Circuit Description
The engine control module (ECM) supplies a voltage near 450 mV between the heated oxygen sensor (HO2S) high signal circuit and the low reference circuit. The HO2S varies the voltage over a range from about 1,000 mV when the exhaust is rich, down through about 10 mV when the exhaust is lean.
The ECM monitors and stores the HO2S voltage information. The ECM evaluates the HO2S voltage samples in order to determine the amount of time that the HO2S voltage was out of range. The ECM 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 ECM monitors the HO2S voltage for being fixed above a predetermined voltage. If the ECM detects the voltage is too high, this DTC sets.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0138 HO2S Circuit High Voltage Sensor 2
Conditions for Running the DTC
| • | The engine is operating. |
| • | The ignition 1 voltage is more than 10.5 volts. |
| • | The HO2S heater is at operating temperature for more than 10 seconds. |
| • | The calculated exhaust temperature is between 250-800°C (482-1,472°F). |
| • | DTC P0138 runs continuously once the above conditions are met for more than 90 seconds. |
Conditions for Setting the DTC
| • | The ECM detects that the HO2S signal voltage is more than 1,050 mV. |
| • | The condition exists for more than 5.1 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 4 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
The following table illustrates the ECM connector locations:
ECM Connector Location | |
|---|---|
C2 | C1 |
Front of Vehicle | |
| • | Use the J 35616-A/BT-8637 Connector Test Adapter Kit for any test that requires probing the ECM harness connector or a component harness connector. |
| • | The rear HO2S will not go into closed loop when the engine is idling. Once the HO2S is at operating temperature, and the vehicle is moving, the rear HO2S will go into closed loop. |
| • | For an intermittent condition, refer to Intermittent Conditions . |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
This step determines if the condition exists. If there is an HO2S circuit condition, the HO2S voltage will be more than the specified value.
-
This step isolates the condition. If the voltage is more than the specified with the engine OFF, this indicates there is a circuit condition.
-
This step isolates if the condition is with the signal circuit or the low reference circuit of the HO2S. If the signal circuit voltage is within the specified range, the low reference circuit is short to a voltage.
-
This step tests for a fuel system that is operating rich.
-
This step inspects the harness connector for water intrusion. Moisture in a connector may cause voltage to bleed to other circuits.
-
This step inspects the harness connector for water intrusion. Moisture in a connector may cause voltage to bleed to other circuits.
Step | Action | Values | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views | ||||||||||||
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | |||||||||
Is the voltage more than the specified value? | 1,050 mV | Go to Step 4 | Go to Step 3 | |||||||||
3 |
Did the DTC fail this ignition? | -- | Go to Step 4 | Go to Diagnostic Aids | ||||||||
Is the voltage more than the specified value? | 1,050 mV | Go to Step 5 | Go to Step 8 | |||||||||
Is the voltage within the specified range? | 350-550 mV | Go to Step 6 | Go to Step 7 | |||||||||
6 |
Important: The engine control module (ECM) may be damaged if the circuit is shorted to B+. Test the low reference circuit of the HO2S for a short to voltage. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 10 | ||||||||
7 |
Important: The HO2S may be damaged if the circuit is shorted to B+. Test the signal circuit of the HO2S for a short to voltage. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 10 | ||||||||
Test or inspect for the following conditions:
Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 9 | |||||||||
Test for shorted terminals and poor connections at the HO2S. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 11 | |||||||||
Test for shorted terminals and poor connections at the ECM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 12 | |||||||||
11 | Replace the HO2S. Refer to Heated Oxygen Sensor 2 Replacement . Did you complete the replacement? | -- | Go to Step 13 | -- | ||||||||
12 | Replace the ECM. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | -- | Go to Step 13 | -- | ||||||||
13 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 14 | ||||||||
14 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK | |||||||||
