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TESTING

The best, and most accurate method to test the operation of an Oxygen (O2S) sensor is with the use of either an oscilloscope or a Diagnostic Scan Tool (DST), following their specific instructions for testing. It is possible, however, to test whether the O2S sensor is functioning properly within general parameters using a Digital Volt-Ohmmeter (DVOM), also referred to as a Digital Multi-Meter (DMM). Newer DMM's are often designed to perform many advanced diagnostic functions. Some are constructed to be used as an oscilloscope. Two in-vehicle testing procedures, and 1 bench test procedure, will be provided for the common zirconium dioxide O2S sensor. The first in-vehicle test makes use of a standard DVOM with a 10 megohms impedance, whereas the second in-vehicle test presented necessitates the usage of an advanced DMM with MIN/MAX/Average functions. Both of these in-vehicle test procedures are likely to set Diagnostic Trouble Codes (DTC's) in the engine control computer. Therefore, after testing, be sure to clear all DTC's before retesting the sensor, if necessary.

These are some of the common DTC's which may be set during testing:

NOTE: Because an improperly functioning fuel delivery and/or control system can adversely affect the O2S sensor voltage output signal, testing only the O2S sensor is an inaccurate method for diagnosing an engine driveability problem.

If after testing the sensor, the sensor is thought to be defective because of high or low readings, be sure to check that the fuel delivery and engine management system is working properly before condemning the O2S sensor. Otherwise, the new O2S sensor may continue to register the same high or low readings.

Often, by testing the O2S sensor, another problem in the engine control management system can be diagnosed. If the sensor appears to be defective while installed in the vehicle, perform the bench test. If the sensor functions properly during the bench test, chances are that there may be a larger problem in the vehicle's fuel delivery and/or control system.

Many things can cause an O2S sensor to fail, including old age, antifreeze contamination, physical damage, prolonged exposure to overly-rich exhaust gases, and exposure to silicone sealant fumes. Be sure to remedy any such condition prior to installing a new sensor, otherwise the new sensor may be damaged as well.

NOTE: Perform a visual inspection of the sensor. Black sooty deposits may indicate a rich air/fuel mixture, brown deposits may indicate an oil consumption problem, and white gritty deposits may indicate an internal coolant leak. All of these conditions can destroy a new sensor if not corrected before installation.

Oxygen (O2S) Sensor Terminal Identification

The easiest method for determining sensor terminal identification is to use a wiring diagram for the vehicle and engine in question. However, if a wiring diagram is not available there is a method for determining terminal identification. Throughout the testing procedures, the following terms will be used for clarity:

1-WIRE SENSOR

1-wire sensors are by far the easiest to determine sensor terminal identification, but this is self-evident. On 1-wire O2S sensors, the single wire terminal is the SOUT and the exhaust system is used to provide the sensor ground pathway. Proceed to the test procedures.

2-WIRE SENSOR

On 2-wire sensors, one of the connector terminals is the SOUT and the other is the SGND. To determine which one is which, perform the following:

  1. Locate the Oxygen (O2S) sensor and its pigtail connector. It may be necessary to raise and safely support the vehicle to gain access to the connector.
  2. Start the engine and allow it to warm up to normal operating temperature, then turn the engine OFF.
  3. Using a Digital Volt Ohm Meter (DVOM) set to read 100–900 mV (millivolts) DC, backprobe the positive DVOM lead to one of the unidentified terminals and attach the negative lead to a good engine ground.

    CAUTION
    While the engine is running, keep clear of all moving and hot components. Do not wear loose clothing. Otherwise severe personal injury or death may occur.

  4. Have an assistant restart the engine and allow it to idle.
  5. Check the DVOM for voltage.
  6. If no voltage is evident, check your DVOM leads to ensure that they are properly connected to the terminal and engine ground. If still no voltage is evident at the first terminal, move the positive meter lead to backprobe the second terminal.
  7. If voltage is now present, the positive meter lead is attached to the SOUT terminal. The remaining terminal is the SGND terminal. If still no voltage is evident, either the O2S sensor is defective or the meter leads are not making adequate contact with the engine ground and terminal contacts; clean the contacts and retest. If still no voltage is evident, the sensor is defective.
  8. Have your assistant turn the engine OFF.
  9. Label the sensor pigtail SOUT and SGND terminals.
  10. Proceed to the test procedures.
3-WIRE SENSOR

NOTE: 3-wire sensors are HO2S sensors.

