The heated oxygen sensor (HO2S) is designed to create a voltage relative to the oxygen content in the engine exhaust stream. The controls modules, the powertrain control module (PCM) and the vehicle control module (VCM), supply the HO2S with signal High and Low circuits. Ignition voltage and ground are supplied to the HO2S heater by independent circuits. The oxygen content of the exhaust indicates when the engine is operating lean or rich. When the HO2S detects that the engine is operating rich, the signal voltage is high, and decreases the signal voltage as the engine runs leaner. This oscillation above and below the bias voltage, sometimes referred to as activity or switching, can be monitored with the HO2S signal voltage.

The HO2S contains a heater in order to quickly warm the sensor to the operating temperature and to maintain the operating temperature during extended idle conditions. The HO2S needs to be at a high temperature in order to produce a voltage. Once the HO2S has reached the operating temperature, the controls module monitors the HO2S bias, or reference, voltage and the HO2S signal voltage for closed loop fuel control. During normal closed loop fuel control operation, the controls module will add fuel, or enrich the mixture, when the HO2S detects a lean exhaust content. The controls module will subtract fuel, or "lean out" the mixture when the HO2S detects a rich exhaust condition.

Certain vehicle models use an oxygen sensor behind the catalyst converter in order to monitor the catalyst efficiency.

This DTC determines if the HO2S is functioning properly by checking for an adequate number of HO2S voltage transitions above and below the bias range of 300-600 mV. This DTC sets when the VCM fails to detect a minimum number of voltage transitions above and below the bias range during the test period. Possible causes of this DTC are:

This DTC is designed to detect an HO2S voltage that remains at a low voltage for more than a specified number of seconds during the test conditions. This DTC is set when:

Lean Test Enable

Power Enrichment Lean Test Enable

Lean Test

The O2 sensor voltage remains less than 86 mV for more than 50 seconds.

Power Enrichment Lean Test

The O2 sensor voltage remains less than 598 mV for more than 30 seconds

Important: Never solder the HO2S wires. For proper wire and connection repair, refer to Wiring Repairs or Connector Repairs in Wiring Systems.

Use the scan tool in order to observe the long-term fuel trim (FT) values at the different RPM and air flow conditions. The scan tool also displays the fuel trim cells. You can check the long-term FT values in each cell in order to determine when the DTC may have set. If the conditions for this DTC exist, the long-term FT values measure approximately 158 or more.

Check for the following conditions:

An intermittent may be caused by any of the following conditions:

Thoroughly inspect any circuitry that is suspected of causing the intermittent complaint. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.

If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Wiring Systems.

The numbers below refer to the step numbers on the diagnostic table.

5. Monitor the HO2S voltage of the opposite bank sensor. If the voltage activity of the opposite bank sensor is similar to the voltage activity of the suspect sensor, check for lean conditions that would affect both of the cylinder banks. An opposite bank sensor with normal HO2S voltage activity indicates that the suspect HO2S needs replacement or that a lean condition exists only on the suspect HO2S cylinder bank.