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Home >> Lexus >> 1998 >> SC 300 >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Tests W/Codes >> Diagnostic Tests >> DTC P0505: Idle Control System Malfunction >> Diagnosis & Repair
July 1: So it begins. Don't worry, this is not in the correct legal jurisdiction!

Diagnosis & Repair

  1. Retrieve trouble codes. See RETRIEVING DIAGNOSTIC TROUBLE CODES  under SELF-DIAGNOSTIC SYSTEM. If only DTC P0505 is displayed, go to next step. If other codes are displayed, diagnose and repair those codes first and retest.
  2. Check for loose, damaged or cracked air intake hose between throttle body and air cleaner. Check for loose or damaged dipstick, oil fill cap or PCV hose. Repair as necessary. If all components are okay, go to next step.
  3. Ensure all accessories are off. Ensure throttle linkage moves smoothly. Turn ignition on. Rotate accelerator pedal position sensor lever and listen for running sound from throttle motor. Ensure friction sound does not exist. If running sound exists, go to step 5). If running sound does not exist, check throttle control motor. See SYSTEM/COMPONENT TESTS article.
  4. If throttle control motor is okay, check for open or short circuit in wiring harness between throttle control motor and ECM. See appropriate wiring diagram in SYSTEM WIRING DIAGRAMS article. Repair wiring as necessary. If wiring is okay, go to next step.
  5. Using Lexus scan tool, connect Lexus scan tool to Data Link Connector (DLC) No. 3. See Figure. Switch Lexus scan tool to CURRENT DATA. Ensure MIL does not illuminate. Rotate accelerator pedal position sensor lever to full open position. THROTTLE POS on Lexus scan tool should indicate 60 percent or more. If results are as specified, go to step 7). If results are not as specified, check accelerator pedal position sensor. See SYSTEM/COMPONENT TESTS article.
  6. If accelerator pedal position sensor is okay, check for open or short circuit in wiring harness between accelerator pedal position sensor and ECM. See appropriate wiring diagram in SYSTEM WIRING DIAGRAMS article. Repair wiring as necessary.
  7. Clear trouble codes. See CLEARING DIAGNOSTIC TROUBLE CODES  under SELF-DIAGNOSTIC SYSTEM. Start engine and ensure MIL does not illuminate. Allow engine to reach normal operating temperature. Ensure all accessories are off. Place transmission in Neutral position. Turn A/C on and then off, and check idle speed. Idle speed should be between 700-800 with A/C on and 650-750 with A/C off.
  8. With engine idling, pinch air assist hose located next to throttle motor. See Fig 1. Idle speed should drop and return back to normal. If idle speed changes as specified, go to next step. If idle speed does not change as specified, check throttle body. If throttle body is okay, check for open or short circuit in wire harness. See appropriate wiring diagram in SYSTEM WIRING DIAGRAMS article. Repair wiring as necessary. If wiring is okay, go to next step.
  9. Drive vehicle to ensure vehicle performs properly. Retrieve trouble codes. See RETRIEVING DIAGNOSTIC TROUBLE CODES  under SELF-DIAGNOSTIC SYSTEM. If DTC P0505 is displayed or vehicle does not perform as specified, replace ECM and retest.
Fig 1: Identifying Air Assist Hose Location
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