For 1990-2009 cars
Engine Controls Components
Power, Ground, HO2S 2, and O2S 1
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 is used to convert harmful exhaust gases into harmless water vapors and carbon dioxide.

The PCM has the capability of monitoring this process by using a rear heated oxygen sensor (HO2S 2). The HO2S 2, located in the exhaust stream behind the catalytic converter, produces an output signal which indicates the storage capacity of the catalyst. This in turn indicates the catalysts 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).

### Conditions for Running the DTC

• | The following DTCs are not set: P0105, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0171, P0201, P0202, P0203, P0204, P0300, P0335, P0440, P0442, P0446, P0506, P0507, P0601, P0602, or P1441.

• | The air flow is more than 5.5 g/s.

• | The engine coolant temperature (ECT) is more than 40°C (104°F).

• | The throttle position (TP) angle is between 5 percent and 50 percent for 4 seconds.

### Conditions For Setting The DTC

The HO2S 2 voltage is more than 1042 mV for 50 to 75 seconds.

### Action Taken When the DTC Sets

• | The Malfunction Indicator Lamp (MIL) will illuminate after 2 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.

### Conditions for Clearing the DTC

• | The MIL will turn off after 3 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.

• | Clear the DTC with a 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 refer to step numbers on the diagnostic table.

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

2. 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 an HO2S 2 voltage below 100 mV . If the signal voltage is still high, the this indicates that the PCM is malfunctioning.

3. The replacement PCM must be programmed and the Crankshaft Position System Variation Learn procedure must be performed.

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

Step | Action | Values | 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 | Is the HO2S 2 sensor voltage more than the specified value? | 1042 mV | Go to Step 4 | Go to Step 3
3 | Operate the vehicle within the Freeze Frame conditions and the Conditions for Running the DTC as specified in the supporting text. Is the sensor voltage more than the specified value? | 1042 mV | Go to Step 4 | Go to Step 9
4 |

1. Turn off the ignition.
2. Disconnect the PCM.

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

3. Disconnect the HO2S 2 connector.
4. Connect a DMM to ground.
5. Turn on the ignition, leaving the engine off.
6. Probe the HO2S 2 sensor signal circuit.

Does the voltage equal the specified value?

0 V

Go to Step 5

Go to Step 6

5 | Connect a fused jumper between the HO2S 2 signal circuit and the HO2S 2 low circuit at the connector. Is the voltage measure less than the specified value? | 100 mV | 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. Did you complete the repair? | -- | 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 Heated Oxygen Sensor Replacement . Did you complete the repair? | -- | Go to Step 9 | --
8 | Important:: The replacement PCM must be programmed. Refer to Powertrain Control Module Replacement/Programming . Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the repair? | -- | Go to Step 9 | --
9 |

1. Use a scan tool to clear the DTCs.
2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC reset?

--

Go to Step 2

System OK