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DTC P0505: Idle Control System Malfunction

NOTE: If DTC P1508 is set, trouble shoot DTC P1508 before trouble shooting DTC P0505.
  1. Start engine and run at 3000 RPM with no loads and transmission in Park or Neutral, until radiator fan comes on. Allow engine to idle. Using scan tool, check engine speed with no loads (headlights, blower motor, rear defogger, radiator cooling fan and A/C off). See IDLE SPEED SPECIFICATION  . If engine speed is not within specification, go to next step. If engine speed is within specification, problem is intermittent, system is okay at this time.
  2. If engine speed is not 650 RPM or less, go to step 3). If engine speed is 650 RPM or less, turn ignition off. Disconnect IAC valve connector. If engine speed does not drop or engine does not stall, replace IAC valve. If engine speed drops or engine stalls, adjust base idle speed. See the ADJUSTMENTS article. If engine speed does not adjust to specification, clean throttle body ports.
  3. Turn ignition off. Remove air intake duct from throttle body. Start engine and allow it to idle. Place a finger on lower port in throttle body. If engine speed does not decrease, go to next step. If engine speed decreases, check engine coolant level and for air in cooling system. If cooling system is okay, replace fast idle thermo valve.
  4. Place a finger on upper port in throttle body. If engine speed does not decrease, check for vacuum leaks and ensure throttle valve is completely closed. Repair as necessary. If idle speed decreases, adjust base idle speed to specification. See the ADJUSTMENTS article. If idle speed will not adjust to specification, replace IAC valve.
    IDLE SPEED SPECIFICATIONS

    Application RPM
    Prelude (1)(2)(3) 500-600
    (1)(2)(4) 650-750
    (1)(2)(5)(6) 740-840
    (1) USA models.
    (2) Canadian models.
    (3) With Idle Air Control (IAC) valve disconnected, transmission in Neutral or Park and no load (headlights, blower motor, rear defogger, A/C and cooling fan off). On Canadian models, set parking brake and ensure headlights are off.
    (4) Turn ignition off and reset PCM by removing CLOCK RADIO fuse, located in underhood fuse/relay box for 10 seconds. With Idle Air Control (IAC) valve connected, transmission in Neutral or Park and no load (headlights, blower motor, rear defogger, A/C and cooling fan off). On Canadian models, set parking brake and ensure headlights are off.
    (5) Check after one minute with headlights on low.
    (6) Turn headlights off and check after one minute with blower motor on HIGH and A/C on.
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