Note on testing:

We test every transformer before shipping with a 20 watt 12 volt halogen light to make sure the bulb lights and the transformer works.

If you want to check the transformer when you receive it, we recommend to connect halogen light 20w 12v directly to output to confirm it works.
Testing with a voltmeter: To test the transformer, you must apply a minimum 10 watt/12v load before measuring
AND you must use a special "true RMS" voltmeter.

A standard AC voltmeter cannot measure the output of a high frequency electronic transformer.
If you test without loading the transformer or without using a "true RMS" voltmeter, you may get no voltage or an incorrect low voltage (i.e. 2.3 VAC or 2.92 VAC).
Only when you connect load to output the check by tester will display 12v output.