Circuit Description
Heated oxygen sensors (HO2S) are used for fuel control and catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content of the exhaust stream. When the vehicle is started, the control module operates in an Open Loop mode, ignoring the HO2S signal voltage while calculating the air-to-fuel ratio. The control module supplies the HO2S with a reference, or bias, voltage of approximately 450 mV. While the engine runs, the HO2S heats up and begins to generate a voltage within a range of 0-1,000 mV. This voltage will fluctuate above and below the bias voltage. Once sufficient HO2S voltage fluctuation is observed by the control module, Closed Loop is entered. The control module uses the HO2S voltage to determine the air-to-fuel ratio. An HO2S voltage that increases above bias voltage toward 1,000 mV indicates a rich fuel mixture. An HO2S voltage that decreases below bias voltage toward 0 mV indicates a lean fuel mixture.
The heating elements within each HO2S heat the sensor. The powertrain control module (PCM) controls the HO2S heater control circuit. The HO2S heater diagnostic monitors the current draw through the HO2S output driver module (ODM) when the engine is running. This allows the system to enter Closed Loop earlier and the control module to calculate the air-to-fuel ratio sooner.
The PCM commands the heater ON or OFF to maintain a specific HO2S operating temperature range. The PCM determines the temperature by measuring the current flow through the heater. When the heater is in the ON state, the PCM will pulse the heater OFF for a duration of 50 milliseconds, once per second. When the heater is in the OFF state, the PCM will pulse the heater ON for a duration of 50 milliseconds, once per second.
If the PCM detects that the HO2S 2 voltage remains above a calibrated voltage for an excessive amount of time, DTC P0138 will set.
Each HO2S 2 has the following circuits:
| • | An HO2S 2 high signal circuit |
| • | An HO2S 2 low signal circuit |
| • | An HO2S 2 heater ignition voltage circuit |
| • | An HO2S 2 heater low control circuit |
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0138 HO2S Circuit High Voltage Bank 1 Sensor 2
Conditions for Running the DTC
The following conditions must be met:
| • | DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0120, P0125, P0128, P0201, P0202, P0203, P0204, P0205, P0206, P0220, P0442, P0443, P0446, P0449, P0451, P0452, P0453, P0454, P0455, P0464, P0496, P2135 are not set. |
| • | DTCs P0401, P0420, and P2A01 are not commanding fuel. |
| • | The system voltage is between 9-18 volts. |
| • | The scan tool special functions are not active. |
| • | The TP Sensor parameter is between 3 percent and 40 percent. |
| • | The Air Fuel Ratio parameter is between 12:1 and 16.5:1. |
| • | The Loop Status parameter is closed. |
| • | The above conditions must be met for at least 3 seconds. |
Conditions for Setting the DTC
| • | The PCM detects that the HO2S 2 signal voltage is more than 924 mV for at least 2 minutes. |
| • | This diagnostic runs continuously. |
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. |
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 | |||||||||||
2 |
Does the voltage vary above and below the specified range? | 390-520 mV | Go to Step 3 | Go to Step 4 | ||||||||||
3 |
Did the DTC fail this ignition? | -- | Go to Step 4 | Go to Testing for Intermittent Conditions and Poor Connections | ||||||||||
4 |
Is the voltage more than the specified value? | 500 mV | Go to Step 5 | Go to Step 6 | ||||||||||
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 | Inspect for the following conditions that may affect the HO2S operation: Notice: Refer to Silicon Contamination of Heated Oxygen Sensors Notice in the Preface section.
Notice: Refer to Heated Oxygen and Oxygen Sensor Notice in the Preface section.
Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 7 | ||||||||||
7 | Test for shorted terminals and poor connections at 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 | Test for shorted terminals and poor connections at the powertrain control module (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. 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 | |||||||||||
