Dishwasher won't start; start button blinks 3 times then turns off

My Whirlpool dishwasher (WDF520PADM7, about 6 years old) will not start. I press the start button, it blinks 3 times, then turns off. I have turned off the circuit breaker for 10 minutes or so, hoping this will reset the machine, but no change. I have run the 123-123-123 diagnostic, all the lights on the control panel light up, but the machine does not run through the testing protocol. I have checked the latch and it seems to be a tight fit. The control lock is not on. Do you have any suggestions as to what else I can try for a DIY fix? Thank you.