Heated Oxygen Sensor Monitor

PURPOSE
On Board Diagnostics II (OBD II) regulations require monitoring of the upstream Heated Oxygen Sensors (HO2S) to detect when the deterioration of the sensor has exceeded emission thresholds.

For OBD II vehicles, two additional HO2S were added to determine the efficiency of the catalyst. These sensors are the same type used for fuel control, but they function differently. These sensors are monitored to determine if a voltage is generated. That voltage is compared to a calibrated acceptable range.


COMPONENTS
The HO2S Monitor/Fuel Control System consists of the following components: HO2S, Mass Air Flow (MAF), Intake Air Temperature (IAT), Engine Coolant Temperature (ECT), Profile Ignition Pickup (PIP) (RPM) and Injector(s). 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 Powertrain Control Module (PCM) uses the input from the HO2S sensor to fine tune the air fuel ratio. The HO2S sensors are mounted in the exhaust flow. The sensors work 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 tip temperature of 350°C (662°F).

The test of the upstream sensors is enabled when entry conditions have been met. At that time, a fixed frequency fuel modulation begins. The upstream HO2S(s) are sampled and the results are calculated to determine if the sensor reacts properly.

The voltage envelope of the downstream sensors is monitored for normal switches. If normal switches do not occur, the fuel is commanded rich and/or lean and a decision made on the downstream sensors based on the voltage envelope on the downstream HO2S sensors.

Whenever an overvoltage 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.


Malfunction Indicator Lamp (MIL) and Diagnostic Trouble Codes (DTC's)
After a malfunction has been present for two drive cycles, the DTC is stored in PCM memory and the MIL is turned on. Once the DTC has been stored in memory, the MIL will stay on as long as the malfunction exists. The MIL is turned off after three consecutive trips without the same malfunction being detected providing there are no other fault codes stored that would independently illuminate the MIL. 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.

Typical HO2S Location:

The above diagram illustrates the location of the HO2S(s).

Sensor 11/21 (upstream) is used for Fuel Control.

Sensor 12/22 (downstream) is used for Catalyst testing.