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Idle Air Control (IAC) System Malfunction

DTC P1506 DETECTION CONDITION AND POSSIBLE CAUSE

DTC P1506 IDLE AIR CONTROL (IAC) SYSTEM MALFUNCTION
DETECTION CONDITION 
  • P1506 indicates that the PCM detected an engine idle speed greater than the desired rpm.
POSSIBLE CAUSE 
  • Vacuum leak.
  • Damaged IAC valve.
  • Damaged PCM.
  • Damaged throttle body.
  • IAC circuit short to GND.
  • IAC valve stuck open.
HINT  IAC Valve Description 
  • Controls engine idle speed and provides a dashpot function.
  • Meters intake air around the throttle plate through a bypass within the IAC valve assembly and throttle body.
  • The PCM determines the desired idle speed or bypass air and signals the IAC valve assembly through a specified duty cycle. The IAC valve responds by positioning itself to control the amount of bypassed air. The PCM monitors the engine rpm and increases or decreases the IAC duty cycle in order to achieve the desired rpm.
G03988161Courtesy of MAZDA MOTORS CORP.
DTC P1506 INSPECTION

STEP INSPECTION ACTION
1
  • Was PCM Quick Test performed? 
 Yes Go to next step.
No Perform PCM Quick Test.
2 DTC P1506: CHECK FOR VACUUM LEAKS
  • Key ON, Engine Running (KOER).
  • With the engine at idle, listen for vacuum leaks.
  • Inspect the entire intake air system from the Mass Air Flow (MAF) sensor to the intake manifold for damage or leaks such as:
    • Cracked or punctured intake air tube.
    • Loose or cracked IAC air tubes.
    • Loose intake air tube at the air cleaner housing or throttle body.
    • IAC valve assembly or gasket seal.
    • Intake manifold assembly or gasket seal.
    • EGR valve gasket seal.
    • Vacuum supply connectors and hose.
    • PCV valve, connectors and hoses.
  • Key OFF.
  • Were any leaks detected? 
 Yes Repair or replace components as necessary. Go to step 7.
No Go to next step.
3 CHECK EVAP SYSTEM FOR A STUCK OPEN VALVE
  • Disconnect hoses at EVAP canister purge solenoid.
  • Connect hand vacuum pump to the fuel vapor port of EVAP canister at the EVAP canister purge solenoid.
  • Apply 53 kPa (16 in-Hg) of vacuum to EVAP canister purge solenoid.
  • Does EVAP canister purge solenoid hold vacuum for 20 seconds? 
 Yes Reconnect all components. Go to next step.
No Replace EVAP canister purge solenoid.
See ELECTRICAL COMPONENT LOCATOR .
Go to step 7.
4 CHECK EVAP SYSTEM FOR A STUCK OPEN VALVE
  • Key On, Engine Running (KOER).
  • Bring engine to normal operating temperature.
  • Transmission in Park or Neutral.
  • Disconnect IAC valve connector.
  • Key OFF.
  • Did the rpm drop or engine stall? 
 Yes Go to next step.
No Inspect throttle body for damage. If OK: Replace IAC valve.
If not OK:
Repair or replace throttle body.
See THROTTLE BODY in REMOVAL, INSTALLATION AND OVERHAUL article in Engine Performance .
Go to step 7.
5 CHECK IAC CIRCUIT FOR SHORT TO GROUND
  • Disconnect NGS Tester from DLC.
  • Disconnect PCM.
  • Install Breakout Box, leave PCM disconnected.
  • Measure resistance between PCM test pin 83 (IAC) and PCM test pins 51 and 103(PWR GND).
  • Is resistance greater than 10K ohms? 
 Yes For fast idle symptom currently present: Replace PCM. See COMPUTERIZED ENGINE CONTROLS .
Go to step 7. All others: Go to next step.
No Repair short in IAC SIG circuit. Go to step 7.
6 CHECK IAC SYSTEM FOR INTERMITTENT OPEN OR SHORT
  • Connect NGS Tester.
  • Key On, Engine Running (KOER).
  • Access IAC PID and RPM PIDs with NGS Tester.
  • With engine at normal operating temperature, accessories off and at idle, the IAC duty cycle must be between 20% and 45%.
  • Observe the PIDs for an indication of a fault while performing the following at idle:
    • Lightly tap on IAC valve assembly and wiggle harness connector to simulate road shock.
    • Grasp the vehicle harness closest to the IAC valve assembly. Shake and bend a small section of the harness from the IAC to the dash panel and from the dash panel to the PCM.
  • Do the IAC or RPM PIDs suddenly change in value indicating a fault? 
 Yes Isolate fault and repair as necessary. Go to step 7.
No For idle quality, starting or stalling symptoms currently present: Replace IAC solenoid,
Go to step 7. All others:
See TEST 4: INTERMITTENT with IAC and RPM PID data and list of possible causes.
7 VERIFY TROUBLESHOOTING OF DTCs COMPLETED
  • Make sure to reconnect all disconnected connectors.
  • Clear DTCs from PCM memory using NGS tester generic OBD-II function.
  • Perform PCM Quick Test.
  • Is same DTC present? 
 Yes Replace PCM. See COMPUTERIZED ENGINE CONTROLS .
Repeat PCM QUICK TEST .
No If any other DTC is present, go to applicable DTC inspection. If no DTC is present, troubleshooting complete.
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