The fix was simple due to the comprehensive wiring diagram and explanation of role of switches that is found on a piece of paper inside the control panel. The culprit was the primary interlock door switch that failed in a curious way. With door open the switch button is fully extended and contacts are open (OK), but as the button on the switch is depressed it first goes closed (OK), but if the switch button is fully depressed it goes open again (not OK) in this particular defective switch. You need to physically remove the switch to monitor this type of failure, so it was somewhat tricky. The new switch does not go back to open if the button is fully depressed with a closed door, so repair was quick and easy with a new switch.