Heated Oxygen Sensor Monitor
Heated Oxygen Sensor Monitor
An O2S comprises of a gas-tight zirconium dioxide ceramic tube covered with thin layer of platinum. One end of the tube is open to atmosphere; the other end is sealed and protrudes into the exhaust. When the tube is filled with oxygen rich atmospheric air, and the outer walls are exposed to the oxygen depleted exhaust gases, a chemical reaction takes place and produces a voltage. The voltage output reflects the differences in oxygen concentrations on either side of the ceramic sensor element. As the oxygen content decreases, the voltage increases. As the oxygen content increases, the voltage decreases.
The oxygen content of the exhaust gas stream is directly related to the air fuel mixture supplied to the engine. The voltage output by the O2S is typically 800 to 1000mV for rich mixtures, and around 100mV for lean mixtures.
The ceramic material in the sensor becomes sensitive to the presence of oxygen in the exhaust gas stream at around 315 degree C. An internal heater is used to bring the sensor quickly up to the operating temperature.
The engine management system runs two tests on the upstream and downstream HO2S, one on the sensor operation and one on the sensor's internal heater.
NOTE: Only the rear HO2S are used for fuel control.
Downstream Oxygen Sensors High/Low Input Monitor
The downstream O2S are checked for their maximum and minimum output values. The monitor increments an execution timer if the monitor entry conditions are satisfied. A low voltage failure is judged if the output of the sensor does not exceed a calibrated value prior to the monitor execution timer exceeding its calibrated failure threshold. A high voltage failure is judged if the sensor output remains above a calibrated value after the monitor execution timer has exceeded its calibrated failure threshold or after a defined period of over run fuel cut off has been conducted. Additionally, a high voltage failure is invoked if the sensor voltage exceeds battery short threshold for the required time.
Downstream Oxygen Sensors Heater Circuit High
Heater resistance checks are performed when the heater is commanded on. If resistance values are outside of the limits when the heater is enabled, then a failure judgment is made.
Downstream Oxygen Sensors Heater Circuit Low
Heater resistance checks are performed when the heater is commanded off. If resistance values are outside of the limits, then a failure is flagged.
Downstream Oxygen Sensors No Activity Detected
The monitor is single shot monitor (runs once per trip), which is designed to operate only when the sensor has been lit off (up to operating temperature). The monitor can be sub divided into two sections:
Stuck low
(Output voltage less than calibrated threshold (0.4 volts).
The monitor initially examines the fuelling control to ensure the system is stable, that linear airflow rate closed loop control, and sub feedback execution has been invoked. Once these conditions are satisfied and a calibrated load/airflow has been achieved, a lean stuck timer is incremented. The monitor then checks the output voltage from the sensor and sets a normal end judgment if a calibrated change in sensor output voltage is observed. If the change in sensor voltage is not detected and the lean stuck timer exceeds the failure threshold, and the associated failure conditions are satisfied, then a failure end judgment is made.
Stuck high
(Output voltage greater than calibrated threshold (0.4 volts).
Again, the monitor strategy checks for stable air fuel ratio control prior to commencing the examination of the sensors output voltage. The monitor then utilizes the lean switching characteristics of the sensor during an over run fuel cut off (where the sensors output voltage tends towards 0 volts), to determine its correct operation. Finally, if the duration of the fuel cut off exceeds a calibrated period and the output voltage of the sensor is greater than calibrated threshold, then a failure judgment is set.
Upstream Oxygen Sensors Circuit
This monitors the upstream O2S element current. If the current is above or below a calibrated value, and the stable operating conditions are satisfied, a failure timer is incremented, otherwise a normal timer is incremented. Upon exceeding the calibrated thresholds for either the failure/normal timers, an appropriate failure/normal end judgment is set.
Upstream Oxygen Sensors Slow Response
The failure criteria for this monitor is the measurement of the time taken for the upstream sensor to attain a calibrated air fuel ratio reading following fuel reinstatement after an over run fuel cut off. The slow response monitor measures the response time of the sensor to react when the air fuel ratio changes from a known lean state to a known non-lean state. The monitor operates after fuelling has been reinstated and the engine management system is in ISC mode, following a period of fuel cut off. If all execution conditions are satisfied the monitor increments a response timer, if the timer exceeds a failure threshold prior to the sensor current switching back to a non-lean condition (6.97mA) a failure end judgment flag is set. If the current signal passes through the lean limit prior to the timer exceeding the failure threshold, then a normal end judgment is set. It should be noted that the slow response monitor is a single shot monitor, which only executes once per drive cycle.
Fuel Cut Off Operation
A timer is employed to ensure that a minimum period of fuel cut off is achieved prior to executing the monitor. This allows the sensors to respond to the lean air fuel ratio fuelling shift, which occurs during the period of fuel cut off.
Upstream Oxygen Sensors Heater Circuit
The control module monitors the heater current to be within limits. If a failure is detected, the control module responds by setting the appropriate signal failure code. On detection of a failure code the monitor proceeds to increment a failure timer and a judgment is made if the failure timer exceeds a calibrated threshold. If a failure code is not present, then the monitor increments a normal judgment timer and sets a judgment upon exceeding a calibrated threshold.
Control Module
The control function within the ECM enables hardware checks to be performed on the sensors. These DTCs will reflect sensor open circuit and short circuit faults along with heater faults. In addition to sensor fault monitoring these DTCs will also reflect failures of the control functions themselves. The sensor impedance is also monitored to ensure that its impedance is below the required level for correct operation after the sensor has been active for the required time.
