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Home >> Buick >> 2007 >> Allure CXS >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System & Fuel System - 3.6L - DTC P0135 To P0420 >> DTC P0138 >> Test Description
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Test Description

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

  1. 2:  This step determines if the condition exists. If there is an HO2S circuit condition, the HO2S voltage will be more than the specified value.
  2. 4:  This step isolates the condition. If the voltage is more than the specified with the engine OFF, this indicates there is a circuit condition.
  3. 5:  This step isolates if the condition is with the signal circuit or the low reference circuit of the HO2S. If the signal circuit voltage is within the specified range, the low reference circuit is short to a voltage.
  4. 8:  This step test for a fuel system that is operating rich.
  5. 9:  This step inspects the harness connector for water intrusion. Moisture in a connector may cause voltage to bleed to other circuits.
  6. 10:  This step inspects the harness connector for water intrusion. Moisture in a connector may cause voltage to bleed to other circuits.
DTC P0138

Step Action Values Yes No
Schematic Reference:  Engine Controls Schematics 
Connector End View Reference:  Engine Control Module Connector End Views  or  Engine Controls Connector End Views 
1 Did you perform the Diagnostic System Check - Vehicle? - Go to Step 2  Go to Diagnostic System Check - Vehicle in Vehicle DTC Information
2 
  1. Allow the engine to reach operating temperature.
  2. Observe the HO2S 2 voltage parameter with a scan tool.
Is the voltage more than the specified value?		 1,050 mV Go to Step 4  Go to Step 3 
3
  1. Observe the Freeze Frame/Failure Records for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. 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 Diagnostic Aids
4 
  1. Turn OFF the engine.
  2. Turn ON the ignition, with the engine OFF.
  3. Observe the HO2S voltage parameter with a scan tool.
Is the voltage more than the specified value?		 1,050 mV Go to Step 5  Go to Step 8 
5 
  1. Disconnect the heated oxygen sensor (HO2S).
  2. Measure the voltage from the signal circuit of the HO2S and a good ground.
Is the voltage within the specified range?		 350-550 mV Go to Step 6  Go to Step 7 
6
IMPORTANT: The engine control module (ECM) may be damaged if the circuit is shorted to B+.
Test the low reference circuit of the HO2S for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.Did you find and correct the condition?		 - Go to Step 13  Go to Step 10 
7
IMPORTANT: The HO2S may be damaged if the circuit is shorted to B+.
Test the signal circuit of the HO2S for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.Did you find and correct the condition?		 - Go to Step 13  Go to Step 10 
8  Test or inspect for the following conditions:
Did you find and correct the condition?		 - Go to Step 13  Go to Step 9 
9  Test for shorted terminals and poor connections at the HO2S. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.
Did you find and correct the condition?		 - Go to Step 13  Go to Step 11 
10  Test for shorted terminals and poor connections at the ECM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.
Did you find and correct the condition?		 - Go to Step 13  Go to Step 12 
11 Replace the HO2S. Refer to Heated Oxygen Sensor 2 Replacement .
Did you complete the replacement?		 - Go to Step 13  -
12 Replace the ECM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming.
Did you complete the replacement?		 - Go to Step 13  -
13
  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. 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 14 
14 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
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