On 3-wire sensors, one of the connector terminals is the SOUT, one of the terminals is the HPWR and the other is the HGND. The SGND is achieved through the exhaust system, as with the 1-wire O2S sensor. To identify the 3 terminals, perform the following:

  1. Locate the O2S sensor and its pigtail connector. It may be necessary to raise and safely support the vehicle to gain access to the connector.
  2. Disengage the sensor pigtail connector from the vehicle harness connector.
  3. Using a Digital Volt Ohm Meter (DVOM) set to read 12 volts, attach the DVOM ground lead to a good engine ground.
  4. Have an assistant turn the ignition switch ON without actually starting the engine.
  5. Probe all 3 terminals in the vehicle harness connector. One of the terminals should exhibit 12 volts of power with the ignition key ON; this is the HPWR terminal.
    1. If the HPWR terminal was identified, note which of the sensor harness connector terminals is the HPWR, then match the vehicle harness connector to the sensor pigtail connector. Label the corresponding sensor pigtail connector terminal with HPWR.
    2. If none of the terminals showed 12 volts of power, locate and test the heater relay or fuse. Then, perform Steps 3–6 again.
  6. Start the engine and allow it to warm up to normal operating temperature, then turn the engine OFF.
  7. Have your assistant turn the ignition OFF.
  8. Using the DVOM set to measure resistance (ohms), attach one of the leads to the HPWR terminal of the sensor pigtail connector. Use the other lead to probe the 2 remaining terminals of the sensor pigtail connector, one at a time. The DVOM should show continuity with only one of the remaining unidentified terminals; this is the HGND terminal. The remaining terminal is the SOUT.
    1. If continuity was found with only one of the 2 unidentified terminals, label the HGND and SOUT terminals on the sensor pigtail connector.
    2. If no continuity was evident, or if continuity was evident from both unidentified terminals, the O2S sensor is defective.
  9. All 3-wire terminals should now be labeled on the sensor pigtail connector. Proceed with the test procedures.
4-WIRE SENSOR

NOTE: 4-wire sensors are HO2S sensors.

On 4-wire sensors, one of the connector terminals is the SOUT, one of the terminals is the SGND, one of the terminals is the HPWR and the other is the HGND. To identify the 4 terminals, perform the following:

  1. Locate the O2S sensor and its pigtail connector. It may be necessary to raise and safely support the vehicle to gain access to the connector.
  2. Disengage the sensor pigtail connector from the vehicle harness connector.
  3. Using a Digital Volt Ohm Meter (DVOM) set to read 12 volts, attach the DVOM ground lead to a good engine ground.
  4. Have an assistant turn the ignition switch ON without actually starting the engine.
  5. Probe all 4 terminals in the vehicle harness connector. One of the terminals should exhibit 12 volts of power with the ignition key ON; this is the HPWR terminal.
    1. If the HPWR terminal was identified, note which of the sensor harness connector terminals is the HPWR, then match the vehicle harness connector to the sensor pigtail connector. Label the corresponding sensor pigtail connector terminal with HPWR.
    2. If none of the terminals showed 12 volts of power, locate and test the heater relay or fuse. Then, perform Steps 2–6 again.
  6. Have your assistant turn the ignition OFF.
  7. Using the DVOM set to measure resistance (ohms), attach one of the leads to the HPWR terminal of the sensor pigtail connector. Use the other lead to probe the 3 remaining terminals of the sensor pigtail connector, one at a time. The DVOM should show continuity with only one of the remaining unidentified terminals; this is the HGND terminal.
    1. If continuity was found with only 1 of the 2 unidentified terminals, label the HGND terminal on the sensor pigtail connector.
    2. If no continuity was evident, or if continuity was evident from all unidentified terminals, the O2S sensor is defective.
    3. If continuity was found at 2 of the other terminals, the sensor is probably defective. However, the sensor may not necessarily be defective, because it may have been designed with the 2 ground wires joined inside the sensor in case one of the ground wires is damaged; the other circuit could still function properly. Though, this is highly unlikely. A wiring diagram is necessary in this particular case to know whether the sensor was so designed.
  8. Reattach the sensor pigtail connector to the vehicle harness connector.
  9. Start the engine and allow it to warm up to normal operating temperature, then turn the engine OFF.
  10. Using a DVOM set to read 100–900 mV (millivolts) DC, backprobe the negative DVOM lead to one of the unidentified terminals and the positive lead to the other unidentified terminal.

