Hayward H-Series H400FDN 400,000 BTU Natural Gas Pool Heater tO Error Fix: Temperature Sensor Open Circuit

Temperature sensor open circuit — the temperature sensing circuit has an open connection due to a broken wire, corroded connector, or failed thermistor

Medium severity2 fixes available✨ Free fix available

Quick Answer

Error tO on the Hayward H-Series H400FDN 400,000 BTU Natural Gas Pool Heater means: Temperature Sensor Open Circuit. The top-ranked community fix is “Replace the Temperature Sensor Thermistor” (medium difficulty, ~45 min, DIY cost: $15-$25). 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: $120-$180.

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

Heater Lockout

2 fixes

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

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

Error code tO on the Hayward H-Series H400FDN 400,000 BTU Natural Gas Pool Heater means: Temperature Sensor Open Circuit. Temperature sensor open circuit — the temperature sensing circuit has an open connection due to a broken wire, corroded connector, or failed thermistor

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

The most successful fix is "Replace the Temperature Sensor Thermistor" (medium difficulty, ~45 minutes). Estimated DIY cost: $15-$25. HomeMD has 2 community-ranked fixes for this error.

Is error code tO 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 tO myself?

Yes — the top-ranked community fix ("Replace the Temperature Sensor Thermistor") is rated medium difficulty and takes about 45 minutes. Tools needed: Multimeter, T20 Torx screwdriver, Needle-nose pliers.

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

DIY cost: $15-$25. Professional repair typically costs $120-$180. 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)
LO — Heater Lockout (2 fixes)

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