The pack decides everything the spa does. We read it with a meter before replacing it — because half of 'dead pack' calls are a relay, a fuse or a cable.
Spa packs fail in patterns. Balboa boards weld heater relays and burn traces at the element terminal; Gecko packs lock out on low voltage and sulk when moisture reaches the connectors; topside panels die at the ribbon cable more often than at the display. Knowing the patterns means the diagnosis is a measurement session, not an exchange program.

Where a relay, fuse or connector is the culprit, the repair happens at board level and your pack survives. Where the board is genuinely cooked, the correct replacement pack goes in configured for your pump layout and heater on the spot, so the first power-up behaves like the tub you remember.
Pumps cycle on schedule but the display is dead — ribbon cable or panel, and the cable is cheaper. Tested from the pack side first.
Water sails past the number — a heater relay stuck closed. Repairable on most boards, urgent on all of them.
An open sensor circuit. The probe or the input it plugs into; we test both before buying either.
The low-voltage supply on the board sagging. A failing transformer more often than a failing brain.
Real faults, real fixes — the paperwork version, minus the addresses.
Ribbon cable green with corrosion at the pack plug. Cleaned, sealed, panel lived.
Water at 106° and climbing. Board-level relay swap on a BP501 — pack saved.
Describe the symptom and the brand — the diagnosis starts on the phone and finishes with one written number.