Oxygen Sensor Monitoring
The oxygen sensors are monitored for:
- Operational readiness.
- Connectivity.
- Short circuit to positive.
- Short circuit to ECM ground.
- Wire break.
- Aging of oxygen sensors via the adjustment frequency and adjustment position.
Function of the Oxygen Sensor Diagnosis
To diagnose the oxygen sensor signals, the Engine Control Module (ECM) uses the form and frequency of the respective signal as a measuring value.
- Time of sensor voltage between positive and negative limits
- Maximum recognized sensor voltage value
- Minimum recognized sensor voltage value
- Oxygen sensor value towards rich
- Oxygen sensor value towards lean
- Sensing limits of oxygen sensing
- Period duration of sensor ahead of the three-way catalytic converter
- Sensor voltage ahead of three-way catalytic converter
Determination of Maximal and Minimal Sensor Voltage
After starting the engine, the values for the previous maximum and minimum are erased. During driving, the absolute maximum and minimum is established, based on for the diagnosis predetermined engine speed and load range. These values are recorded by the ECM. If the sensor voltage is higher than the presently stored maximum value, it will be overridden by the current sensor voltage. The same applies to the minimal values.
Calculation of the Time Between Positive and Negative Limits:
If the sensor voltage exceeds the sensor limits, the time measurement is started between the positive and negative limits. If the sensor voltage is lower than the limit range, the time counter is stopped. The time between starting and stopping of the counter is measured.
Example: Illustration of the affect of the duration of the period regarding emission.:
Monitoring (sensor aging)
A contamination or aging of the oxygen sensor affects the sensor voltage or duration of the sensor signal and with that has a negative influence on emission values.
Monitoring (Oxygen sensor ahead of TWC)
Sensor readiness
The sensor voltage has to have passed 400 mV to 600 mV, 200 seconds after starting the engine, and may not be above 1000 mV and not below -150 mV. During this period no relevant blocking conditions may exist.
Short circuit (signal wire to signal ground)
For a cold sensor the voltage must be at 0 V +/- 40 mV. If the sensor is at operating temperature and the sensor voltage after the TWC is >350 mV, the sensor voltage ahead of the TWC must not be longer than 8 seconds between 60 mV and 400 mV.
Wire break
After starting the engine and 200 seconds of sensor heating, the sensor voltage may not stay longer than 5 seconds between 400 mV to 600 mV.
Short circuit to plus
When the heater of the sensor was exited for more than 200 seconds, the sensor voltage may not exceed 1000 mV.
Monitoring (Oxygen sensor after TWC)
Sensor readiness
The sensor voltage has to have passed 400 mV to 600 mV, 200 seconds after starting the engine, and may not be above 1000 mV and not under -150 mV
Short circuit (signal wire to signal ground)
When the oxygen sensor system is activated and the sensor heating was triggered for 200 seconds and if no interference exists such as secondary air, tank ventilation or fault conditions, the sensor voltage must not be longer than 200 seconds between -40 mV and + 60 mV.
Wire break
After starting the engine and 200 seconds of sensor heating, the sensor voltage must have passed through 400 mV to 500 mV within 400 seconds.
Short circuit to plus
When the heater of the sensor was exited for more than 200 seconds, the sensor voltage may not be over 1000 mV.
Short circuit to ground
When the heater of the sensor was exited for more than 200 seconds, the sensor voltage must not be below -150 mV. The cold sensor voltage ahead of the TWC must not be between -360 mV and -160 mV.
Oxygen Sensor Heating
For each oxygen sensor pair ahead of the TWC and for each sensor pair after the TWC, a resistor (shunt) is provided in the ECM.
The monitoring of the heating is accomplished through measuring the sensor heating voltage (depending on the voltage drop at the shunt) and the voltage for the sensor heating. The ECM uses these values to calculate the heating resistance. The resistance must be between 2.5 and 10 ohms.
For the right and left recognition it is checked once with a cold engine, whether the sensor readiness of a sensor pair is within a certain time span. If the left sensor is operationally ready much later than the right sensor, we can conclude that the left sensor has a heating malfunction.