Acura Coupes and Sedans 1994-2000 Repair Information

Oxygen Sensor

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OPERATION



See Figure 1

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Fig. Fig. 1: Cross-sectional view of a heated oxygen sensor.

An Oxygen (O 2 ) sensor is an input device used by the engine control computer to monitor the amount of oxygen in the exhaust gas stream. The information is used by the computer, along with other inputs, to fine-tune the air/fuel mixture so that the engine can run with the greatest efficiency in all conditions. The O 2 sensor sends this information to the computer in the form of a 100-900 millivolt (mV) reference signal. The signal is actually created by the O 2 sensor itself through chemical interactions between the sensor tip material (zirconium dioxide in almost all cases) and the oxygen levels in the exhaust gas stream and ambient atmosphere gas. At operating temperatures, approximately 1100°F (600°C), the element becomes a semiconductor. Essentially, through the differing levels of oxygen in the exhaust gas stream and in the surrounding atmosphere, the sensor creates a voltage signal that is directly and consistently related to the concentration of oxygen in the exhaust stream. Typically, a higher than normal amount of oxygen in the exhaust stream indicates that not all of the available oxygen was used in the combustion process, because there was not enough fuel (lean condition) present. Inversely, a lower than normal concentration of oxygen in the exhaust stream indicates that a large amount was used in the combustion process, because a larger than necessary amount of fuel was present (rich condition). Thus, the engine control computer can correct the amount of fuel introduced into the combustion chambers.

See Figure 2

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Fig. Fig. 2: A cut-away view of a heated oxygen sensor

Since the control computer uses the O 2 sensor output voltage as an indication of the oxygen concentration, and the oxygen concentration directly affects O 2 sensor output, the signal voltage from the sensor to the computer fluctuates constantly. This fluctuation is caused by the nature of the interaction between the computer and the O 2 sensor, which follows a general pattern: detect, compare, compensate, detect, compare, compensate, etc. This means that when the computer detects a lean signal from the O 2 sensor, it compares the reading with known parameters stored within its memory. It calculates that there is too much oxygen present in the exhaust gases, so it compensates by adding more fuel to the air/fuel mixture. This, in turn, causes the O 2 sensor to send a rich signal to the computer, which, then compares this new signal, and adjusts the air/fuel mixture again. This pattern constantly repeats itself: detect rich, compare, compensate lean, detect lean, compare, compensate rich, etc. Since the O 2 sensor fluctuates between rich and lean, and because the lean limit for sensor output is 100 mV and the rich limit is 900 mV, the proper voltage signal from a normally functioning O 2 sensor consistently fluctuates between 100-300 and 700-900 mV.

The sensor voltage may never quite reach 100 or 900 mV, but it should fluctuate from at least below 300 mV to above 700 mV, and the mid-point of the fluctuations should be centered around 500 mV.

To improve O 2 sensor efficiency, newer O 2 sensors were designed with a built-in heating element, and are called Heated O 2 (HO 2 ) sensors. This heating element was incorporated into the sensor so that the sensor would reach optimal operating temperature quicker, meaning that the O 2 sensor output signal could be used by the engine control computer sooner. Because the sensor reaches optimal temperature quicker, modern vehicles enjoy improved driveability and fuel economy even before the engine reaches normal operating temperature.

On-Board Diagnostics second generation (OBD-II), an updated system based on the former OBD-I, calls for additional O 2 sensors to be used after the catalytic converter, so that catalytic converter efficiency can be measured by the vehicle's engine control computer. The O 2 sensors mounted in the exhaust system after the catalytic converters are not used to affect air/fuel mixture; they are used solely to monitor catalytic converter efficiency.

TESTING



The best, and most accurate method to test the operation of an O 2 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 O 2 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 oxygen 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.

See Figure 3

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Fig. Fig. 3: O2 sensor output voltage vs. mixture ratio

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



Open in the O 2 sensor circuit
 
Constant low voltage in the O 2 sensor circuit
 
Constant high voltage in the O 2 sensor circuit
 
Other fuel system problems could set a O 2 sensor code
 

Because an improperly functioning fuel delivery and/or control system can adversely affect the O2sensor voltage output signal, testing only the O2sensor 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 O 2 sensor. Otherwise, the new O 2 sensor may continue to register the same high or low readings.

Often, by testing the O 2 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 O 2 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.

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.

O2Sensor 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:



Vehicle harness connector - this refers to the connector on the wires which are attached to the vehicle, NOT the connector at the end of the sensor pigtail.
 
Sensor pigtail connector - this refers to the connector attached to the sensor itself.
 
O 2 circuit - this refers to the circuit in a Heated O 2 (HO 2 ) sensor which corresponds to the oxygen-sensing function of the sensor; NOT the heating element circuit.
 
Heating circuit - this refers to the circuit in a HO 2 sensor which is designed to warm the HO 2 sensor quickly to improve driveability.
 
Sensor Output (SOUT) terminal - this is the terminal which corresponds to the O 2 circuit output. This is the terminal that will register the millivolt signals created by the sensor based upon the amount of oxygen in the exhaust gas stream.
 
Sensor Ground (SGND) terminal - when a sensor is so equipped, this refers to the O 2 circuit ground terminal. Many O 2 sensors are not equipped with a ground wire, rather they utilize the exhaust system for the ground circuit.
 
Heating Power (HPWR) terminal - this terminal corresponds to the circuit which provides the O 2 sensor heating circuit with power when the ignition key is turned to the ON or RUN positions.
 
Heating Ground (HGND) terminal - this is the terminal connected to the heating circuit ground wire.
 

