501 lid switch is situated under the top deck. There is a magnet moulded into the lid, closing the lid should activate the switch. Remove the top deck (couple of screws at the back of the control panel) for access. Check for wet, corroded wiring and connectors; switch itself may be faulty.
Lid switch and harness is p/no 426457 if required.
501 Lid Switch Differs from Later Models
The Lid Switch is integrated into the display model
If You remove Lid you will see the a Little red Striker That Activates Lid Switch,Check to see if this little striker has not come out or is worn