Idle Air Control (IAC), Checking

Idle Air Control (IAC), Checking

IAC function, checking
- Connect VAG1551 or VAG1552 Scan Tool (ST), and press buttons -0- and -1- to insert "Engine Electronics" address word 01 (engine running at idle). [1][2]Scan Tool Connecting and Initial Checks

Indicated on display


Rapid data transfer HELP
Select function XX


- Press buttons -0- and -8- to select "Read Measuring Value Block" function 08, and press -Q- button to confirm input.

Indicated on display


Read Measuring Value Block
Input display group number XXX


- Press buttons -0-, -0- and -4- to input display group number 4 (004), and press -Q- button to confirm input.

Indicated on display


Read Measuring Value Block ->
1 2 3 4


- Check specified values for basic functions at engine idle speed (display field 4):







- Press -C- button.

Indicated on display


Read Measuring Value Block
Input display group number XXX


- Press buttons -0-, -0- and -6- to input display group number 6 (006), and press -Q- button to confirm input.
- Press button -3- to advance to display group 7.

Indicated on display


Read Measuring Value Block 6 ->
1 2 3 4


- Check specified values for idle air control at engine idle speed (display fields 1-4):







- Check specified values for idle air control at engine idle speed (display fields 1-4):







Note on display field 3:
Increased temperature of the coil in the Idle Air Control (IAC) valve changes the internal resistance. Idle air control, and the opening of the IAC valve that is calculated by the Engine Control Module (ECM), requires a precise voltage signal. If this voltage is reduced because of increased coil resistance due to heat, the IAC valve duty cycle has to be changed until the calculated voltage is reached. With the engine cold (room temperature) the control factor is approx. 1.0. As the coil heats up the duty cycle increases. Values above 1.2 indicate resistance in the wiring between the ECM and the IAC valve.


Note on display field 4:
The voltage required to maintain the necessary air flow through the IAC valve is calculated by the ECM. If the air flow through the valve is too low (e.g. due to increased air resistance) the IAC valve must be opened farther to compensate. This deviation is recognized by the ECM, which compensates by applying a different control factor (learning process). Values that deviate significantly indicate air flow restrictions or sticking of the IAC valve.


- Press button to advance program sequence.
- Press buttons -0- and -6- to select "End Output" function 06, and press -Q- button to confirm input.