Heated Oxygen Sensor Monitor
DESCRIPTION
OBD II regulations require monitoring of upstream Heated Oxygen Sensors (HO2Ss) to detect when deterioration of the sensor has exceeded emission thresholds. Two additional HO2S(s) are located downstream to determine the efficiency of the catalyst. Although the downstream HO2S(s) are the same type used for fuel control, they function differently. They are monitored to determine if a voltage is generated. That voltage is compared to a calibrated acceptable range.
Hardware
The HO2S Monitor/Fuel Control System consists of HO2S(s), Mass Air Flow (MAF) sensor, Intake Air Temperature (IAT) sensor, Engine Coolant Temperature (ECT) sensor, Profile Ignition Pickup (PIP) rpm and the injectors. The function of the fuel control system is to adjust the quantity of fuel passing through the injectors and maintain an air/fuel ratio of 14.7:1.
Operation
The PCM uses input from upstream HO2S to fine tune the air/fuel ratio. The HO2Ss are mounted in the exhaust flow. The sensors operate between zero and one volt output depending on the oxygen in the exhaust mixture. Each sensor acts as a voltage source and series resistance. Lean of stoichiometric (14.7:1) air/fuel ratio, the sensor will output a voltage between zero and 0.40 volt. Rich of stoichiometric, the sensor will output voltage between 0.60 and 1.0 volt at a HO2S top temperature of 350°C (662°F).
Upstream HO2S Testing:
HO2S Testing:
The test of the upstream sensors is enabled when entry conditions have been met. The time between HO2S signal switches is continuously monitored during vehicle operation. Excessive time between switches indicates a faulty sensor.
A special closed loop high frequency fuel control procedure tests the response rate of the HO2S. The procedure will vary the fuel in order to generate a change around stoichiometry at a known frequency. The frequency is high enough that a slow response sensor will not be able to keep up and will exhibit a reduced amplitude. The signal is also monitored for over-voltage.
The peak voltage and total amplitude of the signal from the HO2S are measured. Signal voltage that exceeds a maximum threshold or signal amplitude below a minimum threshold indicates a faulty sensor response.
In order to reduce driveability concerns during the high frequency fuel test, the fuel to the opposite bank is varied in the opposite phase.
Downstream HO2S Testing:
The rear HO2S is the Catalyst Monitor Sensor (CMS). The CMS signal is constantly monitored for excessive voltage by comparison to a maximum threshold. Peak rich and lean voltages are compared to given thresholds to determine if the HO2S is disconnected or malfunctioning.
If the PCM does not observe switching in the signal under normal operating conditions, the PCM will force the air/fuel mixture rich or lean to verify a rear HO2S failure.
The PCM is constantly looking for a switch around stoichiometry from the CMS. If it does not see this switch, the PCM calls the CMS failed.
Whenever an over-voltage occurs on any HO2S, a malfunction is identified to the Diagnostic Executive and no further testing of the malfunctioning sensor will take place.
In addition, the HO2S heaters will be tested by rapidly turning them On and Off and checking the Output State Monitor (OSM) for proper response. Malfunctions will be identified to the Diagnostic Executive when a faulty heater is found.
DTCs and MIL Operation
When a malfunction has been present for two drive cycles, the DTC is stored in the PCM and the MIL is turned On. The MIL is turned Off after three consecutive Trips without the same malfunction being detected provided no other DTCs are stored that would independently turn the MIL On. The DTC will be erased from memory after 40 warm-up cycles without the malfunction being detected after the MIL is turned Off. Another method of erasing the DTC is initiating a Powertrain Control Module (PCM) Reset.