To get you started...
I'm assuming you know the three targets must be lit before the diverter will open.
Check to see that the diverter works in coil test. That will help determine if it is a coil issue, a switch issue, or a mechanical issue.
Mechanical issue: Is the linkage broken under the playfield?
If the coil doesn't fire in coil test, look for a broken wire.
Switch test: the diverter opens when the ball rolls over the lane switch. If the switch isn't working, then the diverter won't open during game play.
Steve (in Escalon, CA)