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Home >> Buick >> 2004 >> Century Base >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls (DTC P0140 To P0496) >> Engine Controls Diagnosis >> DTC P0171 >> Test Description
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Test Description

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

  1. 5:  If the conditions were not corrected, refer to Fuel System Diagnosis for a possible fuel condition.
  2. 6:  If the conditions were not corrected, a worn cam, worn intake or exhaust valves, or other engine mechanical failures may be at fault.
DTC P0171

Step Action Values Yes No
1 Did you perform the Diagnostic System Check-Engine Controls? - Go to Step 2  Go to Diagnostic System Check - Engine Controls
2
IMPORTANT: If any DTCs are set, other than DTC P0171, refer to those DTCs before continuing.
  1. Install a scan tool.
  2. Start and idle the engine at the normal operating temperature in Closed Loop.
  3. Record the long term fuel trim data.
  4. Turn OFF the engine.
  5. Turn ON the ignition, with the engine OFF.
  6. Review the Freeze Frame/Failure Records and record the displayed data for this DTC.
Does the scan tool indicate that the long term fuel trim is greater than the specified value?		 23% Go to Step 3  Go to Diagnostic Aids
3
  1. Operate the engine at idle.
  2. Observe the HO2S parameters with a scan tool.
Does the scan tool indicate that the values are within the specified range and fluctuating?		 200-800 mV Go to Step 4  Go to Step 5 
4
  1. Turn OFF the engine.
  2. Visually and physically inspect the following conditions:
Did you find and correct the condition?		 - Go to Step 7  Go to Step 6 
5 
  1. Turn OFF the engine.
  2. Inspect for the following conditions:
    • Check the HO2S for proper installation.
    • Ensure that the electrical connectors and the wires are secure and not contacting the exhaust system.
    • Test for continuity between the signal circuit and the low reference circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?		 - Go to Step 7  Go to Fuel System Diagnosis
6 
  1. Operate the engine at idle.
  2. Inspect for the following conditions:
    • Check for missing, loose, or leaking exhaust components forward of the HO2S.
    • Check for vacuum leaks at the intake manifold, the throttle body and the injector O-rings.
    • Check the air induction system and the air intake ducts for leaks.
    • Check the secondary air injection (AIR) system for leaks, improper air delivery, and for the shut-off valve not closing.
    • Check the crankcase ventilation system for leaks. Refer to Crankcase Ventilation System Inspection/Diagnosis in Engine Mechanical.
Did you find and correct the condition?		 - Go to Step 7  Go to Symptoms - Engine Mechanical in Engine Mechanical
7
IMPORTANT: After any of the above repairs, use the scan tool Fuel Trim Reset function in order to reset the long term fuel trim.
  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 8 
8 Observe Capture Info with a scan tool.
Are there any DTCs that have not been diagnosed?		 - Go to Diagnostic Trouble Code (DTC) List System OK
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