the ground (gray) and signal (black) and the other two are for the heater (white).

On Vehicle Test

Plug the DMM into the sensorís wiring. This is probably easiest to do at the sensor connector, but may be possible at the computer. Testing at the computer includes all the wiring too. Set the DMM to read DC voltages below 1 volt. Start the engine and wait for it to warm up. You may have to rev the engine a bit to speed this up. The O2 sensor will indicate no voltage when it is cold. It warms up in 20 seconds and will start to indicate a voltage between 0 and 1 volt. 0v is a lean condition and 1 is rich. A value of .45 is optimum. After the engine has reached normal operating temperature, the voltage value should fluctuate around the .45V value. It should respond instantly to changes in the mixture. This can be tested by first removing the vacuum line from the fuel pressure regulator and watching for the voltage to approach 1V. With the vacuum hose reattached, disconnect the EGR valve from its hose. This should make the sensor read lean and well below .45V. If the sensor passes these checks, it is probably good. If this does not produce the required results, check the wiring for continuity. If that checks out OK, remove the sensor from the vehicle and run a bench test on it.

O2 Sensor Bench Test

Remove the sensor from the vehicle and attach a DMM to the Grey and Black leads. Heat the sensor with a propane torch. Within 20 seconds the sensor should produce a voltage when in the flame. Now pull it from the flame. The voltage should drop off to near zero within a second. Now put it back into the flame and check that the voltage climbs rapidly. If it passes so far, let it set in the flame for several minutes. If its reading stays consistent, its good. If it fails these bench tests, its time for a new one.

O2 sensor replacement

Before you replace this sensor, try to determine the cause of failure. In most cases contamination kills the thing. Non-sensor safe RTV, coolant and excessive carbon quickly kill these things. For best results use the exact replacement part. These are more expensive than the generic Bosch or Borg/Warner, but they are more accurate.

-Old Army

Last edited by Old Army; 05-17-2003 at 09:42 AM.
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