The diagnostic LED (D20, or LED203 on WPC-95) blinks a set number of times instead of flashing steadily after power-on.

CPU diagnostic LED blinks a code instead of flashing steadily

Cause: One blink: game ROM (U6, G11) bad. Two blinks: U8 CMOS RAM bad. Three blinks: U9 WPC custom chip bad before WPC-S, or the G10 security PIC bad on WPC-S and later.

Checks

• Count the blinks after the blanking LED goes out.

Fix

• Reseat or replace the part the code points to.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• Reading the CPU board LEDs at power-on
• WPC CPU board LED meanings
• Won't boot after a ribbon cable was fitted one row off: unplug boards to find the damage
• What a WPC game reset is
• Reboots at start-up and when flipping: flaky power connector to the CPU
• Factory settings restored at every power-up after new batteries: do a factory reset
• Resets: +5 V at the driver board test point is low
• Game keeps resetting during play: poor J101/J102 power connections

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part Three, Clay Harrell (pinrepair.com)