DTC P0138

Circuit Description

To control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), a three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting the NOx to nitrogen. The powertrain control module (PCM) has the ability to monitor this process using the post-catalyst heated oxygen sensor (HO2S). This sensor produces an output signal which indicates the oxygen storage capacity of the catalyst, this in turn indicates the catalysts ability to convert exhaust gases efficiently. If the post HO2S voltage remains excessively high for an extended period of time, this diagnostic trouble code (DTC) will set.

Conditions for Running the DTC

DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0201, P0202, P0203, P0204, P0205, P0206, P0410, P0440, P0442, P0443, P0446, P0449, or P1441 are not set.
The engine is in Closed Loop, with the commanded air/fuel ratio between 14.5 and 14.8.
The accelerator pedal position (APP) indicated angle is between 5-40 percent.

Conditions for Setting the DTC

The post-HO2S signal voltage remains more than 999 mV during normal Closed Loop operation.

The post-HO2S signal voltage remains more than 200 mV during deceleration fuel mode operation.

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.

Diagnostic Aids

Inspect for the following conditions:

High fuel pressure--An excessively rich fuel mixture can cause this DTC to set. Refer to Fuel System Diagnosis .
Fuel injectors--Refer to Fuel Injector Coil Test .
Fuel saturation of the EVAP canister--If the EVAP canister is full of fuel, inspect canister control and hoses. Refer to Evaporative Emission (EVAP) System Cleaning .
A rich condition--Disconnect the MAF sensor and see if the rich condition is corrected. If so, replace the MAF sensor. Refer to Mass Air Flow (MAF) Sensor Replacement .
A leaking fuel pressure regulator diaphragm--Inspect for a leaking fuel pressure regulator diaphragm by inspecting the vacuum line to the fuel pressure regulator for the presence of fuel.
An intermittent TP sensor output--An intermittent TP sensor output will cause the system to go rich, due to a false indication of the engine accelerating.
A shorted HO2S--If the HO2S is internally shorted the HO2S voltage displayed on the scan tool will be over 1 volt. Try disconnecting the affected HO2S and jumper the HO2S low signal circuit to ground with the key ON. If the displayed HO2S voltage changes from over 1,000 mV to around 450 mV, replace the HO2S. Silicon contamination of the HO2S can also cause a high HO2S voltage to be indicated. This condition is indicated by a powdery white deposit on the portion of the HO2S exposed to the exhaust stream. If contamination is evident, replace the affected HO2S. Refer to Heated Oxygen Sensor (HO2S) Replacement Bank 1 Sensor 2 , or to Heated Oxygen Sensor (HO2S) Replacement Bank 1 Sensor 1 , or to Heated Oxygen Sensor (HO2S) Replacement Bank 2 Sensor 1 .
Open HO2S High Signal or HO2S Low Signal Circuit or Poor HO2S--A poor connection or open in the HO2S high signal or low signal circuit can cause the DTC to set during deceleration fuel mode. An HO2S which is faulty and not allowing a full voltage swing between the rich and lean thresholds can also cause this condition. Operate the vehicle while monitoring the HO2S voltage with a scan tool. If the HO2S voltage is limited within a range between 300-600 mV, inspect the HO2S high and low signal circuit wiring and associated terminal connections. If the wiring and connections are OK, replace the HO2S. Refer to Heated Oxygen Sensor (HO2S) Replacement Bank 1 Sensor 2 , Heated Oxygen Sensor (HO2S) Replacement Bank 1 Sensor 1 , or Heated Oxygen Sensor (HO2S) Replacement Bank 2 Sensor 1 .

If the condition is intermittent, refer to Intermittent Conditions .

Test Description

The numbers below refer to the step numbers on the diagnostic table.

This DTC setting during deceleration fuel mode operation may indicate a condition described in the Diagnostic Aids above. If this DTC test passes while the Failure Records conditions are being duplicated, an intermittent condition is indicated. Review the Failure Records vehicle mileage since the diagnostic test last failed. This may help determine how often the condition that caused the DTC to be set occurs.
The new PCM must be programmed.

Post HO2S High Voltage

Step	Action	Values	Yes	No
Schematic Reference: Engine Controls Schematics
1	Did you perform the Diagnostic System Check-Engine Controls?	--	Go to Step 2	Go to Diagnostic System Check - Engine Controls
2	Important During this step, observe the pre-catalyst HO2S voltage parameters. If the pre-catalyst parameters remain above 600 mV, refer to Diagnostic Aids. Operate the vehicle within the Conditions for Setting parameters. With a scan tool, observe the HO2S voltage parameter. Does the HO2S voltage remain more than the specified value?	999 mV	Go to Step 4	Go to Step 3
3	Operate vehicle within Failure Records conditions. Does the scan tool indicate this DTC failed this ignition?	--	Go to Step 4	Go to Diagnostic Aids
4	Disconnect the HO2S. Jumper HO2S low signal circuit to a good ground. Does scan tool indicate HO2S voltage near the specified value?	450 mV	Go to Diagnostic Aids	Go to Step 5
5	Turn OFF the ignition. Disconnect the PCM. Turn ON the ignition, with the engine OFF. Measure the voltage of the HO2S high signal circuit at the PCM harness connector using a DMM that is connected to a good ground. Does the voltage measure more than the specified value?	999 mV	Go to Step 6	Go to Step 7
6	Repair short to voltage in the HO2S high signal circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair?	--	Go to Step 8	--
7	Replace the PCM. Refer to Powertrain Control Module (PCM) Replacement . Did you complete the replacement?	--	Go to Step 8	--
8	Use the scan tool in order to clear the DTCs. Turn OFF the ignition for 30 seconds. Operate the vehicle within the Conditions for Running the DTC. Does the DTC run and pass?	--	Go to Step 9	Go to Step 2
9	With a scan tool, observe the stored information in Capture Info. Does the scan tool display any DTCs that you have not diagnosed?	--	Go to Diagnostic Trouble Code (DTC) List	System OK