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DTC P0505 (ISC): Idle Control System Malfunction

  1. Start and operate engine at idle. Check intake manifold, idle air control (IAC) solenoid 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 DVOM connected to chassis ground, backprobe ECM connector terminal No. 13. See appropriate wiring diagram in WIRING DIAGRAMS article. If voltage reading is 3 volts or more, go to next step. If voltage reading is less than 3 volts, go to step 8).
  3. Backprobe ECM wiring terminal No. 14. If voltage reading is 3 volts or more, go to next step. If voltage reading is less than 3 volts, go to step 8).
  4. Turn ignition off. Remove air intake boot. Disconnect IAC connector. Turn ignition on. Backprobe ECM connector terminal No. 13. If voltage reading is 10 volts or more, repair short in wiring between ECM and IAC. After repair, replace ECM. If voltage is less than 10 volts, go to next step.
  5. Backprobe ECM connector terminal No. 14. If voltage reading is 10 volts or more, check for short in wiring between ECM and IAC connector. After repair, replace ECM. If voltage reading is less than 10 volts, check for poor contact in ECM connector. If contact is poor, repair contact in ECM connector. If contact is okay, go to next step.
  6. Turn ignition off. Check resistance between IAC connector terminals No. 1 and 2, and terminals No. 2 and 3. If resistance is 20 ohms or more, replace IAC. If resistance is 5 ohms or less, replace IAC and ECM. If resistance is more than 5 ohms, but less than 20 ohms, go to next step.
  7. Remove IAC. Connect battery voltage to IAC terminal No. 2. Ground terminal No. 1. If IAC 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.
  8. Turn ignition off. Disconnect IAC connector. Turn ignition on. Check voltage between IAC connector terminal No. 2 and chassis ground. If voltage reading is 10 volts or more, go to next step. If voltage reading is less than 10 volts, repair open in wiring between IAC connector and ECM connector.
  9. Turn ignition off. Disconnect ECM connector. Check resistance between ECM connector terminal No. 14 and IAC connector terminal No. 1, and between ECM connector terminal No. 13 and IAC connector terminal No. 3. If resistance is more than one ohm, repair open in wiring between ECM and IAC connectors. If resistance is one ohm or less, go to next step.
  10. Check resistance between ground and ECM connector terminal No. 13 and No. 14. If resistance is 10 ohms or more, ensure IAC connector terminals are okay. Repair connector terminals as necessary. If resistance is less than 10 ohms, repair short in wiring between ECM and IAC connector.
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