You probably don't need a course on this.
Look at some manufacturers instructions and learn components and function of parts.
Get familiar with sequences...power, switch on, fan, pump, venturi, air-pressure switch, ignition, soft-light, sensor, board, full-ignition...etc.
Seek out fixes to common problems - like cleaning venturis, changing HW diaphragms, and looking how pins move in and out to make micro switches.
Get as much free manufacturer training as possible.
And, get someone to run through safe use of a multi-testor with you. Look toward your peers, workers and forums to get the information - but don't waste your money on more courses.