Display shows F4 and heater will not engage; spa may run pumps but not heat.

Safety first: Turn OFF power at the breaker and verify no voltage before opening the control box.

• Visual inspection of heater and wiring:
  • Open the equipment bay and locate the stainless steel heater tube.
  • Check heater terminals for signs of burning, melted insulation, or loose nuts.
  • Tighten heater terminal nuts gently if loose (do not overtighten).
• Check heater resistance:
  • With power OFF and heater wires disconnected, measure resistance across the heater element terminals.
  • Typical spa heater elements read between 8–15 Ω depending on wattage; OL (open) indicates a failed element.
• Inspect control board heater relay:
  • Open the USSPA control box cover.
  • Locate the heater relay (often a large relay near heater terminals).
  • Look for burn marks, cracked solder joints, or melted plastic.
• Corrective actions:
  • If the heater element is open or shorted, replace the complete USSPA heater assembly or heater element as specified by the manufacturer.
  • If wiring is burned, replace damaged wires with appropriately rated spa heater wire and new ring terminals.
  • If the relay is visibly damaged or F4 persists with a good heater, replace the main control board or have it professionally rebuilt.
• Reassemble and test:
  • Reconnect all wires according to the wiring diagram.
  • Restore power and command heat from the topside.
  • Verify that F4 clears and that the heater indicator and actual heating are present.