DTC P0138. GMC Truck Canyon Pickup

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 2005 🢒 Canyon Pickup - 2WD 🢒 Engine 🢒 Engine Controls - 3.5L (L52) 🢒 DTC P0138

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 Bank 1 Sensor 2

Conditions for Running the DTC

Rich Test Enable

•	DTCs P0068, P0101, P0102, P0103, P0106, P0107, P0108, P0117, P0118, P0120, P0201, P0202, P0203, P0204, P0205, P0220, P0442, P0443, P0446, P0449, P0452, P0453, P0455, P0496, P1516, P2101, P2135, 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 0-50 percent.

•	DTC P0138 runs 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, P0201, P0202, P0203, P0204, P0205, P0220, P0442, P0443, P0446, P0449, P0452, P0453, P0455, P0496, P1516, P2101, P2135, 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 cut-off (DFCO) is active.

•	DTC P0138 runs the decel fuel cutoff test continuously when the above conditions are met for 2 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 within 100 seconds during the rich test 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 within 10 seconds during the decel fuel cutoff test 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.

If the voltage does not change more that the specified value, the condition is present.

Step

Action

Value(s)

Yes

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Powertrain Control Module Connector End Views or Engine Controls Connector End Views

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

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 .

Start the engine.
Allow the engine to reach operating temperature. Refer to Scan Tool Data List .
Operate the engine at 1,500 RPM for 30 seconds.
While observing the HO2S 2 voltage parameter with a scan tool, quickly cycle the throttle from closed throttle to wide open throttle, 3 times.

Did the HO2S 2 voltage parameter change more than the specified value?

200 mV

Go to Step 3

Go to Step 4

Observe the Freeze Frame/Failure Records for this DTC.
Turn OFF the ignition for 30 seconds.
Start the engine.
Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

Go to Step 4

Go to Intermittent Conditions

Turn OFF the ignition.
Disconnect the heated oxygen sensor (HO2S) 2.
Turn ON the ignition, with the engine OFF.
Observe the HO2S 2 voltage parameter with a scan tool.

Is the HO2S 2 voltage parameter more than the specified value?

800 mV

Go to Step 6

Go to Step 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 in Wiring Systems.

Is the voltage more than the specified value?

2 V

Go to Step 7

Go to Step 8

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 , Heated Oxygen Sensor Wiring Repairs , and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 17

Go to Step 14

Test the HO2S 2 low signal circuit for a short to voltage. Refer to Circuit Testing , Heated Oxygen Sensor Wiring Repairs , and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 17

Go to Step 14

Connect a 3-amp fused jumper wire between the high signal circuit of the HO2S 2 harness connector on the engine harness side and a good ground.
Observe the HO2S 2 voltage parameter with a scan tool.

Is the HO2S 2 voltage parameter less than the specified value?

100 mV

Go to Step 9

Go to Step 11

Remove the jumper wire from the previous step.
Connect a 3-amp fused jumper wire between the high signal circuit of the HO2S 2 harness connector on the engine harness side and the low signal circuit of the HO2S 2 harness connector on the engine harness side.
Observe the HO2S 2 voltage parameter with a scan tool.

Is the HO2S 2 voltage parameter less than the specified value?

100 mV

Go to Step 12

Go to Step 10

Test the HO2S 2 low signal circuit for an open or high resistance. Refer to Circuit Testing , Heated Oxygen Sensor Wiring Repairs , and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 17

Go to Step 14

Test the HO2S 2 high signal circuit for an open high resistance. Refer to Circuit Testing , Heated Oxygen Sensor Wiring Repairs , and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 17

Go to Step 14

The HO2S 2 may be detecting a rich exhaust condition or may be contaminated. Inspect for the following conditions:

Notice: Refer to Silicon Contamination of Heated Oxygen Sensors Notice in the Preface section.

•	A silicon contaminated HO2S 2

•	Any water intrusion into the HO2S 2 connector

•	Engine oil contaminated with fuel

•	An EVAP canister purge condition

•	An incorrect fuel pressure--Refer to Fuel System Diagnosis .

•	Any rich fuel injectors--Refer to Fuel Injector Balance Test with Special Tool .

•	An air intake restriction or collapsed air intake duct

Repair any of the above or similar engine conditions as necessary.

Did you find and correct the condition?

Go to Step 17

Go to Step 13

Test for shorted terminals and for poor connections at the HO2S 2. Refer to Testing for Intermittent Conditions and Poor Connections , Heated Oxygen Sensor Wiring Repairs , and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 17

Go to Step 15

Test for shorted terminals and for poor connections at the powertrain control module (PCM). Refer to Testing for Intermittent Conditions and Poor Connections , Heated Oxygen Sensor Wiring Repairs , and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 17

Go to Step 16

Notice: Refer to Heated Oxygen Sensor Resistance Learn Reset Notice in the Preface section.

Replace the HO2S 2. Refer to Heated Oxygen Sensor 1 Replacement or Heated Oxygen Sensor 2 Replacement .

Did you complete the replacement?

Go to Step 17

Replace the PCM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming.

Did you complete the replacement?

Go to Step 17

Clear the DTCs with a scan tool.
Turn OFF the ignition for 30 seconds.
Start the engine.
Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

Go to Step 2

Go to Step 18

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information

System OK