Display shows PSo or PSO, and the spa will not start the heater cycle.

Safety first: Turn off power at the GFCI breaker.

PSo/PSO means that the pressure switch is not closing when the pump is running, so the control system will not allow the heater to energize.

• Verify water level and filter: Fill the spa to the proper level and clean or replace the filter cartridge.
• Bleed air: Loosen pump and heater unions to release trapped air until water flows, then retighten.
• Check pump performance: Restore power and run the pump. Confirm strong flow at the jets and heater return. Weak flow suggests a failing pump or a blocked impeller.
• Inspect and test pressure switch: With power off, remove the pressure switch wires and test continuity while manually running the pump (if possible) or by applying gentle air pressure. Replace the switch if it does not close reliably.

If PSo persists after ensuring good flow and replacing the switch, the control board input may be faulty and should be checked by a spa technician.