    CAUTION
    While the engine is running, keep clear of all moving and hot components. Do not wear loose clothing. Otherwise severe personal injury or death may occur.

  11. Have an assistant restart the engine and allow it to idle.
  12. Check the DVOM for voltage.
    1. If no voltage is evident, check your DVOM leads to ensure that they are properly connected to the terminals. If still no voltage is evident at either of the terminals, either the terminals were accidentally marked incorrectly or the sensor is defective.
    2. If voltage is present, but the polarity is reversed (the DVOM will show a negative voltage amount), turn the engine OFF and swap the 2 DVOM leads on the terminals. Start the engine and ensure that the voltage now shows the proper polarity.
    3. If voltage is evident and is the proper polarity, the positive DVOM lead is attached to the SOUT and the negative lead to the SGND terminals.
  13. Have your assistant turn the engine OFF.
  14. Label the sensor pigtail SOUT and SGND terminals.
In-Vehicle Tests

WARNING
Never apply voltage to the O2S circuit of the sensor, otherwise it may be damaged. Also, never connect an ohmmeter (or a DVOM set on the ohm function) to both of the O2S circuit terminals (

SOUT and

SGND) of the sensor pigtail connector; it may damage the sensor.

Test 1 makes use of a standard DVOM with a 10 megohms impedance, whereas Test 2 necessitates the usage of an advanced Digital Multi-Meter (DMM) with MIN/MAX/Average functions or a sliding bar graph function. Both of these in-vehicle test procedures are likely to set Diagnostic Trouble Codes (DTC's) in the engine control computer. Therefore, after testing, be sure to clear all DTC's before retesting the sensor, if necessary. The third in-vehicle test is designed for the use of a scan tool or oscilloscope. The 4th test (Heating Circuit Test) is designed to check the function of the heating circuit in a HO2S sensor.

NOTE: If the O2S sensor being tested is designed to use the exhaust system for the SGND, excessive corrosion between the exhaust and the O2S sensor may affect sensor functioning.

The in-vehicle tests may be performed for O2S sensors located in the exhaust system after the catalytic converter. However, the O2S sensors located behind the catalytic converter will not fluctuate like the sensors mounted before the converter, because the converter, when functioning properly, emits a steady amount of oxygen. If the O2S sensor mounted after the catalytic converter exhibits a fluctuating signal (like other O2S sensors), the catalytic converter is most likely defective.

Fig. 2: To test the O2S sensor, locate it and its connector (inset), which should be positioned away from the exhaust system to prevent heat damage.

TEST 1 - DIGITAL VOLT-OHMMETER

This test will not only verify proper sensor functioning, but is also designed to ensure the engine control computer and associated wiring is functioning properly as well.

  1. Start the engine and allow it to warm up to normal operating temperature.

    NOTE: If you are using the opening of the thermostat to gauge normal operating temperature, be forewarned: a defective thermostat can open too early and prevent the engine from reaching normal operating temperature. This can cause a slightly rich condition in the exhaust, which can throw the O2S sensor readings off slightly.

  2. Turn the ignition switch OFF, then locate the O2S sensor pigtail connector.
  3. Perform a visual inspection of the connector to ensure it is properly engaged and all terminals are straight, tight and free from corrosion or damage.
  4. Disengage the sensor pigtail connector from the vehicle harness connector.
  5. On sensors equipped with a SGND terminal (sensors which do not use the exhaust system for the sensor ground pathway), connect a jumper wire to the SGND terminal and to a good, clean engine ground (preferably the negative terminal of the battery).
  6. Using a Digital Volt Ohm Meter (DVOM) set to read DC voltage, attach the positive lead to the SOUT terminal of the sensor pigtail connector, and the DVOM negative lead to a good engine ground.

