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Home / DTC / P1507 — ISC rationality high

P1507 — ISC rationality high

Detailed page for trouble code P1507.

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P1507

MITSUBISHI P — Powertrain

ISC rationality high

Brand: MITSUBISHI
Type: P — Powertrain
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Stuck-open or seized ISC valve (idle air bypass/stepper)
  • Carbon-clogged throttle bore or ISC passages
  • Intake manifold vacuum leak (hose, gasket, PCV, EGR)
  • Faulty ISC valve electrical connection, wiring short or open
  • Incorrect throttle body or throttle plate adjustment; sticky throttle
  • Faulty throttle position sensor (TPS) or inaccurate TPS signal

Symptoms

  • High or unstable idle speed
  • Idle hunting or surging
  • Extended cranking or stalling at idle
  • Check Engine Light (CEL) with P1507 stored
  • Poor driveability at low speed or in neutral

What to check

  • Read freeze frame and live data (ISC actual step/position, ISC command/target, RPM, TPS, MAP/MAF, coolant temp)
  • Verify battery voltage is within range during tests
  • Visually inspect ISC valve, throttle body, intake hoses, and vacuum lines for leaks or carbon build-up
  • Back-probe ISC connector and confirm power, ground and command signals
  • Check TPS values at closed throttle and during movement for correct range and smoothness
  • Smoke test intake for vacuum leaks if visual inspection inconclusive

Signal parameters

  • ISC command/target step count or percent
  • ISC actual/feedback step count or percent
  • Engine RPM (idle)
  • Throttle Position Sensor (TPS) % or voltage
  • Manifold Absolute Pressure (MAP) or MAF flow reading
  • Engine Coolant Temperature (ECT)

Diagnostic algorithm

  1. Record freeze frame and live data to capture conditions when code set
  2. Perform visual inspection: wiring, connectors at ISC valve, vacuum hoses, throttle body and intake gasket
  3. With engine off, remove ISC valve for visual inspection and check for carbon/obstruction; clean throttle body and ISC passages if dirty
  4. Bench-test ISC valve (resistance, stepper movement or duty response per service manual) and compare to spec
  5. With scanner, command ISC steps (if supported) and observe actual movement and idle response
  6. Check TPS for correct closed throttle voltage and smooth change; replace or adjust if out of spec
  7. Perform vacuum/smoke test to find leaks; repair any leaks found
  8. If wiring or connector suspect, measure continuity and repair/replace as needed
  9. Replace ISC valve if mechanical/electrical failure confirmed and retest
  10. After repairs, clear codes, run vehicle to verify ISC actual aligns with target and code does not return

Likely causes

  • Carbon build-up in throttle body/ISC passages allowing extra air
  • Failed or mechanically binding ISC valve failing to close
  • Intake vacuum leak (cracked hose, loose clamp, leaking gasket)
  • Open/short in ISC harness or corroded connector
  • Incorrect TPS reading causing inappropriate ISC demand

Fault status

⚠️ Status
PCM/ECU logged P1507: ISC (idle air control) actual position/step count higher than commanded or outside rational limits. PCM could not achieve commanded idle with normal ISC range — indicates excess air or ISC control fault.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1-4 hours

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