Monitoring Procedure

Primary detection is based upon;

1. Downstream O2 sensor high frequency amplitude.
2. The response delay time between upstream and downstream change.

   A calculation of the average value of the downstream O2 sensor high frequency amplitude is made; this is normal if the value falls below a given threshold. Should a failure judgement occur, the main feedback frequency will be adjusted and the response delay time from the upstream to downstream feedback signal monitored. If the response delay time is greater than the threshold then a normal judgement will be made, less will incur a failure judgement.

   Oxygen Sensor (O2) 

   Fig 1: Identifying Locating Of Oxygen Sensor (O2

   Courtesy of JAGUAR CARS, INC.

   Operation 

   Fig 2: Identifying Oxygen Sensor

   Courtesy of JAGUAR CARS, INC.

   The upstream heated oxygen sensor is located at the catalytic converter inlet and detects the concentration of oxygen in the raw exhaust gases; an internal heater reduces the warm up time of the sensor output.

   An oxygen sensor (without heater) is located at the converter outlet.

   The ECM receives input signals from the sensors and varies the fuel injector duration (mixture) to provide optimum gas emissions.

   Monitoring Procedure 

   The sensor outputs are monitored during steady driving with a fully warm engine.

   If the downstream sensors indicate lean Air / Fuel Ratio (AFR) and the fuel system has judged that the fueling is rich, then the upstream sensor is judged to have failed high voltage.

   The upstream sensor is judged to have failed low voltage by comparison of its output with that of the downstream sensor.

   Upstream slow response judgements are made when any of the following switching rates remain above a threshold: low to high switch time from one mid point of the switching cycle to the next mid point.

   Downstream sensor slow, high and no activity judgements are similar to upstream but the mapped values are different. If no activity is seen the AFR is enriched to force a response, and a failure judgement is made should this be unsuccessful.

   Downstream high and low voltage judgements are made after the sensor has remained above or below predetermined thresholds for a long period.

   Heater circuit judgements are made by comparing the expected heater drive state with the actual state. If these states are different for too long then the heater circuits are judged to be faulty.

   For all the above diagnostics, the relevant DTC is stored if the failure judgement is made on two successive trips.

   Fig 3: Circuit Diagram, O2 Sensors/ECM

   Courtesy of JAGUAR CARS, INC.

   INSPECTION AND VERIFICATION 

   Vehicle Harness Check 

3. Check for open and short circuit in harness and connector faults between the O2 sensor and ECM.
4. Check for approximately 12V between heater harness

   Heater Resistance Check 

5. Verify that the resistance between the heater terminals is between 4OHM and 10OHM at -20 °C to 100°C.

   Performance Check 

6. Run the engine at 2500 RPM for two minutes, to heat up the O2 sensor.
7. Check the voltage of the sensor output: Alternates between less than 0.4V at feed back engine conditions and in excess of 0.5V, see graph.
   Fig 4: Sensor Output Voltage Graph

   Courtesy of JAGUAR CARS, INC.

   These voltages are for reference only.

8. Check that cycle of the upstream sensor output is in accordance with the appropriate pinpoint test.
9. Check that cycle of the downstream sensor output is in accordance with the appropriate pinpoint test.