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Home › Lexus › 1998 › SC 400 › Repair and Diagnosis › Engine Performance › System › Engine Controls - Tests W/Codes › Diagnostic Tests › DTC P0505: Idle Control System Malfunction › Diagnosis & Repair

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. Remove engine cover. 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. Ensure all accessories are off. If not using Lexus scan tool, go to step 7). If 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 9). 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. Backprobing, measure voltage between ECM E16 connector terminals No. 21 (Yellow/Red wire) and No. 18 (Brown wire). With ignition switch in ON position and throttle fully closed, voltage should be .3-.9 volt. With ignition switch in ON position and throttle fully open, voltage should be 3.2-4.8 volts. If voltage is as specified, go to next step. If voltage is not as specified, check accelerator pedal position sensor. See SYSTEM/COMPONENT TESTS article.
  8. 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.
  9. 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. Place transmission in Neutral position. Turn A/C on and then off and check idle speed. Idle speed should be 700-800 RPM.
  10. 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.
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