Chevrolet Trailblazer Service Manual page 12

Attach the clip to ground then crank the engine to check for
This spark tester is the easiest to use. Just place it on a
plug wire and the spark voltage is detected; the bulb on the
NOTE: The best way to perform this procedure is to use a spark tester (available at most
automotive parts stores). Three types of spark testers are commonly available:
The Neon Bulb type is connected to the spark plug wire and flashes with each
ignition pulse.
spark.
top will flash with each pulse.