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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 12 Error Fix: Flame Failure

Flame was established but extinguished during operation. The flame rod lost its signal while the burner was firing. This can be caused by gas pressure fluctuations, a dirty or misaligned flame rod, inadequate venting, wind downdraft, or condensation on the flame rod. Clean the flame rod with fine sandpaper. Check gas pressure at the unit — it should be within the rating plate specifications. Inspect venting for proper installation and clearances.

High severity2 fixes available

Quick Answer

Error 12 on the A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater means: Flame Failure. The top-ranked community fix is “Check Gas Pressure and Inspect Venting System” (hard difficulty, ~90 min, DIY cost: $10-$20). 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:

This is a high severity error — continued use may cause damage or a safety hazard
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: $200-$300.

Other A.O. Smith Models with Error 12

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

Combustion Abnormality

2 fixes

Overheat Protection Activated

2 fixes

Over-Temperature Warning

2 fixes

Reversed Polarity / Neutral-Ground Cross

2 fixes

Inlet Water Temperature Sensor Failure

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 12

What does error code 12 mean on a A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater?

Error code 12 on the A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater means: Flame Failure. Flame was established but extinguished during operation. The flame rod lost its signal while the burner was firing. This can be caused by gas pressure fluctuations, a dirty or misaligned flame rod, inadequate venting, wind downdraft, or condensation on the flame rod. Clean the flame rod with fine sandpaper. Check gas pressure at the unit — it should be within the rating plate specifications. Inspect venting for proper installation and clearances.

How do I fix error code 12 on my A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater?

The most successful fix is "Check Gas Pressure and Inspect Venting System" (hard difficulty, ~90 minutes). Estimated DIY cost: $10-$20. HomeMD has 2 community-ranked fixes for this error.

Is error code 12 on a A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater serious?

This error is rated high severity. You should address it promptly — continued use could cause damage or safety concerns.

Can I fix A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater error 12 myself?

Yes — the top-ranked community fix ("Check Gas Pressure and Inspect Venting System") is rated hard difficulty and takes about 90 minutes. Tools needed: Phillips screwdriver, Manometer, Adjustable wrench.

How much does it cost to repair A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater error 12?

DIY cost: $10-$20. Professional repair typically costs $200-$300. 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), 13 (Combustion Abnormality), 14 (Overheat Protection Activated), 16 (Over-Temperature Warning), 29 (Reversed Polarity / Neutral-Ground Cross), and 3 more.

Which other A.O. Smith models have the 12 error?

Error code 12 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)
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)
31 — Inlet Water Temperature Sensor Failure (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 12 error?

Error code 12 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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