A controller for a fuel-fired appliance comprising a microprocessor that is able to detect an interruption in power, and immediately store errors and historical diagnostic information in a non-volatile memory before the microprocessor powers down. Upon restoration of power, the microprocessor would flash or display the most recent error codes on the controller's diagnostic LED or display. The microprocessor is also able to detect an open fuse condition, and flash or display the fuse error on the display.