Pressure switch

The Pressure Switch (Plastic, 1/8" MPT, Model 3902) is the small safety switch that lives on top of your spa's heater. It senses water flow through the heater chamber and tells the controller whether it's safe to fire the heater. When this switch fails, the most common error code on the topside controller is FLC ("flow check") — the controller can't confirm flow, so it locks out heating to protect the heater element from a dry-fire.

When you'll need to replace it: Replace the pressure switch when the topside throws an FLC error code that doesn't clear after a filter clean and pump check. Pressure switches eventually fail from corrosion of the internal contacts, the diaphragm losing flexibility, or the connection threads cracking with age. The switch is cheap and the test is simple — swap it out before assuming the heater itself has failed.

How to install: Drain the spa to below the heater, unscrew the old switch from its 1/8" MPT port, apply Teflon tape to the new switch threads, hand-tighten into place, and reconnect the wiring (typically two spade-terminal wires). Refill, restart, and confirm the FLC code clears.

Quick tips:

• Order the matching cable at the same time — Pressure Switch Cable (Global) for Global motherboards or Pressure Switch Cable 72" (Gecko) for Gecko
• If the FLC error returns after a fresh pressure switch and cable, the heater itself or the circ pump may be the underlying problem — book a service tech