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 temperature 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 for HO2S 2.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0138 HO2S Circuit High Voltage Sensor 2
Conditions for Running the DTC
Rich Test Enable
| • | DTCs P0068, P0101, P0102, P0103, P0106, P0107, P0108, P0117, P0118, P0120, P0125, P0128, P0201, P0202, P0203, P0204, P0205, P0220, P0442, P0443, P0446, P0449, P0452, P0453, P0455, P0496, P1516, P2101, P2176 are not set. |
| • | The Loop Status parameter is closed. |
| • | The Ignition 1 Signal parameter is between 11-18 volts. |
| • | The Fuel Level Sensor parameter is more than 10 percent. |
| • | The TP Sensor parameter is between 15-50 percent. |
| • | DTC P0138 runs during the rich test continuously when the above conditions are met for 2 seconds. |
Decel Fuel Cutoff Test Enable
| • | DTCs P0068, P0101, P0102, P0103, P0106, P0107, P0108, P0117, P0118, P0120, P0125, P0128, P0201, P0202, P0203, P0204, P0205, P0220, P0442, P0443, P0446, P0449, P0452, P0453, P0455, P0496, P1516, P2101, P2176 are not set. |
| • | The Loop Status parameter is closed. |
| • | The Ignition 1 Signal parameter is between 11-18 volts. |
| • | The Fuel Level Sensor parameter is more than 10 percent. |
| • | The Engine Run Time parameter is more than 200 seconds. |
| • | The decel fuel cutoff is active. |
| • | DTC P0138 runs during decel fuel cut-off when the above conditions are met for 3 seconds. |
Conditions for Setting the DTC
Rich Test
| • | The PCM detects that the HO2S 2 voltage parameter is more than 1,000 mV. |
| • | DTC P0138 sets during the rich test within 100 seconds when the above condition is met. |
Decel Fuel Cutoff Test
| • | The PCM detects that the HO2S 2 voltage parameter is more than 350 mV. |
| • | DTC P0138 sets during decel fuel cut-off within 10 seconds when the above condition is met. |
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 number below refers to the step number on the diagnostic table.
Step | Action | Value(s) | 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 | |||||||||||||||
|
Important: With the engine running, observe the HO2S 1 voltage parameter with a scan tool. The voltage should vary from below 300 mV to above 600 mV. If the voltage is not varying, refer to DTC P0131 .
Did the HO2S 2 voltage parameter change more than the specified value? | 200 mV | Go to Step 3 | Go to Step 4 | |||||||||||||||
3 |
Did the DTC fail this ignition? | -- | Go to Step 4 | Go to Intermittent Conditions | ||||||||||||||
4 |
Is the HO2S 2 voltage parameter more than the specified value? | 800 mV | Go to Step 6 | Go to Step 5 | ||||||||||||||
5 | Measure the voltage from the low signal circuit of the HO2S 2 harness connector on the engine harness side to a good ground with a DMM. Refer to Circuit Testing . Is the voltage more than the specified value? | 2 V | Go to Step 7 | Go to Step 8 | ||||||||||||||
6 |
Important: The sensor may be damaged if the circuit is shorted to a voltage source. 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 17 | Go to Step 14 | ||||||||||||||
7 | Test the HO2S 2 low signal circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | -- | Go to Step 17 | Go to Step 14 | ||||||||||||||
8 |
Is the HO2S 2 voltage parameter less than the specified value? | 100 mV | Go to Step 9 | Go to Step 11 | ||||||||||||||
9 |
Is the HO2S 2 voltage parameter less than the specified value? | 100 mV | Go to Step 12 | Go to Step 10 | ||||||||||||||
10 | Test the HO2S 2 low signal circuit for an open or high resistance. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | -- | Go to Step 17 | Go to Step 14 | ||||||||||||||
11 | Test the HO2S 2 high signal circuit for an open or high resistance. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | -- | Go to Step 17 | Go to Step 14 | ||||||||||||||
12 |
Notice: Refer to Silicon Contamination of Heated Oxygen Sensors Notice in the Preface section.
Did you find and correct the condition? | -- | Go to Step 17 | Go to Step 13 | ||||||||||||||
13 | Test for shorted terminals and for 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 17 | Go to Step 15 | ||||||||||||||
14 | Test for shorted terminals and for 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 17 | Go to Step 16 | ||||||||||||||
15 |
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 17 | -- | ||||||||||||||
16 | Replace the PCM. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | -- | Go to Step 17 | -- | ||||||||||||||
17 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 18 | ||||||||||||||
18 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK | |||||||||||||||
