Now and then, when the ball drains, the knocker fires instead of the outhole kicker and the ball is not served. After the game is off for a while it may behave again.

Knocker fires instead of the outhole kicker: bad solenoid driver J4 connector

Cause: Solenoids on these games are selected by a four-wire coded bus from the MPU to the solenoid driver board. A poor connection on one bus line (at J4 on the solenoid driver board here) changes the code, so the wrong solenoid fires.

Checks

• Run solenoid test and confirm each coil fires on its own number.
• Wiggle and inspect the connectors between the MPU and the solenoid driver board, especially J4 on the driver board.
• If connections are good, a failing PIA on the MPU can do the same.

Fix

• Repin the bad connector (new terminals or a new housing), and reflow the header pins on both boards if they show cracked joints.

Systems

• Bally MPU AS-2518-35

Games

• Fireball Classic

Related tips

• Coils go weak or the wrong coil fires mid-game: reseat the solenoid driver plugs
• Drop targets score but no longer light features: cracked switch matrix connector
• Pop bumpers fire nonstop after a solenoid expander swap: board jumper set wrong
• MPU gives 3 flashes after reassembly: two 15-pin plugs swapped
• Pop bumpers, slings and post weak, flippers fine: bad coil ground return plug
• 12 V at the rectifier board but not at the MPU
• Three flashes and 8s or 9s in the hundreds digit
• Rectifier board J3 (to backbox boards) pins

Source: Pinball Repair help group!! (Facebook group)