Hidden hallway multiballs and Frenzy start at random during play.

Hallway multiball and Frenzy start on their own: one bad switch diode on row 5

Cause: A failed diode on the lower ramp switch. With it bad, the rest of switch row 5 stopped reading, and closing that switch upset the whole row, so the game saw false switch hits.

Checks

• Use switch edge test and the switch matrix chart: if most switches in one row never register, look for one switch on that row that behaves differently.
• Check the diode on that switch.

Fix

• Replace the diode on the faulty switch; the rest of the row returns to normal.

Systems

• WPC Alpha-Numeric

Games

• Funhouse

Related tips

• Game suddenly ends mid-ball: false slam tilt from a matrix fault
• Testing CPU switch rows and columns with a diode and clip lead
• A row wire on the wrong end of a switch diode creates ghost switches
• Testing a switch diode out of circuit
• Switch diode failures and what they look like
• Testing a leaf switch diode in place
• Testing CPU switch rows with the switch levels test
• "Check Switch #" message at start-up

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