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Home >> GMC >> 2003 >> Safari Van Cargo, AWD >> Repair and Diagnosis >> External Pages >> Different car >> Section 1144 (Engine Controls Diagnosis - 6.0L - Alternative Fuels (DTC P0005-DTC P2665)) >> Diagnosis >> DTC P0171 or P0174: Fuel Trim System Lean Bank 1/2 >> Test Description

Test Description

WARNING: This page is about a different car, the 2004 GMC Sierra and 2004 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.

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

  1. 5:  The PCM will adjust fuel trim based upon fuel rail pressure (FRP) sensor values. A false high pressure FRP value will cause the PCM to inaccurately lean the fuel trim.
  2. 8:  If conditions were not corrected, a worn cam, worn intake or exhaust valves, or other engine mechanical failure may be the problem.
  3. 9:  The fuel injector control module (FICM) uses the Injector Monitor circuits in order to calculate CNG fuel injector amperage. An inaccurate even bank circuit amperage reading will cause a cylinder #6 misfire, bank 1 sensor 1 voltage to be near 900 mV and bank 2 sensor 1 voltage to be near 0 mV. An inaccurate odd bank circuit amperage reading will cause a cylinder #1 misfire, bank 1 sensor 1 voltage to be near 0 mV and bank 2 sensor 1 voltage to be near 900 mV.
DTC P0171 or P0174

Step Action Values Yes No
1 Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls? - Go to Step 2  Go to Diagnostic System Check - Alternative Fuels (Engine Controls)
2 Is this vehicle equipped with the dedicated compressed natural gas (CNG) option (KL8)? - Go to Step 4  Go to Step 3 
3 Review the Freeze Frame/Failure Records or interview the vehicle operator in order to determine if the DTC set while operating on CNG.
Did this DTC set while operating on CNG?		 - Go to Step 4  Go to DTC P0171 or P0174 in Engine Controls in the C/K Pickup Service Manual
4
IMPORTANT: If any DTCs other than P0171 or P0174 are set, refer to those DTCs before continuing.
  1. Install the scan tool.
    2. IMPORTANT: Ensure the engine is operating on CNG during all testing.
  2. Start and idle the engine at the normal operating temperature in Closed Loop.
  3. Record the long term fuel trim.
  4. Turn OFF the ignition and wait 30 seconds.
  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?		 28% Go to Step 5  Go to Diagnostic Aids
5 
  1. Operate the engine at idle.
  2. Observe the fuel rail pressure (FRP) sensor parameter with a scan tool.
Does the scan tool display fuel rail pressure more than the value specified?		 620 kPa (90 psi) Go to DTC P0191  Go to Step 6 
6 Observe the scan tool display for the following conditions:
  • Sensor 1 heated oxygen sensor (HO2S) voltage of one bank fixed very high
  • Sensor 1 HO2S voltage of the other bank fixed very low
  • A cylinder #1 OR a cylinder #6 misfire

Are ALL of the above conditions present?		 - Go to Step 9  Go to Step 7 
7
  1. Turn OFF the engine.
  2. Visually and physically inspect the following items:
    • The vacuum hoses for splits, kinks, and proper connections. Refer to the Underhood Emission Hose Routing label.
    • The HO2S sensors for proper installation.
    • The HO2S electrical connectors and the wires are secure, and not contacting the exhaust system.
    • Test for continuity between the HO2S signal circuit and the low reference circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.
Did you find and correct the condition?		 - Go to Step 12  Go to Step 8 
8 
  1. Operate the engine at idle.
  2. Inspect for any missing, loose, or leaking exhaust components forward of the HO2S.
  3. Inspect for vacuum leaks at the intake manifold, throttle body, and injector O-rings.
  4. Inspect the air induction system and the air intake ducts for leaks.
  5. Inspect the crankcase ventilation system for leaks. Refer to Crankcase Ventilation System Inspection/Diagnosis in Engine Mechanical in the C/K Pickup Service Manual.
Did you find and correct the condition?		 - Go to Step 12  Go to Symptoms - Engine Mechanical in Engine Mechanical in the C/K Pickup Service Manual
9 
  1. Turn the ignition OFF.
  2. Disconnect the fuel injector control module (FICM) connector. Refer to Fuel Injection Control Module Replacement .
  3. Test the odd bank or even bank injector monitor circuit for an open or high resistance condition. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.
Did you find and correct the condition?		 - Go to Step 12  Go to Step 10 
10 Test for an intermittent and for a poor terminal connection at the FICM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems in the C/K Pickup Service Manual.
Did you find and correct the condition?		 - Go to Step 12  Go to Step 11 
11 Replace the FICM. Refer to Fuel Injection Control Module Replacement .
Did you complete the replacement?		 - Go to Step 12  -
12
IMPORTANT: After 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 13 
13 Observe the 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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