Hayward Pool Heater LO Error Code

What LO means

Safety String Open / Low Pressure Safety string open—water pressure switch, limit switch, or vent pressure switch not satisfied. Heater refuses to start or shuts down quickly. Low flow through heater (pump off, dirty filter, bypass wide open), water pressure switch out of adjustment or failed, tripped high limit switch from previous overheat, vent pressure switch issue. Verify pump is running with solid prime. Clean or backwash filter. Check that bypass valve sends adequate flow through heater. Press high limit reset button if tripped. Check water pressure switch wiring.

First checks before service

This page currently documents the safe reset triage. If the code returns, use the linked source manual or a qualified technician for code-specific diagnosis.

1. 1. One-time fault event (power disturbance or transient sensor reading) common DIY

   Check: Power-cycle at the breaker/disconnect (or unplug for CPAP), restore, and run the system while watching for the code.

   Fix: If the code does not return after a full run cycle, the fault was transient — monitor, no repair needed.

2. 2. Fault persists after a clean reset occasional pro only

   Check: If the code returns immediately or every session, note the conditions (startup, heating, after rain).

   Fix: A persisting code points at the component the manual assigns to this code — that repair is technician territory.