Do the easy stuff first. Check the connectors for the optos under the playfield.
Those little disconnects may just need to be re-seated. Simply unplug them and re-connect during switch test while checking for operation.
I had two switches on the Powerfield which weren't registering - tracked the problem back to the connector. I may re-do the Molex pins just to make sure the problem doesn't re-occur. Problem solved (and solved simply) for now.

I'm not discouraging the purchase of a cool diagnostic tool but there are simpler ways if you're not doing this type of work often.
http://techniek.flipperwinkel.nl/wpc/index3.htm#opto

The longer I think about it.......I'll pose a question.
How often does an "opto" actually go bad?
After all, it is a paired, specialized, LED combination. LEDs take a pretty serious beating before they quit so I'm genuinely curious as to how often these LEDs actually fail. It seems to be the "go to" repair advice for a TZ - "check the optos" - yet I've rarely found the optos to be at fault unless they're just dirty.
I'm willing to guess that a little cleaning may help out too. The gumball area tends to collect a bit of dirt and dust. Recently, I had that area apart and it was pretty filthy under the escutcheon. I wound up dis-assembling and washing the gumball machined while I was at it - lots of black dust.