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Home >> Buick >> 2001 >> Century Limited >> Repair and Diagnosis (Single Page) >> Engine Performance >> Testing & Diagnosis >> Engine Controls Self-Diagnostics - 3.1L & 3.4L >> Diagnostic Tests >> DTC P0506: Idle Speed - Low >> Diagnostic Procedure

Diagnostic Procedure

  1. Perform powertrain diagnostic system check. See POWERTRAIN DIAGNOSTIC SYSTEM CHECK  under SELF-DIAGNOSTIC SYSTEM. After performing powertrain diagnostic system check, go to next step.
  2. Start engine. Ensure all accessories are turned off. Using scan tool RPM control function, slowly increment engine speed to 1800 RPM, then to 600 RPM, then to 1800 RPM. If engine speed stabilizes to within 100 RPM of commanded RPM during test, go to next step. If engine speed does not stabilize to within 100 RPM of commanded RPM during test, go to step  4.
  3. Using scan tool, observe FREEZE FRAME/FAILURE RECORDS data for this DTC. Turn ignition off for 30 seconds. Start engine. Operate the vehicle within conditions for setting DTC or as close to FREEZE FRAME/FAILURE RECORDS data as possible. If DTC fails this ignition cycle, go to next step. If DTC does not fail this ignition cycle, go to DIAGNOSTIC AIDS .
  4. Turn ignition off. Disconnect IAC valve harness connector located on throttle body. Connect IAC Motor Driver (J 37027-A) to IAC valve. Start engine. Using IAC motor driver, command IAC valve in until about 600 RPM is reached, then out until about 1800 RPM is reached. Return engine speed to desired idle speed as indicated on scan tool. If engine speed steadily increased and decreased to specified speeds when commanded, go to next step. If engine speed did not steadily increase and decrease to specified speeds when commanded, go to step  11.
  5. Turn ignition off. Using Connector Test Adapter Kit (J 35616-A), connect a test light between ground and one of the IAC valve harness connector terminals. Start engine. Using IAC motor driver, command low RPM while observing scan tool until IAC counts start to increment. Using IAC motor driver, command high RPM while observing scan tool until IAC counts start to increment. While IAC counts are incrementing, observe test light. Return engine speed to desired idle speed as indicated on scan tool. Repeat procedure for remaining 3 IAC valve circuits. If test light remained on (never flashing) while IAC counts were incrementing for any of the IAC valve circuits, go to step  10. If test light did not remain on, go to next step.
  6. If test light remained off (never flashing) while IAC counts were incrementing for any of the IAC valve circuits, go to step  9. If test light did not remain off, go to next step.
  7. Using Connector Test Adapter Kit (J 35616-A), connect test light between IAC valve harness connector terminals "C" (Light Blue/Black wire) and "D" (Light Blue/White wire). Using IAC motor driver, command low RPM while observing scan tool until IAC counts start to increment. Using IAC motor driver, command high RPM while observing scan tool until IAC counts start to increment. While IAC counts are incrementing, observe test light. Return engine speed to desired idle speed as indicated on scan tool. Repeat procedure with test light connected between IAC valve harness connector terminals "A" (Light Green/Black wire) and "B" (Light Green/White wire). If test light remained on (never flashing) while IAC counts were incrementing for either test, go to next step. If test light did not remain on, go to step  15.
  8. Inspect IAC valve harness connector for poor connections. Repair connector as necessary, then go to step  16. If connector is okay, go to DIAGNOSTIC AIDS .
  9. Turn ignition off. Disconnect PCM harness connectors. See PCM LOCATION . Check for open or short to ground in IAC valve circuit where test light remained off. See IAC VALVE CIRCUIT IDENTIFICATION table. Repair circuit as necessary, then go to step  16. If circuit is okay, go to step  12.
  10. Turn ignition on. Check for short to voltage in IAC valve circuit where test light remained on. See IAC VALVE CIRCUIT IDENTIFICATION table. Repair circuit as necessary, then go to step  16. If circuit is okay, go to step  12.
  11. Inspect throttle body for damage and tampering. Check throttle lever screw for tampering (if equipped). Check for restricted air intake system. Remove IAC valve. Check for clogged IAC passages, excessive deposits on throttle plate, throttle bore or IAC valve pintle. If a problem is found, repair as necessary, then go to step  16. If no problem is found, go to step  13.
  12. Inspect PCM harness connector for poor connections. If a problem is found, repair as necessary, then go to step  16. If no problem is found, go to step  15.
  13. Inspect IAC valve harness connector for poor connections. Repair connector as necessary, then go to step  16. If connector is okay, go to next step.
  14. Replace IAC valve. After repairs, go to step  16.
  15. Replace PCM. Program replacement PCM using required equipment. See PROGRAMMING . After repairs, go to next step.
  16. Using scan tool, clear DTCs. Turn ignition off and wait 30 seconds. Start engine and operate vehicle as close to conditions previously recorded in FREEZE FRAME as possible. If DTC resets, go to step  2. If DTC does not reset, go to next step.
  17. Using scan tool, observe all stored information. If scan tool displays any DTCs that have not been diagnosed, diagnose those DTCs. See DIAGNOSTIC TROUBLE CODE DEFINITIONS . If scan tool does not display any DTCs, system is okay.
IAC VALVE CIRCUIT IDENTIFICATION

IAC Valve Harness Connector Terminal Wire Color (1) PCM Harness Connector/Terminal No.
A Light Green/Black C2/49
B Light Green/White C1/38
C Light Blue/Black C2/44
D Light Blue/White C2/7
(1) See Figure or Figure .
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