2000 Mazda 626 4-cylinder check engine light won't go off, Code p1506. I can't pass inspection till the check engine light is sorted. Local shop already tried the following - replaced IAC valve, IAC gasket, PCV valve, vacuum leak, adjusted/moved throttle body. (In addition they did a new muffle, new front left ball joint w/ control arm and sway bar link, oil change/filter). Any suggestions short of replacing throttle body ($900usd part)? Shop said something about plugging up one vacuum leak can cause a vacuum leak somewhere else. Is this true? Already sunk $1385.95USD into it, really don't want to sink any more $ into it.
Here are the description and procedures for diagnosing and rectifying the problem. Go through them to see if you have missed out anything.
Why would solving a vacuum leak result in another?
DTC P1506: IDLE AIR CONTROL (IAC) VALVE
DTC indicates that IAC system has reached overspeed (engine speed is 200 RPM more than requested RPM) malfunction. Possible causes are as follows: IAC circuit short to ground.
IAC assembly stuck open.
Air intake leaks or restrictions.
Damaged throttle body.
Contaminated or damaged IAC valve assembly.
Faulty Powertrain Control Module (PCM).
1. Check System For Vacuum Leaks
Ensure freeze frame data is recorded. Check entire system for vacuum leaks. Repair or replace as necessary. If no vacuum leaks are found, go to next step.
2. Check EVAP System
Turn ignition off. Disconnect hoses from EVAP canister purge valve or solenoid. Attach vacuum pump with gauge to carbon canister hose port. Using vacuum pump, apply 16 in. Hg to port. If vacuum bleeds off within 20 seconds, replace EVAP canister purge valve or solenoid and repeat QUICK TEST. If vacuum is not bled off immediately, go to next step.
3. Check IAC Solenoid Function
Start engine and allow to idle. Ensure transmission is in Park or Neutral and engine is warmed to normal operating temperature. Disconnect IAC solenoid wiring harness connector. If engine speed drops, go to next step. If engine speed does not drop, check throttle body for damage. If throttle body is okay, replace IAC solenoid and repeat QUICK
4. Check IAC Circuit For Short To Ground
Turn ignition off. Disconnect scan tool from DLC. Disconnect PCM wiring harness connector. Inspect pins for damage and repair as necessary. Install Breakout Box (014-00950), leaving PCM disconnected. Measure resistance between test pin No. 83 (IAC) and test pins No. 51 and 103 (PWR GND) at breakout box. If resistance is greater than 10 k/ohms and idle speed is normal, go to next step. If resistance is greater than 10 k/ohms and high idle speed is present, replace PCM and repeat QUICK TEST. If resistance is less than 10 k/ohms, repair circuit short to ground and repeat QUICK TEST.
5. Check IAC System For Intermittent Open Or Short Circuit
Connect scan tool to DLC. Ensure all accessories are off and engine is warmed to normal operating temperature. Turn ignition on. Using scan tool, access IAC PID and RPM PID. IAC duty cycle should be 20-40 percent. Observe IAC PID and RPM PID for indication of fault while wiggling and bending wiring harness between IAC solenoid and PCM. Fault will be indicated by sudden change in IAC PID or RPM PID value. If any faults are found, isolate and repair as necessary. Repeat QUICK TEST. If no faults are found, problem cannot be duplicated at this time. Ensure all applicable connections are clean and tight, and go to DIAGNOSTIC TEST Z (INTERMITTENT).