GM Cadillac Deville_Fleetwood_ELD_Seville 1990-1998

Oxygen Sensor

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OPERATION



The oxygen (O2) sensor is a device that produces an electrical voltage when exposed to the oxygen present in the exhaust gases. The sensor is mounted in the exhaust system, usually in the manifold or a boss located on the down pipe before the catalyst. Most of the oxygen sensors used on the sophisticated systems of today are heated internally for faster reaction when the engine is started cold. The oxygen sensor produces a voltage between zero and one volt. When there is a large amount of oxygen present (lean mixture), the sensor produces a low voltage (less than 0.4v). When there is a lesser amount present (rich mixture) it produces a higher voltage (0.6 -1.0v). The stoichiometric or correct air to fuel ratio will cause the voltage to fluctuate between 0.4 and 0.6v. By monitoring the oxygen content and converting it to electrical voltage, the sensor acts as a rich-lean switch. The voltage is transmitted to the PCM.

Some models have two or more sensors, before the catalyst and after. This is done for a catalyst efficiency monitor that is a part of the OBD-II engine controls that are on all models from the 1996 model year on. The sensor before the catalyst measures the exhaust emissions right out of the engine, and sends the signal to the PCM about the state of the mixture as previously talked about. The second sensor reports the difference in the emissions after the exhaust gases have gone through the catalyst. This sensor reports to the PCM the amount of emissions reduction the catalyst is performing.

The oxygen sensor will not work until a predetermined temperature is reached, until this time the PCM is running in OPEN LOOP operation. OPEN LOOP means that the PCM has not yet begun to correct the air-to-fuel ratio by reading the oxygen sensor. After the engine comes to operating temperature, the PCM will monitor the oxygen sensor and correct the air/fuel ratio from the readings of the sensor. This is known as CLOSED LOOP operation.

A heated oxygen sensor (O2 S) has a heating element that keeps the sensor at proper operating temperature during all operating modes. Maintaining correct sensor temperature at all times allows the system to enter CLOSED LOOP operation sooner.

In CLOSED LOOP operation the PCM monitors the sensor input (along with other inputs) and adjusts the injector pulse width accordingly. During OPEN LOOP operation, the PCM ignores the sensor input and adjusts the injector pulse to a preprogrammed value based on other inputs.



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Fig. Oxygen sensor and wiring lead

TESTING



Single Wire Sensor

This type oxygen sensor is found on the earlier model vehicles. Later years went to a heated O2 sensor because it would react quicker, allowing the PCM to switch into closed loop operation faster.

  1. Visually inspect the pigtail for proper routing and connection.
  2.  
  3. Check for an adequate air supply, and a clean unclogged air filter.
  4.  
  5. Poor PCM to engine block grounds.
  6.  
  7. Fuel Injectors; faulty or sticky fuel injectors can cause a false reading or false DTC indicating an O2 sensor problem.
  8.  
  9. Fuel pressure; the system will go lean if pressure is too low. The PCM can compensate for some decrease. Likewise, the system will go rich if pressure is too high. The PCM can compensate for some increase. However, if fuel pressure is not in spec, a DTC may be set.
  10.  
  11. Vacuum leaks. Check for disconnected or damaged vacuum hoses and for vacuum leaks at the intake manifold, throttle body, EGR system, and crankcase ventilation system.
  12.  
  13. Exhaust leaks. An exhaust leak may cause outside air to enter the exhaust gas stream going past the O2 sensor, causing the system to appear lean. Check for exhaust leaks that may cause a false lean condition.
  14.  
  15. Fuel contamination; Water, even in small amounts, can be delivered to the fuel injectors. The water passing through the system can cause a lean exhaust. Excessive alcohol in the fuel can also cause this condition.
  16.  
  17. Check the EVAP canister for fuel saturation. If the canister is full of fuel, check the canister control and hoses.
  18.  
  19. Check for a leaking fuel pressure regulator diaphragm by checking the vacuum line to the regulator for the presence of fuel.
  20.  
  21. An intermittent TP sensor output will cause the system to go rich due to a false indication of engine acceleration.
  22.  
  23. A faulty Manifold Absolute Pressure (MAP) Sensor can affect O2 operation.
  24.  
  25. Connect a DVOM between the signal wire and ground. With the engine at normal operating temperature and the engine running (1200 to 1500) RPM, the O2 sensor should be generating a fluctuating signal between zero and 1 volt. If the reading stays above or below .5 volts and does not fluctuate, the sensor could be bad. If the sensor is slow in switching, or acts lazy in changing its voltage reading, it could be bad.
  26.  
  27. If the sensor is operating within specifications, check the circuits to the PCM for continuity.
  28.  
  29. If the sensor and circuits are functional, the PCM may be bad.
  30.  