    CAUTION
    While the engine is running, keep clear of all moving and hot components. Do not wear loose clothing. Otherwise severe personal injury or death may occur.

  7. Have an assistant start the engine and hold it at approximately 2000 rpm. Wait at least 1 minute before commencing with the test to allow the O2S sensor to sufficiently warm up.
  8. Using a jumper wire, connect the SOUT terminal of the vehicle harness connector to a good engine ground. This will fool the engine control computer into thinking it is receiving a lean signal from the O2S sensor,, therefore, the computer will richen the air/fuel ratio. With the SOUT terminal so grounded, the DVOM should register at least 800 mV, as the control computer adds additional fuel to the air/fuel ratio.
  9. While observing the DVOM, disconnect the vehicle harness connector SOUT jumper wire from the engine ground. Use the jumper wire to apply slightly less than 1 volt to the SOUT terminal of the vehicle harness connector. One method to do this is by grasping and squeezing the end of the jumper between your forefinger and thumb of one hand while touching the positive terminal of the battery post with your other hand. This allows your body to act as a resistor for the battery positive voltage, and fools the engine control computer into thinking it is receiving a rich signal. Or, use a mostly-drained AA battery by connecting the positive terminal of the AA battery to the jumper wire and the negative terminal of the battery to a good engine ground. (Another jumper wire may be necessary to do this.) The computer should lean the air/fuel mixture out. This lean mixture should register as 150 mV or less on the DVOM.
  10. If the DVOM did not register millivoltages as indicated, the problem may be either the sensor, the engine control computer or the associated wiring. Perform the following to determine which is the defective component:
    1. Remove the vehicle harness connector SOUT jumper wire.
    2. While observing the DVOM, artificially enrich the air/fuel charge using propane. The DVOM reading should register higher than normal millivoltages. (Normal voltage for an ideal air/fuel mixture is approximately 450-550 mV DC). Then, lean the air/fuel intake charger by either disconnecting one of the fuel injector wiring harness connectors (to prevent the injector from delivering fuel) or by detaching 1 or 2 vacuum lines (to add additional non-metered air into the engine). The DVOM should now register lower than normal millivoltages. If the DVOM functioned as indicated, the problem lies elsewhere in the fuel delivery and control system. If the DVOM readings were still unresponsive, the O2S sensor is defective; replace the sensor and retest.

      NOTE: Poor wire connections and/or ground circuits may shift a normal O2S sensor's millivoltage readings up into the rich range or down into the lean range. It is a good idea to check the wire condition and continuity before replacing a component that will not fix the problem. A voltage drop test between the sensor case and ground which reveals 14-16 mV or more, indicates a probable bad ground.

  11. Turn the engine OFF, remove the DVOM and all associated jumper wires. Reattach the vehicle harness connector to the sensor pigtail connector. If applicable, reattach the fuel injector wiring connector and/or the vacuum line(s).
  12. Clear any DTC's present in the engine control computer memory, as necessary.
TEST 2 - DIGITAL MULTI-METER

This test method is a more straight-forward O2S sensor test, and does not test the engine control computer's response to the O2S sensor signal. The use of a DMM with the MIN/MAX/Average function or sliding bar graph/wave function is necessary for this test. Don't forget that the O2S sensor mounted after the catalytic converter (if equipped) will not fluctuate like the other O2S sensor(s) will.

  1. Start the engine and allow it to warm up to normal operating temperature.

    NOTE: If you are using the opening of the thermostat to gauge normal operating temperature, be forewarned: a defective thermostat can open too early and prevent the engine from reaching normal operating temperature. This can cause a slightly rich condition in the exhaust, which can throw the O2S sensor readings off slightly.

  2. Turn the ignition switch OFF, then locate the O2S sensor pigtail connector.
  3. Perform a visual inspection of the connector to ensure it is properly engaged and all terminals are straight, tight and free from corrosion or damage.
  4. Backprobe the O2S sensor connector terminals. Attach the DMM positive test lead to the SOUT terminal of the sensor pigtail connector. Attach the negative lead to either the SGND terminal of the sensor pigtail connector (if equipped, refer to the terminal identification procedures earlier in this section for clarification) or to a good, clean engine ground.
  5. Activate the MIN/MAX/Average or sliding bar graph/wave function on the DMM.

