Coins add credits, but pressing start does nothing. The coin lockout relay does not pull in when the game is switched on.

Credits register but the start button does nothing

Cause: A wire on the reset relay had broken off its lug while still looking attached, breaking the start circuit.

Checks

• Note whether the coin lockout relay engages at power on.
• Follow the start circuit on the schematic from the start button through the reset relay.
• Gently tug each wire on the relays in that path; a broken joint can hide under the insulation.
• A bridged zero switch on the credit unit may be an intentional free-play change, not a fault.

Fix

• Resolder the broken wire to its lug.

Turn the game off and unplug it before working in the cabinet or backbox; EM games carry line voltage.

Systems

• Electro-mechanical

Games

• Space Mission

Related tips

• Start button does nothing but the start relay works by hand: dirty inertia switch
• Start button adds players 1 to 3 but never the 4th: adjust the coin unit end switch
• Quieting a buzzing coin lockout relay
• Start button gives a two-player game: wire on a latch relay touching another
• Game resets but the outhole relay never fires: feed wire hanging by a strand
• Ball count runs to 10 and the game resets endlessly: dirty ball count stepper
• Game won't start though credits and reels reset: bad joint on the latch coil
• Game Over relay trips again right after start: dirty Lock relay switch

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