Our shop procedure for a game that's been out in the field for several years is to strip the playfield down to just posts, rails, and topside mechs. Take off all plastic and rubbers, and put the nuts, etc, back where they belong, so you know where they go. You can put colors across any under playfield plugs you have to unplug if you're worried you might confuse them. Clean the heck out of the playfield with novus #2. Take a toothbrush and brush out all the little nooks and stuff with novus #1. GI should be more or less fully exposed, so replace all lights. While everything is exposed, go through solenoids and flashers one by one, and see how they work. Fix any solenoid/flasher problems at this point and change as many sleeves as you can. LTG's method is fine, but don't do the tests any time after this. Otherwise you'll have to re-strip to access things. Flip the playfield up and rebuild your flippers, and replace all lamps. Check the lamp test and fix any lamp issues. Put on rubbers. Test each switch with a ball, and a copy of the switch matrix in front of you, so you make sure you get them all. Fix switch issues. Run any special mech tests. Fix mech issues. Clean the removed rails and plastics, and replace them on the playfield. If there are any lamps, solenoids, or flashers that you had to remove, test those now. Run sound tests. Display should be pretty obvious by now if there's issues. Do the lamps/flashers/solenoids on the backbox or in the cabinet. Examine your balls and replace them if needed. Playtest. Cointest. Clean the outside and wipe down the inside and the glass. Am I missing anything?

In my neck of the woods, non-working service buttons usually mean some idiot spilled pop down the front. So I'd flush it with contact cleaner first. If you already ordered a new set, though, that'll take care of it.

Sadly common for a game that's been either in someone's house, or left in the field too long. Glad you figured it out though!