In order to control emissions, a catalytic converter is used to convert 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 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).

If the O2S pigtail wiring, connector, or terminal are damaged the entire O2S assembly must be replaced. DO NOT attempt to repair the wiring, connector, or terminals. In order for the sensor to function properly, it must have a clean air reference provided to it. This clean air reference is obtained by way of the O2S wires. Any attempt to repair the wires, connectors, or terminals could result in the obstruction of the air reference and degrade oxygen sensor performance. Refer to Section 8A-5, Heated Oxygen Sensor (HO2S) Repair.

Malfunction indicator lamp (MIL) illuminates after two consecutive driving cycles with the fault active.

Inspect for the following:

The number below refers to the step number on the diagnostic table.

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

2. Disconnecting HO2S 2 and jumpering CKTs 1668 and 1669 to ground should cause the scan tool to display HO2S 2 voltage below 0.2 volt (200 mV). If the signal voltage is still high, the PCM is faulty.