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A.O. Smith · GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater

A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater 31 Error Fix: Inlet Water Temperature Sensor Failure

The inlet (cold) water temperature sensor has failed — open or short circuit. The unit cannot measure incoming water temperature, which is needed to calculate the required temperature rise. Disconnect power and check the sensor connector on the control board. Measure sensor resistance — it should read approximately 10K-50K ohms depending on water temperature. Replace the inlet temperature sensor if readings are abnormal.

Medium severity2 fixes available

Quick Answer

Error 31 on the A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater means: Inlet Water Temperature Sensor Failure. The top-ranked community fix is “Replace Inlet Water Temperature Sensor” (medium difficulty, ~40 min, DIY cost: $20-$30). 2 community-ranked fixes available below.

Published February 2026 · 2 fixes · water 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 water 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: $110-$160.

Other A.O. Smith Models with Error 31

ATI-540H-N 0.54 GPM 199,900 BTU High Efficiency Tankless Gas Water Heater

water_heater · 2 fixes

Other A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater Error Codes

Ignition Failure

2 fixes

Flame Failure

2 fixes

Combustion Abnormality

2 fixes

Overheat Protection Activated

2 fixes

Over-Temperature Warning

2 fixes

Reversed Polarity / Neutral-Ground Cross

2 fixes

Outlet Water Temperature Sensor Failure

2 fixes

Heat Exchanger Temperature Sensor Failure

2 fixes

People Also Ask About A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater Error 31

What does error code 31 mean on a A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater?

Error code 31 on the A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater means: Inlet Water Temperature Sensor Failure. The inlet (cold) water temperature sensor has failed — open or short circuit. The unit cannot measure incoming water temperature, which is needed to calculate the required temperature rise. Disconnect power and check the sensor connector on the control board. Measure sensor resistance — it should read approximately 10K-50K ohms depending on water temperature. Replace the inlet temperature sensor if readings are abnormal.

How do I fix error code 31 on my A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater?

The most successful fix is "Replace Inlet Water Temperature Sensor" (medium difficulty, ~40 minutes). Estimated DIY cost: $20-$30. HomeMD has 2 community-ranked fixes for this error.

Is error code 31 on a A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water 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 A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater error 31 myself?

Yes — the top-ranked community fix ("Replace Inlet Water Temperature Sensor") is rated medium difficulty and takes about 40 minutes. Tools needed: Phillips screwdriver, Multimeter.

How much does it cost to repair A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater error 31?

DIY cost: $20-$30. Professional repair typically costs $110-$160. Many homeowners save money by trying the top-ranked community fix first.

What other error codes does the A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater have?

The A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater has 8 other known error codes, including: 11 (Ignition Failure), 12 (Flame Failure), 13 (Combustion Abnormality), 14 (Overheat Protection Activated), 16 (Over-Temperature Warning), and 3 more.

Which other A.O. Smith models have the 31 error?

Error code 31 also appears on 1 other A.O. Smith model: ATI-540H-N 0.54 GPM 199,900 BTU High Efficiency Tankless Gas Water Heater. The fix may vary by model, so check each model's specific page for tailored repair steps.

What other error codes does the A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater have?

The A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater has 8 other known error codes:

11 — Ignition Failure (2 fixes)
12 — Flame Failure (2 fixes)
13 — Combustion Abnormality (2 fixes)
14 — Overheat Protection Activated (2 fixes)
16 — Over-Temperature Warning (2 fixes)
29 — Reversed Polarity / Neutral-Ground Cross (2 fixes)
32 — Outlet Water Temperature Sensor Failure (2 fixes)
33 — Heat Exchanger Temperature Sensor Failure (2 fixes)

Which other A.O. Smith models have the 31 error?

Error code 31 also appears on 1 other A.O. Smith model:

ATI-540H-N 0.54 GPM 199,900 BTU High Efficiency Tankless Gas Water Heater (water heater) · 2 fixes

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