Display shows OH or OHH, spa shuts down all functions; water in the tub feels very hot (often above 40–42°C / 104–108°F).

Safety first: Turn off power at the GFCI breaker. Do not enter the spa if water is excessively hot.

• Cool the water: Remove the cover and allow the spa to cool naturally. You can add some cool water to speed cooling, but do not shock the shell with very cold water if it is extremely hot.
• Check temperature setting: After cooling, restore power and verify the set temperature on the topside control. Lower it to a safe level (e.g., 37–38°C / 99–100°F).
• Inspect temperature sensor: Locate the main temperature sensor in the heater tube or sensor well. Ensure it is fully inserted and the cable is intact. A loose or corroded sensor can misread.
• Test sensor: With power off, disconnect the sensor from the control board and measure resistance with a multimeter. Compare to Arctic Spas thermistor resistance charts for the current water temperature. Replace if readings are far off.
• Check for stuck heater: If the heater continues to heat even when the spa is at or above set temperature, the heater relay on the control board may be stuck closed. This requires control pack service or replacement.
• Environmental causes: In hot climates, ensure the spa is shaded and the cover is vented slightly during heat waves to prevent passive overheating.