Before anything else: shut the breaker off before you touch any part of your hot tub. A tripping GFCI is a safety device doing exactly what it was designed to do — telling you electricity is finding a path it shouldn't.
What it looks like
The pattern tells you a lot. A breaker that trips the instant you reset it — or won't set at all — usually points to a wiring fault or a dead short. A breaker that runs fine for a while and then trips, maybe during a heating cycle or when the jets kick on, points to a component failing under load. Related symptoms worth noting: a burning smell, a blank topside display, or a tub that has power at the panel but nothing at the controls.
The usual suspects
In our experience, two culprits account for the overwhelming majority of calls:
The ozone generator. This is the single most common cause we find. The check valve on the ozone line fails, water backflows from the tub all the way down the line into the generator, and the unit dies — taking your breaker with it. That check valve should be replaced annually. It almost never is.
The heater. Pull the element and look at it. Calcium scale, rust, or green corrosion on the tube and sensors causes electrolysis, and that trips the breaker. A heater that looks crusty is the problem.
After those two, in rough order: pumps and motors (uncommon, but it happens), aftermarket stereo AC/DC converters that got wet, a cracked topside control letting water into the electronics, and the light (very rare).
And one that isn't a failure at all: bad wiring. We see supposedly professional installs where the GFCI pigtail — the white neutral coming off the breaker — was landed else where the instead of the panel's neutral bar. If the tub has never worked since installation, suspect the wiring before you suspect the tub.
How to find it
With the breaker off, open the equipment pack and disconnect everything one at a time: heater, pump one, pump two, ozone, and light. Then reconnect one component at a time, flipping the breaker on after each.
The moment it trips, you've found your culprit. If it holds, shut the breaker back off and move to the next component. Also worth checking: the fuses inside the pack — including the 30-amp fuse at center — which you can test for continuity with a basic multimeter.
Where DIY stops
Diagnosing which component is at fault is something most homeowners can do. But if the breaker won't set at all, if you smell burning, if you find scorched or melted wiring, or if the fault traces back to the sub-panel or house wiring — stop and call a professional. Replacing a blown fuse without finding out why it blew just means you'll be doing it again next week.