Heated Oxygen Sensor
  1. Visually check the connector. Make sure it is clean tight and secure. The sensor pigtail may be routed incorrectly and contacting the exhaust system.
  2.  
  3. Poor PCM to engine block grounds.
  4.  
  5. Fuel Injectors; faulty or sticky fuel injectors can cause a false reading or false DTC indicating an O2 sensor problem.
  6.  
  7. Fuel pressure; the system will go lean if pressure is too low. The PCM can compensate for some decrease. Likewise, the system will go rich if pressure is too high. The PCM can compensate for some increase. However, if fuel pressure is not in spec, a DTC may be set.
  8.  
  9. Vacuum leaks. Check for disconnected or damaged vacuum hoses and for vacuum leaks at the intake manifold, throttle body, EGR system, and crankcase ventilation system.
  10.  
  11. Exhaust leaks. An exhaust leak may cause outside air to enter the exhaust gas stream going past the O2 sensor, causing the system to appear lean. Check for exhaust leaks that may cause a false lean condition.
  12.  
  13. MAF sensor; disconnect the MAF sensor and see if the condition is corrected. A faulty MAF sensor can give a false reading (lean or rich) to the O2 sensor.
  14.  
  15. Fuel contamination; Water, even in small amounts, can be delivered to the fuel injectors. The water passing through the system can cause a lean exhaust. Excessive alcohol in the fuel can also cause this condition.
  16.  
  17. Check the EVAP canister for fuel saturation. If the canister is full of fuel, check the canister control and hoses.
  18.  
  19. Check for a leaking fuel pressure regulator diaphragm by checking the vacuum line to the regulator for the presence of fuel.
  20.  
  21. An intermittent TP sensor output will cause the system to go rich due to a false indication of engine acceleration.
  22.  
  23. An internally shorted Heated Oxygen Sensor will indicate voltage output of over 1 volt.
  24.  
  25. If the sensor and circuits are functioning properly, the PCM may be faulty.
  26.  



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Fig. Typical Heated Oxygen Sensor (HO2S) schematic -4 wire

REMOVAL & INSTALLATION



The oxygen sensor uses a permanently attached pigtail and connector. This pigtail should not be removed from the oxygen sensor. Damage or removal of the pigtail connector could effect the proper operation of the oxygen sensor. Use caution when handling the oxygen sensor. The in-line electrical connector and louvered end must be kept free of grease, dirt, or other contaminants. Also, avoid using cleaner solvent of any type. Do not drop or roughly handle the oxygen sensor. It is not recommended to clean or wire brush an oxygen sensor when in question, it is recommended that a new oxygen sensor be used.


NOTE
The oxygen sensor may be difficult to remove when the engine temperature is below 120°F (48°C). Excessive force may damage the threads in the exhaust manifold of exhaust pipe.

  1. Start the engine and let it warm up to 120°F (48°C), stop the engine and disconnect the negative battery cable.
  2.  
  3. Detach the electrical connector from the oxygen sensor.
  4.  
  5. Using a special, oxygen sensor socket, remove the oxygen sensor from the exhaust manifold or exhaust pipe.
  6.  


NOTE
A special anti-seize compound is used on the oxygen sensor threads. The compound consists of liquid graphite and glass beads. The graphite will burn away, but the glass beads will remain, making the sensor easier to remove. New or service sensors will already have the compound applied to the threads. If an oxygen sensor is removed from the engine and if for any reason, it is to be reinstalled, the threads must have this anti-seize compound applied before installation.

  1. Coat the threads of the oxygen sensor with anti-seize compound 5613695 or equivalent.
  2.  
  3. Install the sensor and tighten it to 30 ft. lbs. (40 Nm).
  4.  
  5. Reattach the electrical connector to the sensor and the negative battery cable.
  6.  



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Fig. Location of the rear O2 sensor



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Fig. Location of the the front O2 sensor



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Fig. Use a wrench or special socket to loosen the O2 sensor



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Fig. When replacing or reinstalling an oxygen sensor, be sure to lube the threads sufficiently

 
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