Code
P1508
CHRYSLER
P — Powertrain
IAC System Low RPM
Views:
UK: 28
EN: 35
RU: 31
AI status
Completed
Completed
100%
Causes
- Carbon/clogged throttle body or IAC passage
- Sticking or failed IAC valve (stepper or pintle)
- Vacuum leaks or intake air leaks
- Faulty IAC wiring, connector corrosion, open or short circuits
- Weak battery or poor power/ground to IAC circuit
- Incorrect throttle plate position or binding
Symptoms
- Low idle RPM (engine idles below specified RPM)
- Engine stalls at idle or immediately after startup
- Rough or unstable idle
- Idle improves or changes when throttle is blipped or pedal pressed
- Check Engine Light (MIL) illuminated with P1508 stored
- Possible hard starting or hunting idle when accessories engage
What to check
- Scan for stored codes and view freeze-frame/fuel trim data
- Monitor IAC commanded position/steps and actual RPM response with a scan tool
- Visual inspection of IAC valve, throttle body and intake for heavy carbon
- Inspect IAC connector and wiring for corrosion, damage or loose pins
- Check battery voltage and ground connections with engine off and cranking
- Perform a smoke or propane test to detect vacuum leaks
Signal parameters
- IAC actuator position/step count or duty cycle (commanded vs actual) — vehicle-specific scale (commonly 0–100% or 0–255 steps)
- Engine idle speed (RPM) — typically expected idle ~600–1000 RPM depending on engine/spec
- Supply voltage to IAC (approx. battery voltage when key on/run)
- IAC control signal frequency or PWM (if used) — monitored with scan tool/oscilloscope on some models
- TPS voltage/reference (for idle checking) — typically around 0.5–1.0 V at closed throttle on many systems
Diagnostic algorithm
- Connect a capable scan tool, read and record P1508 plus any other stored/related codes and freeze-frame data.
- Attempt to reproduce condition while monitoring IAC commanded position/steps and engine RPM. Note commanded changes and actual RPM response.
- Visually inspect and test battery/charging system and engine grounds. Ensure adequate system voltage before testing actuator.
- Inspect IAC valve, throttle body and intake passages for carbon build-up. Clean throttle body and IAC passages per service procedures; do not damage pintle or stepper.
- Perform a vacuum/leak test (smoke or spray) around intake manifold, throttle body gaskets, vacuum hoses and brake booster.
- Backprobe IAC connector: check for proper supply voltage, ground continuity and control signal while activating IAC with scan tool. Compare to manufacturer's expected values.
- If the IAC does not move or readings are out of range, bench-test or substitute a known-good IAC (if available) to confirm actuator failure.
- Verify related sensors (TPS, MAF) are within specification; an incorrect TPS can prevent proper idle control.
- If hardware checks good and wiring/grounds are sound, reprogram or update PCM software if a TSB applies, or consider PCM diagnosis.
- After repair or cleaning, perform an idle relearn / adaptation procedure per manufacturer instructions, erase codes and recheck for recurrence.
Likely causes
- Dirty throttle body/IAC passages restricting airflow
- IAC actuator mechanically seized or electrically failed
- Vacuum leak downstream of the MAF/IAT causing low idle
- Connector corrosion or broken wire in IAC harness
- Low system voltage or poor ground affecting IAC response
Fault status
Status
IAC System Low RPM — Idle Air Control not achieving expected control range; engine idle lower than specified.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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