Circuit Description
Heated oxygen sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content in the exhaust stream. The HO2S must reach operating temperature to provide an accurate voltage signal. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature. The powertrain control module (PCM) supplies the HO2S with a reference, or bias, voltage of about 450 mV. When the engine is first started the PCM operates in Open Loop, ignoring the HO2S voltage signal. Once the HO2S reaches operating temperatures and Closed Loop is achieved, the HO2S generates a voltage within a range of 0-1,000 mV that fluctuates above and below bias voltage. High HO2S voltage indicates a rich exhaust stream. Low HO2S voltage indicates a lean exhaust stream.
If the PCM detects an HO2S voltage that stays above a specified value, DTC P0138 will set.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0138 HO2S Circuit High Voltage Sensor 2
Conditions for Running the DTC
| • | DTCs P0068, P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0120, P0122, P0123, P0125, P0128, P0201, P0202, P0203, P0204, P0205, P0206, P0220, P0300-P0306, P0315, P0336, P0442, P0446, P0452, P0453, P0455, P0483, P0496, P0506, P0507, P0601, P0602, P0604, P0606, P060D, P060E, P0641, P0651, P1516, P1621, P2101, P2120, P2125, P2135, P2138, P2176 are not set. |
| • | The Engine Coolant Temperature (ECT) Sensor parameter is more than 70°C (158°F). |
| • | The Ignition 1 Signal parameter is more than 10 volts. |
| • | The Fuel Level Sensor parameter is more than 10 percent. |
| • | The Engine Run Time parameter is more than 10 seconds. |
| • | The Loop Status parameter is closed. |
| • | The Throttle Position (TP) Indicated Angle parameter is between 15-50 percent. |
| • | The Manifold Absolute Pressure (MAP) Sensor parameter is more than 25 kPa. |
| • | The Accelerator Pedal Position (APP) Indicated Angle parameter is more than 1.2 percent. |
| • | DTC P0138 runs continuously when the above conditions are met for 4 seconds. |
Conditions for Setting the DTC
| • | The PCM detects that the HO2S 2 parameter is more than 1,042 mV. |
| • | DTC P0138 sets within 50 seconds when the above condition is set. |
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. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
When the system is operating correctly, the HO2S 2 voltage should toggle above and below the bias voltage.
-
This step tests the HO2S 2 high signal circuit for a short to voltage.
Step | Action | Values | Yes | No | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics Connector End View Reference: Powertrain Control Module Connector End Views or Engine Controls Connector End Views | ||||||||||||||||
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | |||||||||||||
Does the scan tool indicate that the HO2S 2 voltage is more than the specified value? | 1,042 mV | Go to Step 4 | Go to Step 3 | |||||||||||||
3 |
Did the DTC fail this ignition? | -- | Go to Step 4 | Go to Intermittent Conditions | ||||||||||||
4 |
Does the voltage measure within the specified value? | 350-550 mV | Go to Step 6 | Go to Step 5 | ||||||||||||
Test the HO2S 2 high signal circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs. Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 8 | |||||||||||||
6 | The HO2S 2 is detecting a rich exhaust condition or may be contaminated. Inspect for one of the following conditions:
Repair any of the above or similar engine conditions as necessary. Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 7 | ||||||||||||
7 | Inspect for poor connections at the harness connector of the HO2S 2. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs. Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 9 | ||||||||||||
8 | Inspect for poor connections at the harness connector of the PCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs. Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 10 | ||||||||||||
9 |
Notice: Refer to Heated Oxygen Sensor Resistance Learn Reset Notice in the Preface section. Important: Before replacing a contaminated HO2S, determine and repair the cause of the contamination. Replace the HO2S 2. Refer to Heated Oxygen Sensor 2 Replacement . Did you complete the replacement? | -- | Go to Step 11 | -- | ||||||||||||
10 | Replace the PCM. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | -- | Go to Step 11 | -- | ||||||||||||
11 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 12 | ||||||||||||
12 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK | |||||||||||||
