After being switched off for a while the game will not boot: GI lights, but the dot matrix stays dark and the LEDs on the CPU and display boards stay solidly lit instead of blinking. A known-good CPU from another System 3 game does the same.

Suddenly won't boot, CPU and DMD LEDs solid, display dark: reseat playfield ground plugs

Cause: A poor ground connection in the plugs under the playfield. Fuses, 5 V and the backbox connectors were all fine.

Checks

• Measure 5 V at the boards (members suggest about 5.2 V on these games) and check it for AC ripple.
• Unplug everything from the CPU except power and the ribbon and see if it boots.
• Swap in a known-good CPU board; if it still won't boot, look outside the backbox.
• Check and reseat the ground connectors under the playfield.

Fix

• Reseat and clean the ground plugs under the playfield, then power up; repin any that are loose or burnt.

Systems

• Gottlieb System 3

Games

• Rescue 911

Related tips

• Garbled replay score setting after a battery change: repeat the factory reset
• Where the boards sit in a System 3 dot matrix backbox
• Gottlieb System 3 CPU passes test ROM, then watchdog trips: torn trace and wrong parts
• Low battery / Bad U3 or U6 error and beep loop after a coin cell swap
• Low battery error comes back a month after a new coin cell: the new cell was bad
• System 3 games with 128x32 dot matrix displays
• Nonsense message at boot, game stuck in a loop

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