Idle air control valves can be tested for resistance, mechanical function and valve triggering with a digital multimeter. An idle air valve is important for maintaining correct idle engine speed.
With your car running, be on the lookout for dropping RPMs — a common sign of idle air control valve problems. When the RPMs start to drop, lightly tap on the accelerator with your foot. Watch the RPMs as you do so; if they go back up to an appropriate level and drop again when you let your foot off of the pedal, it can be a signal of a valve issue.
Before checking the valve, the ignition should be switched off, the valve's harness electrical connector disconnected and the valve extracted with an appropriate tool.
Step 1: Test resistance
Use a digital multimeter to test resistance between the valve's terminals. It should read 7-11 Ohms. If not, replace and reconnect the valve. No humming with the ignition on is indicative of a problem with the engine control module or harness.
Step 2: Check mechanical function
Remove the valve again and check it for wear, particularly on the rotary slide. Without replacing the valve, reconnect the harness and run the scan tool "Output Diagnostic Test Mode" function 03 to trigger the valve. The rotary slide should move freely between stops. If it does not, proceed to the next step.
Step 3: Check triggering
Provided the valve is functioning electrically, slide up the boot and use the scan tool "Output Diagnostic Test Mode" function 03 again. Connect a VAG 1527B voltage tester to both terminals individually (between the engine ground and terminal 1 and between the battery positive voltage and terminal 2). If the voltage tester lights up, triggering is functional.