Milk jug has a safety sensor which when blocked will stop the ball conveyor from running. When the sensor is blocked the game will generate an error 20. There is a small adjustment potentiometer located on the back of the board to increase and decrease the sensitivity of the board. The safety sensor has an indicator LED that tells you the state of the optics. When the game is in attract mode it will always be flashing, when the game is coined up the LED will go dark, when the sensor is blocked the led will light be solid on. You can use a digital multi meter to test the sensor signal lines for proper operation when referenced to ground the signal line will be 5v when the sensor is aligned (indicator LED off) 0 vdc when the sensor is blocked (indicator LED on), the enable line when referenced to 12vdc will pulse high to low. Please see the document below for steps to access and adjust the sensor.