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Home >> Toyota >> 1999 >> Corolla LE, Automatic >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System Self-Diagnostics >> Diagnostic Tests >> DTC P0505: Idle Air Control (IAC) System Circuit >> Diagnosis & Repair (Using OBD-II Scan Tool)
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Diagnosis & Repair (Using OBD-II Scan Tool)

  1. Start engine and warm to normal operating temperature. Turn all accessories off. Ensure A/C is off. Shift transaxle lever into Neutral. Connect scan tool to Data Link Connector (DLC) No. 3. See Figure. Note engine RPM. Using a jumper wire, connect terminals TE1 and E1 at DLC No. 1 in engine compartment. See Fig 1. Note engine RPM and compare RPM readings. If difference in engine speed is more than 100 RPM, go to step  8. If difference in engine speed is 100 RPM or less, go to next step.
  2. Access ECM behind stereo on instrument panel, just in front of center console. Connect oscilloscope between ground and terminal No. 10 (Black/Blue wire) at ECM harness connector E4. See Figure. Start engine. Ensure engine is at normal operating temperature. Oscilloscope should display a square wave pattern that toggles from about 4.5 volts at 4 milliseconds to about 0.5 volt at 2 milliseconds. If IAC duty signal output is okay, go to step  6. If IAC duty signal output is not okay, go to next step.
  3. Turn ignition off. Turn ignition on. Using a DVOM, backprobe ECM harness connector and measure voltage between ground and terminal No. 10 (Black/Blue wire) at ECM harness connector E4. If voltage is 9-14 volts, replace ECM. If voltage is not 9-14 volts, go to next step.
  4. Disconnect IAC valve harness connector. Turn ignition on. Measure voltage between ground and terminal No. 2 (Black wire) at IAC valve harness connector. See Figure. If voltage is 9-14 volts, go to next step. If voltage is not 9-14 volts, repair open or short in Black wire IAC valve and fuse box.
  5. Check for an open or short in Black/Blue wire between IAC valve and terminal No. 10 at ECM harness connector E4. See appropriate wiring diagram in WIRING DIAGRAMS article. If problem exists, repair wiring as necessary. If problem does not exist, replace IAC valve.
  6. Check IAC valve operation. See IDLE CONTROL SYSTEM in SYSTEM & COMPONENT TESTING - 4-CYLINDER article. If problem does not exist, go to step  8. If problem exists, go to next step.
  7. Check for open in Brown wire between ground and terminal No. 3 at IAC valve harness connector. If problem exists, repair as necessary. If problem does not exist, replace IAC valve.
  8. Remove IAC valve from throttle body. Check IAC valve and passages for carbon build-up and blockage. If problem exists, repair as necessary. If problem does not exist, go to next step.
  9. Ensure oil dipstick, oil filler cap, PCV system and all other air induction system components are intact and operating properly. If problem exists, repair as necessary. If problem does not exist, an open or short in A/C switch signal circuit may exist. Check A/C amplifier. See appropriate article in AIR CONDITIONING & HEATING. Repair as necessary.
Fig 1: Installing Jumper Wire Between DLC No. 1 Terminals
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