Fuel and air

P1503 — Idle Air Control Valve Control Circuit Low

Severity: medium Drive with care, to a shop

What P1503 means

The engine computer holds idle speed steady by using an idle air control (IAC) valve that lets air bypass the closed throttle. P1503 sets when the voltage in the valve's control circuit is lower than expected. The usual causes are a bad valve or a short in its wiring. The exact definition can differ by manufacturer.

Symptoms

  • Stalling when stopping or slowing down
  • Low idle speed
  • Hunting or surging idle
  • Hard starting or dying right after start
  • Check Engine light on

Possible causes

  1. Failed idle air control valve
  2. Short to ground in the IAC control circuit
  3. Open circuit or poor connection in the wiring or connector
  4. Carbon buildup on the IAC pintle
  5. Faulty engine control module (rare)

How to diagnose

  1. Scan for codes and freeze frame data, clear the code and see if it returns
  2. Inspect the IAC connector and harness for damage, corrosion and chafing
  3. Measure the valve coil resistance and compare it with factory specs
  4. Test the control circuit for a short to ground and for opens
  5. Check for power at the valve and a control signal from the PCM
  6. Remove the valve and throttle body and look for carbon or sticking
  7. Check for vacuum leaks that upset idle control

Common fixes

  • Clean the IAC valve and throttle body
  • Replace the faulty IAC valve
  • Repair the wiring harness or connector
  • Perform an idle relearn after service
  • Diagnose or replace the PCM if everything else checks out

P1503 on different brands

MINI
On MINI, the idle speed control valve closing solenoid control signal stays below the expected range.
Chrysler
Chrysler uses this code for the theft deterrent system, where the password did not match what was expected.
Hyundai · manuals
Hyundai uses it for a fault in the cruise control switch circuit or function.
Kia · manuals
Kia uses it for a malfunction of the cruise control master switch.
Opel · manuals
On Opel, it means an incorrect signal from the immobilizer control module.
VAG
On VAG vehicles, it refers to the alternator load signal rather than the idle air valve.

Questions and answers

Can I drive with P1503?

Short trips are possible, but the engine may stall when you stop, which hurts safety. Get it repaired soon.

What most often causes P1503?

Usually a dirty or failed idle air control valve or a short in its wiring. Start by inspecting the connector and testing the valve resistance.

Will cleaning the IAC valve fix it?

If carbon on the pintle is the cause, cleaning often helps. If the coil or wiring is faulty, you will need a new valve or a harness repair.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.