Diagnostic Aids

WARNING: This page is about a different car, the 1995 Mazda MX-6 and 1995 Mazda 626. However, it is still accessible from the selected car via links, so may be relevant.

IAC valve controls engine idle speed function by regulating the volume of air by-passing throttle plate. IAC valve positioning is determined by signals sent from PCM. A thermowax material is also built into unit to aid operation of IAC valve.

Perform this test when instructed by QUICK TEST or if directed by other test procedures. This test is intended to diagnose:

RPM during SELF-TEST mode.
IAC valve.
Wiring harness circuits (IAC and VPWR).
Powertrain Control Module (PCM).

To prevent replacement of good components, be aware the following non-EEC related areas may be at fault:

Engine temperature outside correct operating range.
A/C input (electrical problem).
Incorrect throttle stop adjustment.
Faulty throttle or cruise control linkage.

1) KOER Code 412 Code 412 indicates engine RPM could not be controlled within upper RPM limit during KOER SELF-TEST. Possible causes for this fault are:

Open or shorted circuit.
Sticking or binding throttle linkage.
Incorrect idle airflow setting.
IAC valve contaminated.
Faulty IAC valve.
Faulty PCM.
Mechanical faults unrelated to EEC-IV which could affect RPM.

Turn ignition off. Connect tachometer to engine. Start engine. Disconnect Idle Air Control (IAC) valve. If RPM drops or engine stalls, go to next step. If RPM does not drop or engine does not stall, go to step 3).

2) Check For Ignition Or EGR Codes If ignition service Code 213, or EGR service Code 327 or 332 is displayed, reconnect IAC valve. Go to appropriate KOER SELF-TEST. See DIAGNOSTIC TROUBLE CODE REFERENCE CHARTS. If these codes are not displayed, go to next step.

3) Check For Heated Oxygen Sensor Codes If Code 172 or 173 is displayed, reconnect IAC valve. Go to appropriate KOER SELF-TEST. See DIAGNOSTIC TROUBLE CODE REFERENCE CHARTS. If these codes are not displayed, go to next step.

4) Measure IAC Valve Resistance Turn ignition off. Disconnect IAC valve. Measure resistance of IAC valve. If resistance is not 7.7-9.3 ohms at 73°F (23°C), replace IAC valve and repeat QUICK TEST. If resistance is 7.7-9.3 ohms at 73°F (23°C), go to next step.

5) Check For Internal Short To IAC Valve Case Turn ignition off. Disconnect IAC valve. Measure resistance from either IAC terminal pin to IAC valve housing. If resistance is more than 10,000 ohms, go to next step. If resistance is 10,000 ohms or less, replace IAC valve. Repeat QUICK TEST.

6) Check VPWR Circuit Voltage Disconnect IAC valve. Turn ignition on. Measure voltage between VPWR terminal at IAC valve harness connector and negative battery terminal. If voltage is more than 10.5 volts, go to next step. If voltage is 10.5 volts or less, repair open in circuit and repeat QUICK TEST.

7) Check IAC Circuit Continuity Turn ignition off. Disconnect IAC valve. Disconnect 60-pin PCM connector and inspect for damaged pins, corrosion and loose wires. Repair as necessary. Install Breakout Box (49-UN01-058), leaving PCM disconnected. Measure resistance between test pin No. 21 and IAC terminal at IAC valve wiring harness connector. If resistance is less than 5 ohms, go to next step. If resistance is 5 ohms or more, repair open circuit. Remove breakout box, reconnect all components and repeat QUICK TEST.

8) Check IAC Circuit For Short To Ground Turn ignition off. Disconnect IAC valve. With breakout box installed and and PCM disconnected, measure resistance between test pin No. 21 and test pins No. 40, 46 and 60 at breakout box. If any reading is 10,000 ohms or less, repair short to ground. Remove breakout box, reconnect all components and repeat QUICK TEST. If all readings are more than 10,000 ohms, go to next step.

9) Check IAC Circuit For Short To Power Disconnect IAC valve. Turn ignition on. With breakout box installed and PCM disconnected, measure voltage between test pin No. 21 at breakout box and chassis ground. If voltage is one volt or more, repair short to power. Remove breakout box, reconnect all components and repeat QUICK TEST. If code or symptom is still present, replace PCM. If voltage is less than one volt, go to next step.

10) Check IAC Signal From PCM Turn ignition off. Reconnect IAC valve. With breakout box installed, connect PCM to breakout box. Connect DVOM between test pins No. 21 and 40 at breakout box. Start engine. Observe DVOM while slowly increasing engine speed to 3000 RPM. If voltage is 7-11 volts, go to next step. If voltage is not 7-11 volts, remove IAC valve to confirm that it is not stuck open. If IAC valve is okay, replace PCM. Remove breakout box and repeat QUICK TEST.

