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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 108 (Engine Controls Self-Diagnostics - 5.7L Corvette) >> Diagnostic Tests >> DTC P1639: 5-Volt Reference B Circuit >> Diagnostic Procedures

Diagnostic Procedures

WARNING: This page is about a different car, the 2002 Chevrolet Corvette. 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, see DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM.
  2. Observe the Freeze Frame and/or Failure Records data for this DTC. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions for Running DTC or as close to the Freeze Frame and/or Failure Records data that you observed. Does the DTC fail this ignition? If yes, go to next step. If no, see INTERMITTENT CONDITIONS  under SELF-DIAGNOSTIC SYSTEM.
  3. Inspect the PCM and engine grounds. Ensure that the grounds are clean and secure. If a problem is found, repair as necessary. Was a problem found? If yes, go to step  12. If no, go to next step.
  4. Turn OFF the ignition. Disconnect the A/C pressure sensor electrical connector. Turn ON the ignition, with the engine OFF. Using a Digital Multimeter (DMM), measure the voltage between the 5-volt reference circuit and engine ground. Is voltage near 5 V? If yes, go to step  6. If no, go to next step.
  5. Is voltage more than 5 V? If yes, go to step  9. If no, go to step  7.
  6. Reconnect A/C pressure sensor. Disconnect FTP sensor. Using a DMM, measure the voltage between the 5-volt reference B circuit and engine ground. Is voltage near 5 V? If yes, see INTERMITTENT CONDITIONS  under SELF-DIAGNOSTIC SYSTEM. If no, go to step  10.
  7. Monitor DMM while disconnecting the FTP sensor. If voltage changes when the sensor is disconnected, replace the component. Refer to Fuel Tank Pressure Sensor Replacement Was a component replaced? If yes, go to step  12. If no, go to next step.
  8. Turn OFF the ignition. Disconnect the PCM. Check the 5-volt reference circuit for a short to ground or any sensor low reference circuit. If a problem is found, repair as necessary. Was a problem found? If yes, go to step  12. If no, go to step  11.
  9. Turn OFF the ignition. Disconnect the PCM. Turn ON the ignition, with the engine OFF. Check the following circuits for a short to voltage:
    • The 5-volt reference circuit.
    • The FTP sensor signal circuit.

    If a problem is found, repair as necessary. Was a problem found? If yes, go to step  12. If no, go to step  11.

  10. Replace the A/C pressure sensor. Is the action complete? After repairs, go to step  12.
  11. Replace the PCM. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. Reprogram PCM. See POWERTRAIN CONTROL MODULE  under PROGRAMMING. After repairs, go to next step.
  12. 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 DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step  2.
  13. Observe the stored information, Capture Info with a scan tool. Does the scan tool display any DTCs that you have not diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay.
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