P0138

DTC P0138 HO2S Circuit High Voltage Sensor 2:

HO2S Circuit:

Circuit Description
The Vehicle Control Module (VCM) supplies a voltage of about 0.45 volt (450 mV) between the HO2S High and HO2S Low circuits. (If measured with a megaohm digital voltmeter, this may read as low as 0.32 volt.) The Heated Oxygen Sensor (HO2S Bank 1 Sensor 1) varies the voltage within the range of about 1.0 volt (1000 mV) if the exhaust is rich, down to about 0.10 volt (100 mV) if the exhaust is lean.

The Sensor is like an open circuit and produces no voltage when it is below 360°C (600°F). The DTC P0137, P0138 or an open sensor circuit or the cold sensor causes the Open Loop operation. The DTC P0138 determines if the HO2S sensor or the circuit is shorted to high by checking for a rich condition during a steady throttle. This DTC is a type B DTC.

Conditions For Setting The DTC
The Following Conditions will set the DTC:

^ No Throttle Position (TP) sensor DTCs.
^ No Evaporation Emission (EVAP) DTCs.
^ No Intake Air Temperature (IAT) sensor DTCs.
^ No Manifold Absolute Pressure (MAP) sensor DTCs.
^ No Engine Coolant Temperature (ECT) sensor DTCs.
^ No Mass Air Flow (MAF) sensor DTCs.
^ No intrusive test in progress.
^ No device controls active.
^ The system voltage measures at least 9.0 volts.

Test Enable Rich Test
^ A Closed Loop.
^ Air to fuel ratio is at least 14.5 but less than 14.8.
^ The TP is greater than 0% but less than 50%.
^ The above must meet for 5 seconds.
^ HO2S (Bank1, Sensor 2) DTCs not activate since the code clear.
^ The HO2S (Bank 1, Bank 2) voltage measures greater than 0.976 volts (976 mV).

Test Enable DFCO Rich Test

^ Decel Fuel cutoff mode is active.
^ A Closed Loop.
^ The time elapsed is at least 2 seconds.
^ HO2S (Bank 1, Sensor 2) not active since the clear code.
^ The HO2S (Bank 1, Sensor 2) voltage measures greater than 0.468 volts (486 mV).

Action Taken When The DTC Sets
The VCM turns on the Malfunction Indicator Lamp (MIL) after 2 consecutive failures.

Conditions For Clearing The MIL/DTC
The VCM turns the MIL off after 3 consecutive driving trips without a fault condition present. A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 40°F from start-up coolant temperature and the engine coolant temperature exceeds 160°F during that same ignition cycle) or the scan tool clearing feature has been used.

Diagnostic Aids
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, the DTC P0138 may set. Refer to Fuel System Pressure Test. System Diagnosis - Fuel System Pressure Test
^ A rich injector. Preform a poppet nozzle test. Refer to Fuel Injector Balance Test. Fuel Injector Balance Test
^ A leaking injector. Refer to Fuel Injector Balance Test.
^ Fuel contaminated oil
^ EVAP canister purge: Check for a fuel saturation. If full of fuel, check the canister control and hoses.
^ 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. A DTC P0138 accompanied by a lean driveability conditions and a powdery white deposit on the sensor indicates a false rich indication.
^ Faulty HO2S (Bank 1, Sensor 2): the HO2S (Bank 1, Sensor 2) is internally shorted, the HO2S (Bank 1, Sensor 2) voltage displayed on a scan tool will be over 1.0 volt (1000 mV). Try disconnecting the HO2S (Bank 1, Sensor 2), if the displayed voltage goes from over 1000 mV to around 450 mV, replace the HO2S (Bank 1, Sensor 2).

NOTE: Never solder the HO2S wires. For proper wire and connector repair, refer to Diagrams.

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

2. This step determines if the conditions necessary for setting the DTC P0138 exist.