This is only intended as a guide and should not be used blindly. Your experience and expertise are of high value when troubleshooting this unit. Do not disregard all of your instincts. The microprocessor furnace control was designed with diagnostic capabilities built in. LEDs are used to flash a fault code which will lead you to 1 of the subsections as listed in the Index. You should ALWAYS begin in the START HERE subsection (see Index for page number) which will guide you to the appropriate sub