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HOMESERVICE MANUALSSUBARU1998FORESTER L, STANDARDREPAIR AND DIAGNOSISENGINE PERFORMANCESYSTEMENGINE CONTROLS - TESTS W/CODESDIAGNOSTIC TESTSDTC P0505 (ISC): IDLE CONTROL SYSTEM MALFUNCTION
1998 Subaru Forester L, Standard

DTC P0505 (ISC): Idle Control System Malfunction

1998 Subaru Forester L, StandardSECTION DTC P0505 (ISC): Idle Control System Malfunction
  1. Start and operate engine at idle. Check intake manifold, idle air control (IAC) valve and throttle body for loose installation and gaskets for cracks. Check by-pass and vacuum hoses for loose connections and cracks. Repair or replace as necessary. If air intake components are okay, go to next step.
  2. Turn ignition on, with engine off. Using voltmeter connected to ground, backprobe PCM connector terminal No. 13 (Black wire). If voltage reading is more than 3 volts, go to next step. If voltage reading is 3 volts or less, go to step 8).
  3. With voltmeter still connected to ground, backprobe PCM connector terminal No. 14 (White/Black wire). If reading is more than 3 volts, go to next step. If reading is 3 volts or less, go to step 8).
  4. Turn ignition off. Disconnect IAC valve connector. Turn ignition on. Backprobe PCM connector terminal No. 13 (Black wire). If voltage is 10 volts or less, go to next step. If voltage reading is more than 10 volts, check for short in wiring between PCM and IAC. After repairing short, check PCM operation and replace if necessary.
  5. Backprobe PCM connector terminal No. 14 (White/Black wire). If voltage reading is more than 10 volts, check for short in wiring between PCM and IAC valve connector. After repairing short, check PCM operation and replace if necessary. If voltage reading is 10 volts or less, check for poor contact in PCM connector. If connection is okay, go to next step.
  6. Turn ignition off. Check resistance of IAC valve by measuring between terminals No. 1 and 2, and terminals No. 2 and 3. See Fig 1. If resistance is between 5 and 20 ohms, go to next step. If resistance is more than 20 ohms, replace IAC valve. If resistance is less than 5 ohms, replace IAC valve and PCM.
  7. Remove IAC valve. Connect battery voltage to IAC valve terminal No. 2. Ground terminal No. 1. If IAC valve does not open fully, clean IAC. If IAC opens fully, ground terminal No. 3. If IAC closes fully, go to next step. If IAC does not close fully, clean IAC valve.
  8. Turn ignition off. Disconnect IAC valve connector. Turn ignition on. Measure voltage between ground and IAC valve connector terminal No. 2 (Yellow/Red wire). If voltage reading is more than 10 volts, go to next step. If voltage reading is 10 volts or less, check for an open in harness between IAC valve connector and main relay connector terminal No. 4.
  9. Turn ignition off. Disconnect PCM connector. Check resistance of harness between PCM connector terminal No. 14 (White/Black wire) and IAC valve connector terminal No. 1 (White wire). Check resistance of harness between PCM connector terminal No. 13 and IAC valve connector terminal No. 3 (Black wire). If resistance is more than one ohm, repair open in wiring between PCM and IAC valve connectors. If resistance is one ohm or less, go to next step.
  10. Check resistance between ground and PCM connector terminal No. 13 (Black wire). Check resistance between ground and PCM connector terminal No. 14 (White/Black wire). If resistance is more than 10 ohms, check IAC valve connector terminals for poor contact. Repair connector terminals as necessary. If resistance is 10 ohms or less, repair short in wiring between PCM and IAC valve connector.
Fig 1: Identifying Terminals Of IAC Valve
G98J12343Courtesy of SUBARU OF AMERICA, INC.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.