Control panel displays OH (overheat) and spa shuts down; water temperature is above set point, often over 108°F.

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

• Cool the spa: Remove the cover and allow the spa to cool naturally. You can add some cool water to speed cooling, but do not add large amounts of very cold water to extremely hot water to avoid shell stress.
• Check set temperature and filtration cycles: Once cooled and powered, verify that the set temperature is reasonable (e.g., 100–102°F) and that filtration cycles are not excessively long in hot weather, which can add heat.
• Inspect heater operation: With the spa calling for heat, observe the heater indicator on the topside. If the heater indicator is OFF but the heater tube is still heating (you may hear boiling or feel excessive heat), the heater relay may be stuck closed.
• Verify sensor accuracy: Compare the displayed temperature to an external thermometer placed in the spa water. If the display is significantly lower than actual until OH trips, the temperature sensor is likely out of calibration and should be replaced.
• Check for flow issues: Dirty filters, low water level, or a weak circulation pump can cause hot spots at the heater. Clean or replace filters and confirm strong circulation.

Important: A stuck heater relay or repeated OH errors require professional service on the Marquis control pack to prevent damage or safety hazards.