Skip to content
GTECH Calibration
Portal

P1508

Idle Air Control Circuit Open.

  • Mazda
  • Powertrain
  • Manufacturer code
  • Medium severity

How this code is built

P
SystemPowertrain
1
Defined byThe manufacturer
5
AreaVehicle speed, idle control and auxiliary inputs
08
FaultThe specific fault in that area

What it means

On a Mazda, P1508 means the engine control unit has detected an open circuit in the idle air control (IAC) valve circuit. The ECU drives the valve to regulate idle speed and sees a voltage or current response that matches a broken or disconnected circuit rather than a working valve. Without proper control, idle speed can become unstable or incorrect.

Symptoms

  • Rough, hunting or surging idle
  • Idle speed too high or too low
  • Engine stalls when coming to a stop or when the air conditioning or electrical loads switch on
  • Difficult starting or stalling shortly after a cold start
  • Engine management warning light on

Possible causes

  • Loose, corroded or damaged connector at the IAC valvelikely
  • Open circuit or chafed wiring between the IAC valve and the ECU, or in the valve's power or ground supplylikely
  • Faulty IAC valve with an open internal coillikely
  • Blown fuse or faulty relay feeding the IAC valve circuit (depends on model and engine)possible
  • Poor ECU connector pin contact or corrosion at the control unit terminalspossible
  • Internal ECU driver faultrare

How to diagnose

  1. Read the stored codes and freeze frame data, note whether other codes are present, and check whether P1508 returns after clearing it.
  2. Inspect the IAC valve connector and nearby wiring for looseness, corrosion, heat damage, chafing or signs of previous repair.
  3. Check the relevant fuse and relay for the IAC circuit using the wiring diagram for the specific model and engine.
  4. With the ignition on, check the supply voltage and ground at the IAC valve connector against the manufacturer's specification.
  5. Measure the IAC valve coil resistance and compare it with the specification for that model; an open reading confirms a failed valve.
  6. With the ECU and valve connectors disconnected and the ignition off, test continuity and resistance to ground of each wire between the valve and the ECU.
  7. Inspect the ECU connector terminals for corrosion or spread pins, and only suspect the ECU once the wiring and valve have passed.
  8. Check for vacuum leaks and a dirty throttle body, as these can mask or compound idle problems once the fault is repaired.

How to fix

  • Clean, secure or replace the damaged IAC valve connector or terminals.
  • Repair or replace open-circuit, chafed or corroded wiring between the valve, the power supply and the ECU.
  • Replace the IAC valve if its coil is open or outside specification, and clean the throttle body and air passages as part of the job.
  • Replace a blown fuse or faulty relay after finding out why it failed.
  • Repair ECU connector terminal problems, and only repair or replace the ECU if a confirmed driver fault remains after everything else has been ruled out.
  • After repair, clear the code and perform any idle relearn procedure specified for the model.

Can I keep driving?

Medium severity

The car can usually be driven short distances, but stalling at junctions or at low speed can be a safety risk. Get it repaired soon, and avoid driving if the engine stalls repeatedly.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.

This code for other makes

Nearby codes