    CAUTION
    While the engine is running, keep clear of all moving and hot components. Do not wear loose clothing. Otherwise severe personal injury or death may occur.

  6. Have an assistant start the engine and wait a few minutes before commencing with the test to allow the O2S sensor to sufficiently warm up.
  7. Read the minimum, maximum and average readings exhibited by the O2S sensor or observe the bar graph/wave form. The average reading for a properly functioning O2S sensor is be approximately 450-550 mV DC. The minimum and maximum readings should vary more than 300-600 mV. A typical O2S sensor can fluctuate from as low as 100 mV to as high as 900 mV; if the sensor range of fluctuation is not large enough, the sensor is defective. Also, if the fluctuation range is biased up or down in the scale. For example, if the fluctuation range is 400 mV to 900 mV the sensor is defective, because the readings are pushed up into the rich range (as long as the fuel delivery system is functioning properly). The same goes for a fluctuation range pushed down into the lean range. The mid-point of the fluctuation range should be around 400-500 mV. Finally, if the O2S sensor voltage fluctuates too slowly (usually the voltage wave should oscillate past the mid-way point of 500 mV several times per second) the sensor is defective. (When an O2S sensor fluctuates too slowly, it is referred to as being "lazy.")

    NOTE: Poor wire connections and/or ground circuits may shift a normal O2S sensor's millivoltage readings up into the rich range or down into the lean range. It is a good idea to check the wire condition and continuity before replacing a component that will not fix the problem. A voltage drop test between the sensor case and ground which reveals 14-16 mV or more, indicates a probable bad ground.

  8. Using the propane method, richen the air/fuel mixture and observe the DMM readings. The average O2S sensor output signal voltage should rise into the rich range.
  9. Lean the air/fuel mixture by either disconnecting a fuel injector wiring harness connector or by disconnecting a vacuum line. The O2S sensor average output signal voltage should drop into the lean range.
  10. If the O2S sensor did not react as indicated, the sensor is defective and should be replaced.
  11. Turn the engine OFF, remove the DMM and all associated jumper wires. Reattach the vehicle harness connector to the sensor pigtail connector. If applicable, reattach the fuel injector wiring connector and/or the vacuum line(s).
  12. Clear any DTC's present in the engine control computer memory, as necessary.
TEST 3 - OSCILLOSCOPE

This test is designed for the use of an oscilloscope to test the functioning of an O2S sensor.

NOTE: This test is only applicable for O2S sensors mounted in the exhaust system before the catalytic converter.

  1. Start the engine and allow it to reach normal operating temperature.
  2. Turn the engine OFF, and locate the O2S sensor connector. Backprobe the scope lead to the O2S sensor connector SOUT terminal. Refer to the manufacturer's instructions for more information on attaching the scope to the vehicle.
  3. Turn the scope ON.
  4. Set the oscilloscope amplitude to 200 mV per division, and the time to 1 second per division. Use the 1:1 setting of the probe, and be sure to connect the scope's ground lead to a good, clean engine ground. Set the signal function to automatic or internal triggering.
  5. Start the engine and run it at 2000 rpm.
  6. The oscilloscope should display a wave form, representative of the O2S sensor switching between lean (100-300 mV) and rich (700-900 mV). The sensor should switch between rich and lean, or lean and rich (crossing the mid-point of 500 mV) several times per second. Also, the range of each wave should reach at least above 700 mV and below 300 mV. However, an occasional low peak is acceptable.
  7. Force the air/fuel mixture rich by introducing propane into the engine, then observe the oscilloscope readings. The fluctuating range of the O2S sensor should climb into the rich range.
  8. Lean the air/fuel mixture out by either detaching a vacuum line or by disengaging one of the fuel injector's wiring connectors. Watch the scope readings; the O2S sensor wave form should drop toward the lean range.
  9. If the O2S sensor's wave form does not fluctuate adequately, is not centered around 500 mV during normal engine operation, does not climb toward the rich range when propane is added to the engine, or does not drop toward the lean range when a vacuum hose or fuel injector connector is detached, the sensor is defective.
  10. Reattach the fuel injector connector or vacuum hose.
  11. Disconnect the oscilloscope from the vehicle.
    Fig. 3: An oscilloscope wave form of a typical good O2S sensor as it fluctuates from rich to lean

