Hayward H-Series H400FDN 400,000 BTU Natural Gas Pool Heater LO Error Fix: Heater Lockout

Heater lockout — the heater has locked out after repeated ignition failures or due to gas pressure issues, requires manual reset or power cycle

Medium severity2 fixes available✨ Free fix available

Quick Answer

Error LO on the Hayward H-Series H400FDN 400,000 BTU Natural Gas Pool Heater means: Heater Lockout. The top-ranked community fix is “Check and Adjust Gas Pressure Regulator” (medium difficulty, ~45 min, DIY cost: $10-$20). 2 community-ranked fixes available below.

Published February 2026 · 2 fixes · pool heater

Content is based on manufacturer service documentation, community repair experience, and OEM technical bulletins. Always verify with your owner's manual.Learn about our editorial standards.

When to Call a Professional

While many pool heater repairs can be done safely at home, consider hiring a qualified technician if:

The repair involves electrical wiring, gas lines, or refrigerant
You've tried the top-ranked fix and the error persists
You smell gas, see sparking, or notice water leaking
The repair requires specialized tools you don't have
Your appliance is still under manufacturer warranty

Average professional repair cost for this issue: $150-$250.

Other Hayward H-Series H400FDN 400,000 BTU Natural Gas Pool Heater Error Codes

Air Proving Switch Closed

Air Operation Error

Board Diagnostic

Bypass Operation

Communication Error

High Limit Switch Tripped

2 fixes

Ignition Failure

2 fixes

Pressure Switch Failure

2 fixes

People Also Ask About Hayward H-Series H400FDN 400,000 BTU Natural Gas Pool Heater Error LO

What does error code LO mean on a Hayward H-Series H400FDN 400,000 BTU Natural Gas Pool Heater?

Error code LO on the Hayward H-Series H400FDN 400,000 BTU Natural Gas Pool Heater means: Heater Lockout. Heater lockout — the heater has locked out after repeated ignition failures or due to gas pressure issues, requires manual reset or power cycle

How do I fix error code LO on my Hayward H-Series H400FDN 400,000 BTU Natural Gas Pool Heater?

The most successful fix is "Check and Adjust Gas Pressure Regulator" (medium difficulty, ~45 minutes). Estimated DIY cost: $10-$20. HomeMD has 2 community-ranked fixes for this error.

Is error code LO on a Hayward H-Series H400FDN 400,000 BTU Natural Gas Pool Heater serious?

This error is rated medium severity. It should be addressed soon, but your appliance may still function in a limited capacity.

Can I fix Hayward H-Series H400FDN 400,000 BTU Natural Gas Pool Heater error LO myself?

Yes — the top-ranked community fix ("Check and Adjust Gas Pressure Regulator") is rated medium difficulty and takes about 45 minutes. Tools needed: T20 Torx screwdriver, Manometer gauge, Adjustable wrench.

How much does it cost to repair Hayward H-Series H400FDN 400,000 BTU Natural Gas Pool Heater error LO?

DIY cost: $10-$20. Professional repair typically costs $150-$250. Many homeowners save money by trying the top-ranked community fix first.

What other error codes does the Hayward H-Series H400FDN 400,000 BTU Natural Gas Pool Heater have?

The Hayward H-Series H400FDN 400,000 BTU Natural Gas Pool Heater has 8 other known error codes, including: AC (Air Proving Switch Closed), AO (Air Operation Error), bd (Board Diagnostic), bo (Bypass Operation), CE (Communication Error), and 3 more.

What other error codes does the Hayward H-Series H400FDN 400,000 BTU Natural Gas Pool Heater have?

The Hayward H-Series H400FDN 400,000 BTU Natural Gas Pool Heater has 8 other known error codes:

AC — Air Proving Switch Closed (2 fixes)
AO — Air Operation Error (2 fixes)
bd — Board Diagnostic (2 fixes)
bo — Bypass Operation (2 fixes)
CE — Communication Error (2 fixes)
HS — High Limit Switch Tripped (2 fixes)
IF — Ignition Failure (2 fixes)
PF — Pressure Switch Failure (2 fixes)

Ad Space — banner

Replace ADSENSE_PUBLISHER_ID in AdUnit.tsx