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DTC P1508: Idle Air Control (IAC) Valve

  1. Perform PCM reset procedure. See SELF-DIAGNOSTICS - INTRODUCTION article. 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 for at least 30 seconds. If DTC P1508 is indicated, go to next step. If DTC P1508 is not indicated, problem is intermittent. System is okay at this time. Check for poor connections or loose wires at Idle Air Control (IAC) valve and PCM connectors. See appropriate wiring diagram in WIRING DIAGRAMS article.
  2. Turn ignition off. Disconnect IAC valve 2-pin connector. Turn ignition on. Measure voltage between ground and IAC valve harness connector terminal No. 2 (Yellow/Black wire). If battery voltage exists, go to next step. If battery voltage does not exist, repair open in Yellow/Black wire between IAC valve connector and PGM-FI main relay. See appropriate wiring diagram in WIRING DIAGRAMS article.
  3. Turn ignition off and reconnect IAC valve 2-pin connector. Disconnect PCM 25-pin connector "B". Turn ignition on. Measure voltage between ground and PCM 25-pin harness connector "B" terminal No. 23 (Black/Blue wire). If battery voltage exists, go to next step. If battery voltage does not exist, repair open or short in Black/Blue wire between IAC valve connector and PCM 25-pin connector "B". See appropriate wiring diagram in WIRING DIAGRAMS article.
  4. Using a fused jumper wire, momentarily connect PCM 25-pin connector "B" terminal No. 2 (Black wire) and terminal No. 23 (Black/Blue wire), several times. See Figure . If IAC valve does not click, replace IAC valve. If IAC valve clicks, substitute a known-good PCM and recheck. See SUBSTITUTING PCM in SELF-DIAGNOSTICS - INTRODUCTION article. If symptom or indication goes away, replace original PCM.
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