Heated Oxygen Sensor (HO2S) Monitor

HEATED OXYGEN SENSOR (HO2S) MONITOR

Heated Oxygen Sensor Monitor:

The HO2S monitor is an on-board strategy designed to monitor the HO2S for damage or deterioration which can affect emissions. The fuel control sensor or HO2S11 is checked for proper output voltage and response rate (the time it takes to switch from lean to rich or rich to lean). The HO2S12 is used for catalyst monitoring. The HO2Ss are also monitored for proper output voltage. Input is required from the cylinder head temperature (CHT), intake air temperature (IAT), mass air flow (MAF) and crankshaft position (CKP) sensors to activate the HO2S monitor. The fuel system monitor and misfire detection monitor must also complete successfully before the HO2S monitor is enabled.

1. The hybrid vehicle is a partial zero emission vehicle (PZEV) which uses 2 separate HO2Ss. The front sensor (HO2S11) is the primary fuel control sensor. The last sensor downstream in the exhaust is used to monitor the light-off catalyst (HO2S12).
2. The HO2S senses the oxygen content in the exhaust flow and outputs a voltage between 0 and 1 volt. Lean of stoichiometric (air/fuel ratio of approximately 14.7:1), the HO2S generates a voltage between 0 and 0.45 volt. Rich of stoichiometric, the HO2S generates a voltage between 0.45 and 1 volt. The HO2S monitor evaluates the HO2S11 (fuel control) and HO2S12 (catalyst monitor) for proper function.
3. The time between HO2S switches is monitored after vehicle startup and during closed loop fuel conditions. Excessive time between switches or no switches since startup indicates a concern. Since lack of switching concern may be caused by HO2S concerns or by shifts in the fuel system, diagnostic trouble codes (DTC)s are stored that provide additional information for the lack of switching concerns. Different DTCs indicate whether the sensor always indicates lean/disconnected (P2195), or always indicates rich (P2196). The powertrain control module (PCM) monitors the HO2S signal for high voltage, in excess of 1.1 volts and store a unique DTC (P0132). An over-voltage condition is caused by a HO2S heater or battery power short to the HO2S signal line.

A functional test of the HO2S12 is done during normal vehicle operation. The peak rich and lean voltages are continuously monitored. Voltages that exceed the calibrated rich and lean thresholds indicate a functional sensor. If the voltages have not exceeded the thresholds after a long period of vehicle operation, the air/fuel ratio may be forced rich or lean in an attempt to get the HO2S12 to switch, This situation normally occurs only with a green catalyst (less than 500 miles). If the sensor does not exceed the rich and lean peak thresholds, a problem with the HO2S is indicated. The HO2S12 signal is monitored for high voltage, in excess of 1.1 volts and stores a unique DTC (P0138). An over-voltage condition is caused by a HO2S heater or battery power short to the HO2S signal line.

4. The malfunction indicator lamp (MIL) is activated after a concern is detected on 2 consecutive drive cycles.

The HO2S monitor sets a DTC as follows:
- P0133 - HO2S slow response rate
- P0132, P0138 - HO2S circuit high voltage
- P0135, P0141 - HO2S heater circuit
- P0053, P0054 - HO2S heater resistance
- P1127 - downstream HO2S not running in on-demand self-test
- P2195, P2196 - HO2S lack of switching
- P2270 - HO2S lack of switching (sensor indicates lean)
- P2271 - HO2S lack of switching (sensor indicates rich)