P0151

CIRCUIT DESCRIPTION
The Heated Oxygen Sensor (HO2S) is a sensor designed to create a voltage relative to the oxygen content in the engine exhaust stream. The control module supplies 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 that is necessary in order to quickly warm the sensor to operating temperature. The heater also maintains the operating temperature during extended idle conditions. The HO2S needs to be at a high temperature in order to produce a voltage. When the HO2S reaches operating temperature, the control module monitors the HO2S bias or reference voltage. The control module also monitors the HO2S signal voltage for closed loop fuel control. During normal closed loop fuel control operation, the control module will add fuel, or enrich the mixture, when the HO2S detects a lean exhaust content. The control module will subtract fuel, or lean out" the mixture, when the HO2S detects a rich exhaust condition. Certain vehicle models utilize an oxygen sensor behind the catalytic converter in order to monitor the catalyst efficiency.

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.

CONDITIONS FOR RUNNING THE DTC
^ DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P01 17, P0118, P0125, P0128, P0200, P0300, P0351-P0358, P0401, P0404, P0405, P0410, P0418, P0443, P0446, P0449, P1120, P1220, P1221, P1258, or P1404 are not set.
^ The Secondary Air Injection (AIR) and the Exhaust Gas Recirculation (EGR) diagnostics are not active.
^ The ignition voltage is more than 9 volts.
^ The fuel system is operating in Closed Loop.
^ The fuel trim learn is enabled.
^ The Accelerator Pedal Position (APP) indicated angle is between 3-70 percent.

CONDITIONS FOR SETTING THE DTC
The HO2S signal voltage remains below 200 mV for 165 seconds or more.

ACTION TAKEN WHEN THE DTC SETS
^ The control module illuminates the Malfunction Indicator Lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
^ The control module records the conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The control module turns OFF the Malfunction Indicator Lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
^ A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
^ A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
^ Use a scan tool in order to clear the MIL and the DTC.

DIAGNOSTIC AIDS
^ Check the Heated Oxygen Sensor (HO2S) wire. The sensor pigtail may be in contact with the exhaust manifold or the exhaust system.
^ An oxygen supply inside the HO2S is necessary for proper operation. The HO2S wires provides the supply of oxygen. Inspect the HO2S wires and connections for breaks or contamination. Refer to Heated Oxygen Sensor (HO2S) Wiring Repairs in Diagrams.
^ Check for intermittent ground in the signal wire between the connector and the sensor. Refer to Testing for Intermittent and Poor Connections in Diagrams.
^ Check for lean injectors. Perform the Injector Balance Test. Refer to Fuel Injector Balance Test with Tech 2. Component Tests and General Diagnostics
^ Check for fuel contamination. Water near the in-tank fuel pump inlet can be delivered to the injectors. The water causes a lean exhaust and can set a DTC. Refer to Alcohol/Contaminants-in-Fuel Diagnosis. Component Tests and General Diagnostics
^ Check the fuel pressure. The system will be lean if fuel the pressure is low. Refer to Fuel System Diagnosis. Component Tests and General Diagnostics
^ Check for exhaust leaks. An exhaust leak near the HO2S can cause a lean condition. Refer to Exhaust Leakage in Exhaust System. Exhaust Leakage
^ Vacuum or crankcase leaks can cause a lean condition.
^ If the above components are OK, the HO2S may be at fault.

For an intermittent, refer to Intermittent Conditions. Intermittent Conditions

TEST DESCRIPTION

Steps 1-5:

Steps 6-13:

The number below refers to the step number 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 cylinder banks. An opposite bank sensor with normal HO2S voltage activity indicates the suspect HO2S needs replacement or a lean condition exists only on the suspect HO2S cylinder bank.