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DTC codes / Chrysler codes / P1508

P1508 · ChryslerIAC System Low RPM

P1508 on a Chrysler vehicle means the powertrain control module (PCM) commanded a target idle speed through the idle air control (IAC) motor but the engine speed stayed below the expected threshold for that command. The PCM monitors idle stability and sets this code when the IAC system cannot raise or maintain idle RPM within its calibrated window.

SystemPowertrain
Code typeChrysler specific
SeverityMedium
Fix difficultyIntermediate
AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

  • Idle speed that hangs low or feels like it is about to stall, especially when cold or after coming to a stop
  • Engine stalling when you release the throttle or shift into gear at a standstill
  • Rough or surging idle that fluctuates instead of holding steady
  • Often no symptoms besides the warning light if the fault is intermittent or only occurs under specific temperature conditions
  • Hesitation or a brief dip in RPM when you turn on the air conditioning or electrical loads at idle

Common causes, most likely first

  1. Carbon buildup or sludge on the IAC valve pintle and its seat, restricting airflow at idle
  2. A sticking or failed IAC motor winding that no longer moves the pintle through its full range
  3. Wiring damage or a poor connection at the IAC valve connector, including corroded terminals or a chafed harness
  4. A vacuum leak downstream of the throttle body that leans out the mixture and drags idle speed down
  5. A dirty or incorrectly adjusted throttle body and throttle plate that limits the IAC's authority over idle speed
  6. A failing coolant temperature sensor or intake air temperature sensor that gives the PCM wrong idle strategy inputs
  7. Low fuel pressure or a restricted fuel filter that prevents the engine from reaching commanded idle RPM

How to diagnose and fix it

  1. Scan all modules for stored and pending codes, then record freeze-frame data for P1508 to see the engine temperature and RPM conditions when it set
  2. Inspect the IAC valve connector and harness for corrosion, pushed pins, or chafed wires, and verify battery voltage and ground at the valve with the key on
  3. Remove the IAC valve and clean the pintle and seat with throttle body cleaner, then reinstall and clear the code to see if it returns
  4. Check for vacuum leaks with a smoke machine or by spraying carb cleaner around intake gaskets and hoses while watching idle RPM
  5. Command the IAC valve with a scan tool through its full range and watch for smooth pintle movement and corresponding RPM change
  6. Measure IAC motor winding resistance and compare to specification, replacing the valve if a winding is open or shorted
  7. If the valve and wiring pass, inspect the throttle body for carbon buildup and verify base idle adjustment before condemning the PCM

What a low idle really tells you

  • A dirty IAC valve is the most common cause on Chrysler engines, but technicians often replace the valve before checking the throttle body bore, which can be carboned shut and starve the IAC of the airflow it needs to raise idle.
  • P1508 can set from a vacuum leak that is small enough to pass a visual inspection, so a smoke test is more reliable than guessing at gaskets and hoses.
  • A weak battery or charging system fault can cause the PCM to lose its learned idle strategy, and the code may return after clearing until the vehicle completes several idle relearn cycles.
  • On some Chrysler models the IAC valve shares a circuit with the throttle position sensor, so a TPS signal that drops out momentarily can set P1508 even though the IAC valve itself is healthy.
  • Do not overlook a restricted exhaust or a failing torque converter clutch that drags the engine down at idle, because the PCM sees low RPM and blames the IAC system.

Can I keep driving with P1508?

You can usually drive the vehicle to a shop, but avoid long periods of idling in traffic and be ready for the engine to stall when you come to a stop. If the engine stalls repeatedly or the idle is so low that power steering and brakes feel weak, park the vehicle and arrange a diagnosis rather than continuing to drive.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for Chrysler vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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