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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 78 (Engine Controls - Self-Diagnostics - 4.8L, 5.3L & 6.0L) >> Diagnostic Tests >> DTC P0178: Fuel Composition Sensor >> Diagnostic Procedure

Diagnostic Procedure

WARNING: This page does not describe the selected car, but rather 11 other vehicles, including the 2002 GMC Yukon XL, 2002 GMC Yukon, 2002 GMC Sierra, 2002 GMC Cab & Chassis Sierra, and 2002 Chevrolet Tahoe. However, it is still accessible from the selected car via links, so may be relevant.
  1. Did you perform the Diagnostic System Check-Engine Controls? If yes, go to next step. If no, go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS .
    1. Start the engine.
    2. Observe the Fuel Comp Sensor Frequency parameter with a scan tool.
    Is the frequency below 45 Hz? If yes, go to Step  4. If no, go to next step.
    1. Observe the Freeze Frame/Failure Records for the 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 failed this ignition? If yes, go to next step. If no, go to Diagnostic Aids .
    1. Turn OFF the ignition.
    2. Disconnect the fuel composition sensor (FCS) harness connector.
    3. Turn ON the ignition, with the engine OFF.
    4. Probe the ignition 1 circuit of the FCS harness connector with a test lamp that is connected to a good ground.
    Does the test lamp illuminate? If yes, go to next step. If no, go to Step  9.
  5. Connect a test lamp between the ignition 1 voltage circuit and the ground circuit of the FCS harness connector. Does the test lamp illuminate? If yes, go to next step. If no, go to Step  10.
  6. Measure the voltage from the fuel temperature/composition signal circuit to a good ground with a DMM. Is the voltage within 4.3-5.2 V? If yes, go to next step. If no, go to Step  8.
    1. Turn OFF the ignition.
    2. Connect the power lead of the Variable Signal Generator (J 38522) to the vehicle.
    3. Connect the red test lead of the Variable Signal Generator to the signal circuit of the FCS connector and the black test lead to a good ground.
    4. Set the Duty Cycle switch of the Variable Signal Generator to the normal position.
    5. Set the Frequency switch of the Variable Signal Generator to 60 Hz.
    6. Set the Signal switch of the Variable Signal Generator to 5 volts.
    7. Start the engine and allow the engine to idle.
    8. Observe the FCS Frequency parameter with a scan tool.
    Is the FCS Frequency parameter at 60 Hz? If yes, go to Step  14. If no, go to Step  11.
  8. Test the fuel temperature/composition signal circuit for the following:
    1. An open
    2. A short to ground
    3. A short to voltage
    Did you find and correct the condition? If yes, go to Step  16. If no, go to Step  12.
  9. Test the ignition 1 voltage circuit for an open or short to ground. Did you find and correct the condition? If yes, go to Step  16.
  10. Test the ground circuit for an open. Did you find and correct the condition? If yes, go to Step  16.
  11. Test the FCS signal circuit for an open or for high resistance. Did you find and correct the condition? If yes, go to Step  16. If no, go to next step.
  12. Test for an intermittent and for a poor connection at the powertrain control module (PCM). Did you find and correct the condition? If yes, go to Step  16. If no, go to next step.
  13. Replace the PCM. Refer to POWERTRAIN CONTROL MODULE . Did you complete the replacement? If yes, go to Step  16.
  14. Test for an intermittent and for a poor connection at the FCS. Did you find and correct the condition? If yes, go to Step  16. If no, go to next step.
  15. Replace the FCS. Did you complete the replacement? If yes, go to next step.
    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? If yes, go to Step  2. If no, go to next step.
  17. Observe the Capture Info with a scan tool. Are there any DTC's that have not been diagnosed? If yes, go to DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, System OK.
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