P0138

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 powertrain control module (PCM) 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 heater quickly warms the sensor to operating temperature. The heater also maintains the operating temperature during extended idle conditions. The HO2S must reach a high temperature to produce a voltage. When the HO2S reaches the operating temperature, the PCM monitors the HO2S bias, or reference, voltage. The PCM also monitors the HO2S signal voltage for Closed Loop fuel control. During normal Closed Loop fuel control operation, the PCM will add fuel, or enrich the mixture, when the HO2S detects a lean exhaust content. The PCM will subtract fuel, or lean out the mixture, when the HO2S detects a rich exhaust condition.

This DTC is designed to detect an HO2S voltage that remains high 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, P0116, P0117, P0118, P0121, P0122, P0123, P0200, P0300, P0440, P0442, P0446, P0452, P0453, or P1441 are not set.
^ The ignition 1 signal is between 9-18 volts.
^ The fuel tank level remaining is more than 10 percent.
^ The loop status is Closed.
^ Intrusive tests are not in progress.
^ The scan tool output controls are not active.

Rich Test Enable
^ The air fuel ratio is between 14.5:1-14.7:1.
^ The throttle position (TP) is between 3-70 percent.

Decel Fuel Cut-off Test Enable
^ The decel fuel cut-off (DFCO) is active.
^ The above condition is met for 5 seconds.

CONDITIONS FOR SETTING THE DTC

Rich Test
The HO2S voltage is more than 975 mV for 450 seconds.

Decel Fuel Cut-off Test
The HO2S voltage is more than 200 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.

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: Never solder the HO2S wires. For the proper wire and connector repair procedures, refer to Wiring Repairs in Diagnostic Aids.

Inspect the following items:
^ The fuel pressure-If the pressure is too high the system will run rich. The PCM can compensate for some increase. However, if the pressure gets too high, the DTC may set. Refer to Fuel System Diagnosis. Component Tests and General Diagnostics
^ A rich injector-Perform a Fuel Injector Balance Test. Refer to Fuel Injector Balance Test with Tech 2. Component Tests and General Diagnostics
^ A leaking injector-Refer to Fuel System Diagnosis. Component Tests and General Diagnostics
^ The engine oil is contaminated by fuel.
^ The evaporative emission (EVAP) canister purge-Inspect for a fuel saturation. If the canister is full of fuel, inspect the canister control and the hoses. Refer to Evaporative Emission (EVAP) Canister Purge Valve Replacement.
^ A leaking fuel pressure regulator diaphragm-Refer to Fuel System Diagnosis. Component Tests and General Diagnostics
^ The TP sensor-An intermittent TP sensor output causes the system to run rich due to a false indication of the throttle moving.
^ A false rich indication due to silicon contamination of the heated oxygen sensor-A DTC P0144 accompanied by lean driveability conditions and a powdery white deposit on the sensor indicates a false rich indication.

An intermittent can be caused by any of the following conditions:
^ A poor connection
^ Rubbed-through wire insulation
^ A broken wire inside the insulation

Thoroughly inspect any circuitry that is suspected of causing the intermittent complaint. Refer to Testing for Intermittent and Poor Connections in Diagnostic Aids. If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Diagnostic Aids.

TEST DESCRIPTION

Steps 1-5:

Steps 6-12:

The numbers below refer to the step numbers on the diagnostic table.
6. 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 inspect for rich conditions that would affect both cylinder banks. An opposite bank sensor with normal HO2S voltage activity indicates the suspect HO2S is defective or a rich condition exists only on the suspect HO2S cylinder bank.
7. Voltage more than the specified value indicates a short to voltage.
9. An HO2S contaminated by silicon will have a white, powdery deposit on the portion of the HO2S that is exposed to the exhaust stream. The usual cause of silica contamination is the use of unapproved silicon RTV engine gasket material or the use of silicon based sprays or fluids within the engine. If the cause of this contamination is not corrected, the replacement HO2S will also be contaminated.