The GI lights and (on DMD games) the display version shows, but the game goes no further.

Game will not boot: GI on, little else happens

Cause: A CPU board fault, low +5 V, or burnt power input connector.

Checks

• Measure +5 V at the CPU board test points beside the battery (+5 and GND): it should be 4.9-5.1 V.
• If the voltage is good, watch the CPU LEDs at power-on and read the code.

Fix

• Follow the LED code entries, or repair the +5 V supply.

Systems

• Data East CPU Version 1/2
• Data East CPU Version 3
• Data East/Sega CPU Version 3b

Related tips

• Display EPROM version appears, then nothing more
• PIA LED lights, goes out, then lights again
• PIA LED stays lit and blanking never comes on
• Normal CPU board LED sequence at power-on
• Odd DMD screens or "command 9c" errors with matching ROMs
• Time Machine: CPU and display EPROM types and latest versions
• Robocop: CPU and display EPROM types and latest versions
• The Simpsons: CPU and display EPROM types and latest versions

Source: Repairing DataEast 1987 (Laser War) to Sega 1995 (Batman Forever), Part Two, Clay Harrell (pinrepair.com)