HEATING CIRCUIT TEST

The heating circuit in an O2S sensor is designed only to heat the sensor quicker than a non-heated sensor. This provides an advantage of increased engine driveability and fuel economy while the engine temperature is still below normal operating temperature, because the fuel management system can enter closed loop operation (more efficient than open loop operation) sooner.

Therefore, if the heating element goes bad, the O2S sensor may still function properly once the sensor warms up to its normal temperature. This will take longer than normal and may cause mild driveability-related problems while the engine has not reached normal operating temperature.

If the heating element is found to be defective, replace the O2S sensor without wasting your time testing the O2S circuit. If necessary, you can perform the O2S circuit test with the new O2S sensor and save yourself some time.

  1. Locate the O2S sensor pigtail connector.
    Fig. 4: The heating circuit of the O2S sensor can be tested with a DMM set to measure resistance

  2. Perform a visual inspection of the connector to ensure it is properly engaged and all terminals are straight, tight and free from corrosion or damage.
  3. Disengage the sensor pigtail connector from the vehicle harness connector.
  4. Using a DVOM set to read resistance (ohms), attach 1 DVOM test lead to the HPWR terminal, and the other lead to the HGND terminal, of the sensor pigtail connector, then observe the resistance readings.
    1. If there is no continuity between the HPWR and HGND terminals, the sensor is defective. Replace it with a new one and retest.
    2. If there is continuity between the 2 terminals, but the resistance is greater than approximately 20 ohms, the sensor is defective. Replace it with a new one and retest.

      NOTE: For the following step, the HO2S sensor should be approximately 75°F (23°C) for the proper resistance values.

    3. If there is continuity between the 2 terminals and it is less than 20 ohms, the sensor is probably not defective. Because of the large diversity of engine control systems used in vehicles today, O2S sensor heating circuit resistance specifications change often. Generally, the amount of resistance an O2S sensor heating circuit should exhibit is between 2-9 ohms. However, some manufacturer's O2S sensors may show resistance as high as 15-20 ohms. As a rule of thumb, 20 ohms of resistance is the upper limit allowable.
  5. Turn the engine OFF, remove the DVOM and all associated jumper wires. Reattach the vehicle harness connector to the sensor pigtail connector.
  6. Clear any DTC's present in the engine control computer memory, as necessary.
Bench Test

NOTE: Utilize one of the in-vehicle tests before performing this test.

This test is designed to test an O2S sensor which does not seem to fluctuate fully beyond 400-700 mV. The sensor is to be secured in a table-mounted vise.

CAUTION
This test can be very dangerous. Take the necessary precautions when working with a propane torch. Ensure that all combustible substances are removed from the work area and have a fire extinguisher ready at all times. Be sure to wear the appropriate protective clothing as well.

  1. Remove the O2S sensor.

    NOTE: Perform a visual inspection of the sensor. Black sooty deposits may indicate a rich air/fuel mixture, brown deposits may indicate an oil consumption problem, and white gritty deposits may indicate an internal coolant leak. All of these conditions can destroy a new sensor if not corrected before installation.

  2. Position the sensor in a vise so that the vise holds the sensor by the hex portion of its case.
  3. Attach 1 lead of a DVOM set to read DC millivoltages to the sensor case and the other lead to the SOUT terminal of the sensor pigtail connector.
  4. Carefully use a propane torch to heat the tip (and ONLY the tip) of the sensor. Once the sensor reaches close to normal operating temperature range, alternately heat the sensor up and allow it to cool down; the sensor output voltage signal should change with the temperature change.

    NOTE: This may also clean a sensor covered with a heavy coat of carbon.

  5. If the sensor voltage does not change with the fluctuation in temperature, replace the sensor with a new one. Install the new sensor and perform one of the in-vehicle tests to rule out additional fuel management system faults.
 
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