Pool and Spa
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This can be a hard one to diagnose as it could be one of several issues, in most cases the 4 dashes means watchdog. This is most likely because the two sensors (high limit, Thermostat) are out of sync. I would remove and replace both sensors and try to restart spa, if this does not work then we might have to think about the circuit board. The high limit sensor is usually located on the heater housing and the thermostat is usually near the filter box. Open the control box to expose the circuit board be sure power is OFF and find on the top of the circuit board where the sensors plug in and trace the wires back to the sensors and replace.