11) Check Base Idle Verify base idle speed is correct. See ADJUSTMENTS - 2.0L article. If base idle speed is within specification, replace IAC valve. Repeat QUICK TEST. If base idle speed is not correct, reset idle speed to specification. Repeat QUICK TEST. If unable to set idle to specification, go to next step.

12) Check For Faults Affecting Idle Speed Check following mechanical items for faults:

Throttle linkage and/or cruise control linkage (sticking or binding).
Throttle body (operation).
Vacuum hoses. Check Vehicle Emission Control Information (VECI) label.
Check for leaks around IAC valve (gaskets, etc.).

If any of these items are faulty, service as necessary. Remove breakout box, reconnect all components and repeat QUICK TEST. If all of these items are okay, replace IAC valve. Repeat QUICK TEST.

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 12) to step 15). No test procedures have been omitted.

15) KOER Code 411 Code 411 indicates engine RPM could not be controlled within lower RPM limit during KOER SELF-TEST. Possible causes for this fault are:

Incorrect idle airflow setting.
Vacuum leaks.
Sticking or binding throttle linkage.
Throttle plates open.
Incorrect ignition timing.
IAC valve contamination.
IAC circuit shorted to ground.
Faulty IAC valve.

If listed items are okay and idle is set to specification, remove IAC valve and inspect for contamination. Replace as necessary. Repeat QUICK TEST. If code or symptom is still present, replace IAC valve. If idle speed is not within specification, adjust as necessary and repeat QUICK TEST. If idle speed cannot be set to specification, go to next step.

16) Check For Conditions Affecting Idle Speed Check the following mechanical components:

Vacuum hoses. Check Vehicle Emission Control Information (VECI).
Throttle linkage and/or cruise control linkage (sticking or binding).
Induction system (vacuum leaks).
Ensure purge solenoid valve is not stuck open.
Ensure base ignition timing is to specification. See ADJUSTMENTS - 2.0L article.If everything checks okay, go to next step. If fault is found, service as necessary and repeat QUICK TEST.

17) Check For Internal Short To IAC Valve Case Turn ignition off. Disconnect IAC valve. Measure resistance from either IAC terminal pin to IAC valve housing. If resistance is more than 10,000 ohms, go to next step. If resistance is 10,000 ohms or less, replace IAC valve. Repeat QUICK TEST.

18) Check IAC Circuit For Short To Ground Turn ignition off. Disconnect IAC valve. Disconnect 60-pin PCM connector and inspect for damaged pins, corrosion and loose wires. Repair as necessary. Install Breakout Box (49-UN01-058), leaving PCM disconnected. Measure resistance between test pin No. 21 and test pins No. 40, 46 and 60 at breakout box. If any reading is 10,000 ohms or less, repair short to ground. Remove breakout box, reconnect components and repeat QUICK TEST. If all readings are more than 10,000 ohms, go to next step.

19) Check IAC Signal From PCM Turn ignition off. With breakout box installed, connect PCM to breakout box. Reconnect IAC valve. Set DVOM on 20-volt scale. Connect DVOM between test pins No. 21 and 40 at breakout box. Start engine. Observe DVOM while slowly increasing engine speed to 3000 RPM. If voltage is 7-11 volts, go to next step. If voltage is not 7-11 volts, replace PCM. Remove breakout box and repeat QUICK TEST.

20) Check For Proper Engine Coolant Flow To Idle Air Control-Bypass Air (IAC-BPA) Valve Check the following:

Verify engine coolant is warming up properly.
Check engine coolant level.
Check coolant hoses to and from IAC-BPA valve for leaks, blockage and kinks.

If any check is not okay, repair as necessary. Remove breakout box and reconnect PCM. If all checks are okay, remove IAC-BPA valve and inspect for contamination. Clean or replace IAC-BPA valve as necessary and repeat QUICK TEST. If code or symptom is still present, replace IAC-BPA valve.

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 20) to step 25). No test procedures have been omitted.

25) Lack Of High Idle With Engine Cold: Verify IAC-BPA Valve Is Not Stuck Closed Turn ignition off. Remove IAC-BPA valve from engine. With IAC-BPA valve at room temperature, blow through air passage. Air should flow freely. If air does not flow freely, replace IAC-BPA valve and check system operation. If air flows freely, no problem is indicated at this time. See SYMPTOMS in TESTS W/O CODES article.