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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 239 (ENGINE CONTROLS SELF-DIAGNOSTICS - 3.1L ENGINE)DIAGNOSTIC TESTSDTC P1635: 5-VOLT REFERENCE 1 CIRCUIT (MALIBU)DIAGNOSTIC PROCEDURE
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedure

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedure
WARNING: This page is about a different car, the 2003 Pontiac Grand Prix, 2003 Chevrolet Malibu, and 2003 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
  1. Perform diagnostic system check - engine controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
  2. Observe the Freeze Frame/Failure Records for this DTC. Turn ignition switch to OFF position for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running Test. See CONDITIONS FOR RUNNING TEST . You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Does the DTC fail this ignition? If yes, go to next step. If no, go to INTERMITTENTS in TROUBLE SHOOTING - NO CODES - 3.1L CENTURY, GRAND PRIX & MALIBU article.
  3. Turn ignition switch to OFF position. Disconnect the Exhaust Gas Recirculation (EGR) valve position sensor harness connector. Turn ignition switch to RUN position, engine OFF. Command the EGR solenoid to 100 percent using a scan tool. Measure the voltage from the 5-volt reference circuit to a good ground using a DMM. Is the voltage 4.8-5.2 volts? If yes, go to step  5. If no, go to next step.
  4. Is the voltage measured in the previous step more than 5.2 volts? If yes, go to step  8. If no, go to step  6.
  5. Connect the EGR valve harness connector. Disconnect the Manifold Absolute Pressure (MAP) sensor. Command the EGR solenoid to 100 percent using a scan tool. Measure the voltage from the 5-volt reference circuit of the MAP sensor to a good ground using a DMM. Is the voltage 4.8-5.2 volts? If yes, go to DIAGNOSTIC AIDS . If no, go to step  11.
  6. Monitor a DMM while disconnecting all other devices connected to the 5-volt reference circuit, one at a time. If voltage changes when one of the devices are disconnected (MAP, FTP or TP sensors), replace the component. Was a component replaced? If yes, go to step  13. If no, go to next step.
  7. Turn ignition switch to OFF position. Disconnect the PCM harness connectors. Test the 5-volt reference circuit for a short to ground or any sensor low reference circuit. Did you find and correct the condition? If yes, go to step  13. If no, go to step  12.
  8. Monitor the DMM while disconnecting the MAP sensor and FTP sensor, one at a time. If the voltage returns to within 4.8-5.2 volts when one of the devices are disconnected, test that sensors signal circuit for a short to voltage. Did you find and correct the condition? If yes, go to step  13. If no, go to next step.
  9. Test the 5-volt reference circuits for a short to voltage. Did you find and correct the condition? If yes, go to step  13. If no, go to next step.
  10. Test for a short between the EGR 5-volt reference circuit and the EGR control high circuit. Did you find and correct the condition? If yes, go to step  13. If no, go to step  12.
  11. Replace the EGR valve. After repair, go to step  13.
  12. Replace PCM. See POWERTRAIN CONTROL MODULE in appropriate REMOVAL & INSTALLATION article. Perform PCM relearn procedure. See POWERTRAIN CONTROL MODULE  under PROGRAMMING. After replacing PCM, go to next step.
  13. Using scan tool, clear DTCs. Turn ignition switch to OFF position for 30 seconds. Start the engine. Operate vehicle within Conditions For Running Test. See CONDITIONS FOR RUNNING TEST . Did the DTC fail this ignition? If yes, go to step  2. If no, go to next step.
  14. Using the scan tool, observe the stored information in CAPTURE INFO. Does scan tool display any DTCs that you have not diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay at this time.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.