Spa will not heat; water stays cold even though pumps run and no error code is displayed.

Safety first: Turn off power at the GFCI breaker and verify no voltage before touching heater terminals.

• Check set temperature: Confirm the set temperature is above current water temperature and that the spa is not in Economy or Sleep mode.
• Observe heater indicator: When the spa calls for heat, the heater icon or indicator light on the topside should turn on. If it does not, suspect a control issue.
• Inspect heater connections: Open the equipment bay and locate the Marquis heater tube. Ensure both power leads are firmly attached to the heater terminals and that there is no sign of burning.
• Test heater resistance: With power OFF and wires removed from the heater, measure resistance across the heater terminals. Typical values are 8–15 ohms depending on wattage. An open circuit (OL) indicates a failed element.
• Check for voltage (advanced): With extreme caution and power ON, a qualified person can measure AC voltage across the heater terminals when the spa is calling for heat. If proper voltage is present but no heating occurs, the heater is bad. If no voltage is present, the relay or board is likely faulty.

Replace the heater assembly if the element is open. If the relay or board is defective, the control pack will need professional repair or replacement.