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Home >> Buick >> 2002 >> Century Custom >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System Self-Diagnostics - 3.1L Engine >> Diagnostic Tests >> DTC P1635: 5-Volt Reference Low >> Diagnostic Procedures
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Diagnostic Procedures

  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 data for this DTC. Turn ignition off for 30 seconds. Start engine. Operate the vehicle within the Conditions for Running the DTC or as close to the Freeze Frame/Failure Records data that you observed. See CONDITIONS FOR RUNNING DTC . Does the DTC fail this ignition? If yes, go to next step. If no, go to 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. Did you find and correct the condition? If yes, go to step  13. If no, go to next step.
  4. Turn ignition off. Disconnect the EGR valve electrical connector. Turn ignition on, with engine off. Using a Digital Multimeter (DMM), measure voltage between the 5-volt reference circuit and engine ground. Does the voltage measure about 5 volts? If yes, go to step  6. If no, go to next step.
  5. Does the voltage measure more than 5 volts? If yes, go to step  9. If no, go to step  7.
  6. Reconnect EGR valve. Disconnect throttle position sensor. Using a DMM, measure voltage between the 5-volt reference 1 circuit and engine ground. Does the voltage measure about 5 volts? If yes, see INTERMITTENT CONDITIONS  under SELF-DIAGNOSTIC SYSTEM. If no, go to step  10.
  7. Monitor DMM while disconnecting all other devices connected to 5-volt reference circuit one at a time. If voltage changes when one of the components are disconnected, replace the component. Was a component replaced? If yes, go to step  13. If no, go to next step.
  8. Turn ignition off. Disconnect the PCM. Inspect the 5-volt reference circuit for a short to ground or any sensor low reference circuit. If a problem is found, repair as necessary. Did you find and correct the condition? If yes, go to step  13. If no, go to step  12.
  9. Turn ignition off. Disconnect the PCM. Turn ignition on, with engine off. Inspect the following circuits for a short to voltage: The 5-volt reference circuit, MAP sensor signal circuit, or FTP sensor signal circuit. If a problem is found, repair as necessary. Did you find and correct the condition? If yes, go to step  13. If no, go to step  12.
  10. Inspect for a short between the EGR control high circuit and the 5-volt reference circuit and repair as necessary. Did you find and correct the condition? If yes, go to step  13. If no, go to next step.
  11. Replace the EGR valve. After repairs, go to step  13.
  12. Replace PCM. Perform PCM relearn procedure. See POWERTRAIN CONTROL MODULE  under PROGRAMMING. After replacing PCM, go to next step.
  13. Use the scan tool in order to clear the DTCs. Turn ignition off for 30 seconds. Start engine. Operate the vehicle within the Conditions for Running the DTC. See CONDITIONS FOR RUNNING DTC . Does the DTC run and pass? If yes, go to next step. If no, go to step  2.
  14. Using scan tool, observe stored information and Capture Info. 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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