Hayward Pool Heater LO Error Code

Updated 2026-08-29 · review status: manual-verified Last verified: 2026-08-29

Quick answer

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

Severity: high · DIY difficulty: intermediate

Safety

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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.

Causes, ranked

  1. 1. Limit string open - safety (limit) device tripped by overheat or low flow common diy-ok

    Check:

    Fix:

  2. 2. Failed limit switch within the safety string occasional pro-only

    Check:

    Fix:

Parts & supplies for Hayward Pool Heater: filters, control boards, sensors, and common replacement parts.

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Commonly searched as: "hayward lo" · "hayward error code lo" · "hayward air conditioner lo"

Sources

  • PoolDial error-code reference (manual-backed) — verified 2026-08-29
  • Hayward H-Series H150FD-H500FD Service & Installation Manual (Figure 27: Error codes) — verified 2026-08-29
  • PoolDial error-code reference (manual-backed) — verified 2026-08-29