One pop bumper never fires.

A pop bumper doesn't work at all

Cause: In rough order: the pop's own under-playfield fuse, the coil, lost +5 V or logic ground to the PBDB, wiring, the skirt switch, or the PBDB itself.

Checks

• Game off: pull and continuity-test the pop's fuse under the playfield (make sure it is the right one).
• Game on and started (or Q relay held in): both coil lugs should show 25 to 40 V DC. Only one lug means a bad coil; neither means the fuse.
• Clip a jumper from the bottom panel ground strap and tap it on the coil's unbanded-diode lug; the coil should fire.
• Read 4.8 to 5.2 V across PBDB pins 5 and 6 (pin 3 is the key). If it is only there with the black lead on the cabinet ground strap, the PBDB ground (CPU A1-J6 pin 9) is lost. +5 V comes from A1-J6 pin 18.
• Briefly bridge PBDB pins 1 and 2: the coil should fire (a spark is normal). If not, the coil-to-board wiring is open.
• Bridge PBDB pins 4 and 6 to fake the skirt switch: if the coil fires, the board is fine and the switch or its wiring is at fault.

Fix

• Repair whatever the checks find; if all pass and it still won't fire, repair the PBDB.

Only bridge PBDB pins 1 and 2 for a moment, or the coil can burn.

Systems

• Gottlieb System 80
• Gottlieb System 80A
• Gottlieb System 80B

Related tips

• A pop bumper never fires
• How the pop bumper driver board fires its coil
• Add power and fire LEDs to a pop bumper driver board
• Pop bumpers fire on their own or work only sometimes
• Bench check of a pop bumper driver board
• Pop bumper board wiring is in each game manual
• Pop bumper fires erratically or several times
• Common failures on the pop bumper driver board

Source: Repairing & Upgrading Gottlieb System 80 Pinballs from 1980 to 1989, Part One, Clay Harrell (pinrepair.com)