Quoted from pinballinreno:
There is a small connector for the monkey sensor at the bottom center edge of the playfield.
Hard to see. You can look at it with the playfied raised.
You can feel it from the bottom rear with the playfield pulled out to its furthest service position.
Its loose. Unplug it and replug it in firmly.
Mine has fallen of before. I widened a couple pins on it and now its snug.
Also check that the little monkey sensor PC board isnt loose and changing position. It's on the rear of the backboard. It just has to be snug, not over tightened
Ive noticed that if the playfield is down almost all the way where the metal hinges under the lock down bar are, if they are resting on the metal, the sensor is normal, if I lower it even about a centimeter, it triggers the sensor on. I cant find anything to be causing it.