Pool Error Codes

Hayward LO: Limit String Open

A Hayward gas heater showing LO has tripped a safety switch somewhere in its limit string and will refuse to fire again until that circuit closes.

Medium severity

Applies to: Hayward Universal H-Series and H-Series (H150FD-H400FD) residential gas pool/spa heaters

What it means

LO isn't a single sensor reporting a single fault — it's a whole circuit. Hayward wires the water pressure switch, the vent/pressure switch, the high-temperature limit, and the flue temperature limit together in series as the "limit string," and the control board only sees whether that combined circuit is open or closed. If any one switch in the chain opens, the display shows LO and the heater won't start a heating cycle, no matter which specific component actually caused it. In practice, the large majority of LO codes trace back to water flow rather than a genuinely tripped high-limit switch, since the pressure switch is the first thing to open when flow drops. Hayward's own heat-exchanger specifications list minimum flow requirements ranging from 20 GPM on the smaller H150FD/H200FD models up to 40 GPM on the largest H400FD, with a system maximum of 125 GPM — flow outside either end of that range is exactly the kind of thing that trips this string.

Likely causes

What you can check yourself

  1. Confirm the pump is running and check that all suction/return valves feeding the heater are fully open
  2. Clean or backwash the filter and clear the pump strainer basket of debris
  3. If on a variable-speed pump, temporarily raise the heater's associated pump speed to see if flow is the issue
  4. Power-cycle the heater at the breaker for 60 seconds to clear a latched fault after flow is restored
Call a professional if…

If flow looks normal (clean filter, open valves, adequate pump speed) and LO still shows, or it returns repeatedly, the pressure switch, a limit switch, or heater wiring likely needs testing and replacement — have a licensed pool heater technician diagnose it rather than bypassing a safety switch yourself.

Sources

Related

This site is an independent reference, not a substitute for a licensed pool professional. Gas pool heaters can leak carbon monoxide and cause fire; pump and heater electrical circuits carry mains voltage. Anything involving the gas line, burner assembly, or house wiring should be handled by a licensed technician or electrician.