Control boxes have several fuses. In this example
there are 2 shown in red circles.
The best way to test a fuse is turn main power to
the spa OFF. Set your meter to OHMS. Place one probe
on each side of the fuse. If your meter showns fuse
contininuity the fuse is fine. If the meter shows no
continuity replace the fuse.
In this example the smaller glass fuse is in
place to protect the board from damage. It would be
the likely fuse to fail. The larger fuse on the
bottom of the box is there to protect the pump
IF your spa has passed both the incoming power test
and the fuse test we then focus on the transformer.