F3 displayed, heater cycles erratically or spa overheats/underheats

Safety: Disconnect power at the breaker before opening the control compartment.

• Power cycle: Turn off the breaker for 2–3 minutes, then restore power to see if F3 clears temporarily. If it returns, continue.
• Inspect sensor wiring: Open the Villeroy & Boch equipment bay and locate the water temperature sensor on the heater tube. Follow its cable to the control board. Ensure the plug is fully seated and free of corrosion or moisture.
• Check for physical damage: Look for cuts, abrasions, or rodent damage on the sensor cable. If damaged, the sensor assembly should be replaced.
• Compare displayed temperature: With the spa running, compare the displayed temperature to an external thermometer placed in the water. If the reading is wildly inaccurate, the sensor is likely faulty.
• Replace sensor if needed: If you are comfortable, drain water below the heater level, remove the old Villeroy & Boch temperature sensor from the heater manifold, install the new sensor with appropriate O-ring, and reconnect to the control board.

If replacing the sensor does not resolve F3, the control board may be defective and should be evaluated by a professional.