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Heated Oxygen Sensor Monitor: Description

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Heated Oxygen Sensor (HO2S) monitor is an on-board strategy designed to monitor HO2S sensors for a malfunction or deterioration which can affect emissions. The fuel control or upstream HO2S is checked for proper output voltage and response rate (time it takes to switch from lean to rich or rich to lean). Downstream HO2S sensors used for catalyst monitor are also monitored for proper output voltage. Input is required from ECT or CHT, IAT, MAF and CKP sensors to activate HO2S monitor. Fuel system monitor and misfire detection monitor must also have completed successfully before HO2S monitor is enabled.

  1. HO2S sensor senses oxygen content in exhaust flow and outputs a voltage between zero and one volt. Lean of stoichiometric (air/fuel ratio of approximately 14.7:1); HO2S will generate a voltage between zero and 0.45 volt. Rich of stoichiometric; HO2S will generate a voltage between 0.45 and one volt. HO2S monitor evaluates both upstream HO2S (fuel control), and downstream HO2S (catalyst monitor) for proper function.
  2. Once HO2S monitor is enabled, upstream HO2S signal voltage amplitude and response frequency are checked. Excessive voltage is determined by comparing HO2S signal voltage to a maximum calibratable threshold voltage. A fixed frequency closed-loop fuel control routine is executed and upstream HO2S voltage amplitude and output response frequency are observed. A sample of upstream HO2S signal is evaluated to determine if sensor is capable of switching or has a slow response rate. An HO2S heater circuit fault is determined by turning heater on and off and looking for a corresponding change in Output State Monitor (OSM) and by measuring current going through heater circuit. HO2S monitor DTCs can be categorized as follows:
    • DTCs P1130, P1131, P1132, P1150, P1151 and P1152 are associated with HO2S lack of switching.
    • DTCs P0133 and P0153 are associated with HO2S slow response rate.
    • DTCs P0131, P0136, P0148, P0151 and P0156 are associated with HO2S signal circuit malfunction.
    • DTCs P0135, P0141, P0155 and P0161 are associated with HO2S heater circuit malfunction.
    • DTC P1127 is associated with downstream HO2S not running in KOER self-test.
    • DTCs P1128 and P1129 are associated with swapped HO2S connectors.
  3. MIL is activated after a fault is detected on 2 consecutive drive cycles.

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