Difficult to test. Easy to fix.
The manufacturer taped the circuit diagram and the chart for the timer under the control console. I took a photograph and blew up the parts I needed.
I tested all of the parts. It took several tries. Tested all of the thermostats and the thermal fuse. Tested the heating element. Because this was an intermittent problem the timer tested good until it failed entirely.
Had to test it for 2 hours total time spread across three occasions across three weeks.
The actual repair, once I found the problem, took 10 minutes to repair.