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 Control Module (PCM/VCM) supplies the HO2S with signal High and Low circuits. Ignition voltage and ground is 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 and 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 operating temperature, the Control Module monitors the HO2S bias, or reference, voltage as well as the HO2S signal voltage for closed loop fuel control. During normal Closed Loop fuel control operation, the Control Module will add fuel (enriches the mixture) when the HO2S detects a lean exhaust content and subtract fuel (leans-out the mixture) when the HO2S detects a rich exhaust condition.

Certain vehicle models utilize an oxygen sensor after the catalyst converter in order to monitor 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 listed below.
^ An open or a short to voltage on either the HO2S signal or HO2S low circuits.
^ A malfunctioning HO2S.
^ A problem in the HO2S heater or its circuit.
^ A poor HO2S ground.

This DTC is designed to detect an HO2S voltage that remains at a high (rich) voltage for more than 100 seconds during a 120 second test period. This DTC is set under the following conditions:
^ There is an HO2S circuit fault that results in a false rich exhaust condition.
^ The HO2S is correctly detecting the rich air/fuel ratio resulting from either a vacuum leak or a fuel control system fault.

Conditions for Running the DTC
^ No active TP sensor DTCs
^ No active EVAP system DTCs
^ No active IAT sensor DTCs
^ No active MAP sensor DTCs
^ No active ECT sensor DTCs
^ No active MAF sensor DTCs
^ No active misfire DTCs
^ No intrusive tests (i.e., EGR or Catalyst) in progress
^ No device control active
^ The system voltage is between 11.7-18 volts
^ The above condition is met for 5 seconds

Rich Test Enable
^ System in closed loop
^ The air/fuel ratio is between 14.5-14.8
^ The throttle position is between 0-50 percent
^ The elapsed time since test enabled is more than 2 seconds

Decel Fuel Cut-off (DFCO) Rich Test Enable
^ DFCO mode active
^ System in closed loop
^ The elapsed time since test enabled is more than 2 seconds

Conditions for Setting the DTC

Rich Test
The O2 sensor voltage is more than 994 mV for more than 110 seconds

DFCO Rich Test
The O2 sensor voltage is more than 469 mV for more than 40 seconds

Action Taken When the DTC Sets
^ The Control Module illuminates the Malfunction Indicator Lamp (MIL) if a failure is detected during 2 consecutive key cycles.
^ The Control Module will set the DTC and records the operating conditions at the time the diagnostic fails. The Control Module stores the failure information in the scan tools Freeze Frame and/or the Failure Records.

Conditions for Clearing the MIL/DTC
^ The Control Module turns OFF the MIL after 3 consecutive drive trips when the test has Run and Passed.
^ A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 22°C (40°F) from the start-up coolant temperature and the Engine Coolant Temperature is more than 70°C (158°F) during the same ignition cycle).
^ Use the scan tool Clear Information function.

Diagnostic Aids

Important: Never solder the HO2S wires.

Check the following items:
^ The fuel pressure: If the pressure is too high, the system will run rich. The VCM can compensate for some increase; however, if the pressure gets too high, this DTC may set. Refer to Fuel System Diagnosis.
^ A rich injector. Perform an Injector Balance test. Refer to Fuel Injector Balance Test with Tech 2.
^ A leaking injector. Refer to Fuel System Diagnosis.
^ Fuel contaminated oil
^ EVAP canister purge: Check for a fuel saturation. If full of fuel, check the canister control and hoses. Refer to EVAP Control System Diagnosis.
^ Leaking fuel pressure regulator diaphragm by checking the vacuum line to the regulator for fuel.
^ The TP sensor: An intermittent TP sensor output causes the system to run rich due to a false indication of the throttle moving.
^ False rich indication due to silicon contamination of the heated oxygen sensor. This DTC, accompanied by a lean driveability condition and a powdery white deposit on the sensor, may indicate a false rich indication. Component Tests and General Diagnostics Component Tests and General Diagnostics Component Tests and General Diagnostics

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

Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis. Symptom Related Diagnostic Procedures

Test Description
The number below refers to the step number on the diagnostic table.
3. In order to determine if the engine is rich during De-acceleration Fuel Cut-Off (DFCO) operate the vehicle up to highway speed conditions and release the accelerator pedal allowing the vehicle to coast in gear. Monitor the scan tool HO2S voltage and the DFCO parameter. A rich condition will cause HO2S voltage to be above 468 mV during DFCO.
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 un-approved 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 get contaminated.
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 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.
6. This step checks the HO2S HIGH signal circuit for a short to voltage.
8. 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 un-approved 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 get contaminated.