CIRCUIT DESCRIPTION
This diagnostic applies only to vehicles built with case grounded heated oxygen sensors (HO2S). In order to identify the HO2S, refer to Computers and Controls Systems Component Views.

The powertrain control module (PCM) supplies a voltage of about 450 mV between the HO2S high and low signal circuits. The HO2S varies the voltage over a range of about 1,000 mV when the exhaust is rich to about 10 mV when the exhaust is lean.

The PCM monitors and stores the HO2S voltage information. The PCM evaluates the HO2S voltage samples in order to determine the amount of time the HO2S voltage was out of range. The PCM compares the stored HO2S voltage samples taken within each sample period and determines if the majority of the samples are out of the operating range.

The PCM monitors the HO2S voltage for being fixed below a predetermined voltage. If the PCM detects the voltage is below a predetermined voltage, a diagnostic trouble code (DTC) sets.

CONDITIONS FOR RUNNING THE DTC

• DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0200, P0300, P0401, P0404, P0405, P0440, P0442, P0446, P0452, P0453, P1120, P1125, P1220, P1221, P1258, P1404, P1441, P1514, P1515, P1516, P1517, or P1518 are not set.
• The vehicle is operating in Closed Loop.
• The ignition 1 signal is between 9-18 volts .
• The fuel tank level remaining is more than 10 percent.
• Intrusive tests are not in progress.
• The scan tool output controls are not active.

Lean Test Enable:

• The air fuel ratio is between 14.5:1-14.7:1.
• For vehicles with throttle-actuated control (TAC), the throttle position (TP) indicated angle is between 3-70 percent more than the value observed at idle.
• For vehicles without TAC, the TP is between 3-70 percent .

Power Enrichment Test Enable:

• Power enrichment is active.
• The above condition is met for 1 second .

CONDITIONS FOR SETTING THE DTC
Lean Test:
The HO2S voltage is less than 200 mV for 165 seconds .

Power Enrichment Test:
The HO2S voltage is less than 360 mV for 10 seconds .

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 operating 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.
• The control module commands Open Loop.

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.
• Clear the MIL and the DTC with a scan tool.

DIAGNOSTIC AIDS

IMPORTANT: Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing or replacing the PCM. Ensure that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM.

• Inspect the HO2S wire. The sensor pigtail may be in contact with the exhaust manifold or the exhaust system.
• Inspect the HO2S oxygen supply. An oxygen supply inside the HO2S is necessary for proper operation. The HO2S wires provide the supply of oxygen. Inspect the HO2S wires and the connections for breaks and for contamination. Refer to Heated Oxygen Sensor (HO2S) Wiring Repairs in Diagnostic Aids.
• Inspect for an intermittent ground in the signal wire between the connector and the sensor. Refer to Testing for Intermittent and Poor Connections in Diagnostic Aids
• Inspect the injectors. Perform the Fuel Injector Balance Test. Refer to Fuel Injector Balance Test with Tech 2 or to Fuel Injector Balance Test with Special Tool. See: Powertrain Management\Fuel Delivery and Air Induction\Fuel Injector\Testing and Inspection
• Inspect for contaminated fuel. 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 (W/O Special Tool) or Alcohol/Contaminants-in-Fuel Diagnosis (W/ Special Tool). See: Powertrain Management\Fuel Delivery and Air Induction\Fuel\Testing and Inspection
• Inspect the fuel pressure. The system will be lean if the fuel pressure is low. Refer to Fuel System Diagnosis. See: Powertrain Management\Computers and Control Systems\Testing and Inspection\Component Tests and General Diagnostics
• Inspect for exhaust leaks. An exhaust leak near the HO2S can cause a lean condition. Refer to Exhaust Leakage in Exhaust System.
• Inspect for vacuum leaks or for crankcase leaks that can cause a lean condition.
• If the above components are OK, the HO2S may be at fault. For an intermittent condition, refer to Intermittent Conditions. See: Powertrain Management\Computers and Control Systems\Testing and Inspection\Initial Inspection and Diagnostic Overview\Diagnostic Strategies\Intermittent Conditions