DIY Error Codes

Hayward Universal H-Series gas pool heater Error LO

Limit string open. It restarts 2 minutes after the limit string closes.

Note the code. It restarts 2 minutes after the limit string closes. If LO comes back, call a qualified pool heater technician.

Start the fix →When to call a pro ↓
Full answer & models

Hayward LO on a Universal H-Series heater means limit string open. If it keeps returning, Hayward says to contact a qualified service company.

Applies to: H150FDH200FDH250FDH300FDH350FDH400FDH500FD
Also searched as: Hayward heater error LO, Hayward H-Series LO code, Hayward pool heater LO

Gas appliance. Service only by a qualified technician; power off before any service.
Why
Hayward, what to do if you smell gas: do not try to light any appliance; do not touch any electrical switch and do not use any phone in your building; immediately call your gas supplier from a neighbor's phone and follow their instructions; if you cannot reach your gas supplier, call the fire department. Installation and service must be performed by a qualified installer, service agency, or the gas supplier. Hayward: before installing or servicing this electrical equipment, turn the power supply off. Hazardous voltage can shock, burn, cause death or serious property damage.

What LO means

Limit string open. It restarts 2 minutes after the limit string closes.

More detail
Hayward's Universal H-Series owner's manual (Figure 4, Error Codes) lists LO as "Limit string open error": If the limit string opens, the heater shuts down and goes into lockout. Automatic restart is 2 minutes after the error condition is corrected and the limit string closes. The manual's instruction for error codes is to contact a qualified service company for assistance.

5-minute checks

Fix it step by step

Step 1 of 2

  1. Note the code and wait

    • 2 min
    • None
    • DIY

    Safety: Gas appliance. Service only by a qualified technician; power off before any service.

    Automatic restart is 2 minutes after the condition is corrected and the limit string closes.

    More detail

    Note the code and wait.

    Write down the code and when it appears. Per Hayward's Figure 4: Automatic restart is 2 minutes after the condition is corrected and the limit string closes.

    Did the code clear?

  2. Call a qualified service company

    • Decision
    • None
    • Pro decision

    Stop here and book a qualified technician.

    More detail

    Call a qualified service company.

    Hayward's owner's manual says to contact a qualified service company for assistance with error codes. Installation and service must be done by a qualified installer, service agency, or the gas supplier. Do not open the heater cabinet yourself.

    Still showing LO?Need a pro → call a pro

Code cleared. Nice work.

Watch the display for a day to make sure it stays clear.

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Fixed after any step? Watch the display for a day to make sure it stays clear. Still LO after step 2? call a pro.

Call a pro if…

  • You smell gas (leave first, then call your gas supplier from a neighbor's phone).
  • LO keeps coming back or the heater stays in lockout.
  • Anything that needs the heater cabinet opened or wiring checked.

FAQ

What does Hayward heater code LO mean?

"Limit string open error", per Figure 4 of the Universal H-Series owner's manual. If the limit string opens, the heater shuts down and goes into lockout. Automatic restart is 2 minutes after the error condition is corrected and the limit string closes.

What does Hayward say to do about LO?

Contact a qualified service company for assistance, per the manual's error-code section.

Source: Hayward Universal H-Series Owner's Manual (H150FD-H500FD), Figure 4 Error Codes, pp. 13-14; safety, pp. 1 and 3. Written by the DIY Error Codes team. Updated 2026-10-16. DIY Error Codes is independent and not affiliated with or endorsed by Hayward.