See Figure 4



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Fig. Fig. 4: Wiring schematic of typical 1-, 2-, 3- and 4-wire oxygen sensor circuits

1-WIRE SENSOR

1-wire sensors are by far the easiest to determine sensor terminal identification, but this is self-evident. On 1-wire O2 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 O2 sensor and its pigtail connector. It may be necessary to raise and safely support the vehicle to gain access to the connector.
  2.  
  3. Start the engine and allow it to warm up to normal operating temperature, then turn the engine OFF.
  4.  
  5. Using a 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.
  6.  


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.

  1. Have an assistant restart the engine and allow it to idle.
  2.  
  3. Check the DVOM for voltage. OFF.
  4.  
  5. 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.
  6.  
  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 O2 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.  
  9. Have your assistant turn the engine OFF.
  10.  
  11. Label the sensor pigtail SOUT and SGND terminals.
  12.  
  13. Proceed to the test procedures.
  14.  

3-WIRE SENSOR
3-wire sensors are HO2 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 O2 sensor. To identify the 3 terminals, perform the following:

  1. Locate the O2 sensor and its pigtail connector. It may be necessary to raise and safely support the vehicle to gain access to the connector.
  2.  
  3. Disengage the sensor pigtail connector from the vehicle harness connector.
  4.  
  5. Using a DVOM set to read 12 volts, attach the DVOM ground lead to a good engine ground.
  6.  
  7. Have an assistant turn the ignition switch ON without actually starting the engine.
  8.  
  9. 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.  
    3. If none of the terminals showed 12 volts of power, locate and test the heater relay or fuse. Then, perform Steps 3-6 again.
    4.  

  10.  
  11. Start the engine and allow it to warm up to normal operating temperature, then turn the engine OFF.
  12.  
  13. Have your assistant turn the ignition OFF.
  14.  
  15. 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.  
    3. If no continuity was evident, or if continuity was evident from both unidentified terminals, the O2 sensor is defective.
    4.  

  16.  
  17. All 3-wire terminals should now be labeled on the sensor pigtail connector. Proceed with the test procedures
  18.  

4-WIRE SENSOR
4-wire sensors are HO2 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 O2 sensor and its pigtail connector. It may be necessary to raise and safely support the vehicle to gain access to the connector.
  2.  
  3. Disengage the sensor pigtail connector from the vehicle harness connector.
  4.  
  5. Using a DVOM set to read 12 volts, attach the DVOM ground lead to a good engine ground.
  6.  
  7. Have an assistant turn the ignition switch ON without actually starting the engine.
  8.  
  9. 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.  
    3. If none of the terminals showed 12 volts of power, locate and test the heater relay or fuse. Then, perform Steps 2-6 again.
    4.  

  10.  

  1. Have your assistant turn the ignition OFF
  2.  
  3. 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.  
    3. If no continuity was evident, or if continuity was evident from all unidentified terminals, the O2 sensor is defective.
    4.  
    5. 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.
    6.  

  4.  



Reattach the sensor pigtail connector to the vehicle harness connector.
 
Start the engine and allow it to warm up to normal operating temperature, then turn the engine OFF.
 
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.

  1. Have an assistant restart the engine and allow it to idle.
  2.  
  3. 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.  
    3. 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.
    4.  
    5. 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.
    6.  

  4.  

  1. Have your assistant turn the engine OFF.
  2.  
  3. Label the sensor pigtail SOUT and SGND terminals.
  4.  

REMOVAL and INSTALLATION




WARNING
The sensors use a pigtail and connector. This pigtail should not be removed from the sensor. Damage or removal of the pigtail or connector could affect proper operation of the oxygen sensor. Keep the electrical connector and louvered end of the sensor clean and free of grease. NEVER use cleaning solvents of any type on the sensor! The sensor may be difficult to remove when the engine temperature is below 120°F (48°C). Excessive removal force may damage the threads in the exhaust manifold or pipe; follow the removal procedure carefully.

  1. Make sure the ignition is OFF, then disconnect the negative battery cable.
  2.  
  3. Locate the oxygen sensor. It protrudes from the exhaust manifold or exhaust pipe (it looks somewhat like a spark plug). It may be necessary to raise and safely support the vehicle to access the sensor.
  4.  
  5. Unplug the sensor electrical connector.
  6.  

See Figures 5, 6, 7 and 8



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Fig. Fig. 5: Loosen the oxygen sensor using a wrench as shown, or use a sensor socket made especially for that purpose



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Fig. Fig. 6: Remove the oxygen sensor by hand once it has been loosened



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Fig. Fig. 7: Typical oxygen sensor location and mounting



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Fig. Fig. 8: Late model Acuras have two oxygen sensors

There are special wrenches, either socket or open-end available from reputable retail outlets for removing the oxygen sensor. These tools make the job much easier and often prevent unnecessary damage.

  1. Carefully unscrew the sensor, then remove the oxygen sensor from the manifold or pipe.
  2.  

To install:
  1. During and after the removal, be very careful to protect the tip of the sensor if it is to be reused. Do not let it to come in contact with fluids or dirt. Do not clean it or wash it.
  2.  
  3. Apply a coat of anti-seize compound to the bolt threads but DO NOT allow any to get on the tip of the sensor.
  4.  
  5. Install the sensor in the manifold or exhaust pipe.
  6.  
  7. Attach the electrical connector and ensure a clean, tight connection.
  8.  
  9. If raised, carefully lower the vehicle.
  10.  
  11. Connect the negative battery cable.
  12.  

 
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