For 1990-2009 cars
Engine Controls Components
Oxygen Sensors
OBD II Symbol Description Notice
Powertrain Control Module Connector End Views
Handling ESD Sensitive Parts Notice

### Circuit Description

In order to control emissions, a catalytic converter converts any harmful exhaust emissions into harmless water vapor and carbon dioxide.

The Powertrain Control Module (PCM) has the capability to monitor this process by using a rear heated oxygen sensor (HO2S 2). The HO2S 2 , located in the exhaust stream past the catalytic converter, produces an output signal which indicates the storage capacity of the catalyst. This in turn indicates the catalyst's ability to convert the exhaust emissions effectively. If the catalyst is functioning properly, the HO2S 2 signal will be far less active than the signal produced by the front oxygen sensor (O2S 1).

Replace the entire HO2S 2 assembly. Do not attempt to repair the HO2S 2 if the following conditions exist:

• | Damaged wiring

• | Damaged electrical connectors

• | Damaged terminals

Obstruction of the air reference and degraded HO2S 2 performance could result from any attempt to repair the above conditions.

The HO2S 2 must have a clean air reference in order for the HO2S 2 to function properly. This clean air reference is obtained by way of the HO2S 2 wires.

### Conditions for Setting the DTC

• | DTCs P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0171, P0200, P0300, P0301, P0302, P0303, P0304, P0341, P0404, P0506, P0507, P0601, P0602 not set.

• | Air flow is greater than 5.5 grams per second.

• | Engine Coolant Temperature (ECT) is greater than 40°C (104°F).

• | Throttle Position (TP) angle is between 5% and 50% for 4 seconds.

• | HO2S 2 voltage is greater than 1042 mV for 50 to 75 seconds.

### Action Taken When the DTC Sets

• | The malfunction indicator lamp (MIL) will illuminate after two consecutive ignition cycles in which the diagnostic runs with the malfunction present.

• | The PCM will record the operating conditions at the time that the diagnostic fails. This information will store in the Freeze Frame and Failure Records buffers.

• | A history DTC stores.

• | The coolant fan turns ON.

### Conditions for Clearing the MIL/DTC

• | The MIL will turn OFF after three consecutive ignition cycles in which the diagnostic runs without a fault.

• | A history DTC will clear after 40 consecutive warm-up cycles without a fault.

• | The MIL/DTCs can be cleared by using the scan tool.

### Diagnostic Aids

Check for the following conditions:

• | A rich exhaust--An overly rich exhaust may load the catalyst, causing high HO2S 2 signal voltages.

• | Silicone contamination--Silicone contamination of HO2S 2 could cause a false rich condition . A powdery white deposit on the sensor will indicate this condition.

• | A malfunctioning HO2S 2--If there is an internal short in the HO2S 2, the HO2S 2 voltage displayed on a scan tool will be more than 1.0 volt. Disconnect the HO2S 2 and connecting a jumper wire between the HO2S 2 low circuit to engine ground. Replace the HO2S 2 if the displayed voltage goes from more than 1000 mV to approximately 450 mV .

• | An intermittent test--Observe the HO2S 2 on the scan tool while moving the related electrical connectors and the wiring harness with the ignition switch turned ON. If the malfunction is induced, the HO2S 2 display will change. This should help isolate the location of the malfunction.

• | Damaged harness--Inspect PCM harness for a short to voltage in the HO2S 2 signal circuit.

An intermittent could be the result of the following conditions:

• | A poor electrical connection

• | Rubbed-- through wire insulation

• | A broken wire inside of the insulation

Replace the entire HO2S 2 assembly. Do not attempt to repair the HO2S 2 if the following conditions exist:

• | Damaged wiring

• | Damaged electrical connectors

• | Damaged terminals

Obstruction of the air reference and degraded HO2S 2 performance could result from any attempt to repair the above conditions.

The HO2S 2 must have a clean air reference in order for the HO2S 2 to function properly. This clean air reference is obtained by way of the HO2S 2 wires.

### Test Description

The numbers below refers to the step numbers on the Diagnostic Table.

1. The Powertrain OBD System Check prompts you to complete some of the basic checks and to store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data captured when the malfunction occurred. The scan tool stores this data for later reference.

2. This step determines if DTC P0138 is the result of a hard malfunction or an intermittent condition.

3. Disconnecting the HO2S 2 and connecting a jumper wire between the HO2S 2 signal circuit and the HO2S 2 low circuit to ground, should cause the scan tool to display HO2S 2 voltage below 100 mV (0.1 Volts). If the signal voltage is still high, the this indicates that the PCM is malfunctioning.

4. Reprogram the replacement PCM and perform the crankshaft position system variation learn procedure. Refer to the latest Techline information for PCM programming and Crankshaft Position System Variation Learn for the crankshaft position system variation learn procedure.

#### DTC P0138-Heated Oxygen Sensor (HO2S) Circuit High Voltage (Sensor 2)

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 |

1. Turn ON the ignition switch leaving the engine OFF.
2. Install a scan tool.

Is the HO2S 2 sensor voltage above the specified value?

1042 mV (1.042 V)

Go to Step 4

Go to Step 3

3 |

1. Review the Freeze Frame data and record the parameters.
2. Start the engine.
3. Operate the vehicle within the Freeze Frame conditions and Conditions For Setting the DTC as specified.

Is the HO2S 2 sensor voltage above the specified value?

1042 mV (1.042 V)

Go to Step 4

Go to Step 9

4 |

1. Turn OFF the ignition switch.

Caution: Avoid contact with moving parts and hot surfaces while working around a running engine in order to prevent physical injury.

2. Disconnect the HO2S 2 electrical connector.
3. Connect a DMM to ground.
4. Turn ON the ignition switch leaving the engine OFF.
5. Probe the HO2S 2 sensor signal circuit.

Does the DMM indicate a voltage of the specified value?

0 V (± 0.5)

Go to Step 5

Go to Step 6

5 |

1. Turn OFF the ignition switch.
2. Reconnect the PCM electrical connectors.
3. Turn ON the ignition switch leaving the engine OFF.
4. Connect a jumper wire at the following circuits at the HO2S 2 electrical connector (PCM side) to ground.

• | HO2S 2 signal circuit

• | HO2S 2 low circuit

Does the scan tool indicate the HO2S 2 sensor voltage below the specified value?

100 mV (0.10 V)

Go to Step 7

Go to Step 8

6 | Repair the short to voltage in the HO2S 2 sensor signal circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 9 | --
7 | Caution: Avoid contact with moving parts and hot surfaces while working around a running engine in order to prevent physical injury. Replace the HO2S 2 sensor. Refer to the Heated Oxygen Sensor Replacement . Is the action complete? | -- | Go to Step 9 | --
8 |

1. Turn OFF the ignition switch.
2. Replace the PCM. Refer to the Powertrain Control Module Replacement/Programming .

Is the action complete?

--

Go to Step 9

--

9 |

1. Clear the DTCs with the scan tool.
2. Start the engine.
3. Idle the engine at the normal operating temperature.
4. Operate the vehicle within the conditions for setting this DTC as specified in the supporting text.

Does the scan tool indicate that this diagnostic ran and passed?

--

Go to Step 10

Go to Step 2

10 | Check to see if any additional DTCs are set. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to applicable DTC table | System OK