An Error 20 is present when the game believes that it is taking too long for the game to cycle the balls through the game, or the lower ball elevators ball switch is not working. This can be caused by one of 3 issues.
1) Not enough balls in the game ( the game is shipped with 7 - 9 balls ). 6 or less balls will cause both elevators to run for an extended period of time and will cause the game to display an error 20 on the LCD monitor.
2) When on first powering on the game, if all the balls for the game are in the base of the game and not already on the elevator you will get an Error 20 until the game feels it is cycling properly, then it will clear itself after a few minutes.
3) Ball switch issue, either the mechanical ball switch is not working properly, you have a loose connection to the switch or the U10 input chip is damaged. If the elevator is moving properly the Error 20 will not be the ball switch.