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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 65 Error Fix: Water Flow Sensor Failure

The water flow sensor (flow meter) has malfunctioned. The unit cannot accurately measure the water flow rate, which is needed to properly modulate the burner. The turbine-type flow sensor may be jammed by debris. Remove and clean the flow sensor — check for mineral deposits or sediment. Spin the turbine manually to verify it moves freely. Replace the flow sensor if cleaning does not resolve the issue.

Medium severity2 fixes available

Quick Answer

Error 65 on the A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater means: Water Flow Sensor Failure. The top-ranked community fix is “Replace the Water Flow Sensor Assembly” (medium difficulty, ~60 min, DIY cost: $50-$70). 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: $180-$250.

Other A.O. Smith Models with Error 65

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

Inlet Water Temperature Sensor Failure

2 fixes

Outlet Water 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 65

What does error code 65 mean on a A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater?

Error code 65 on the A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater means: Water Flow Sensor Failure. The water flow sensor (flow meter) has malfunctioned. The unit cannot accurately measure the water flow rate, which is needed to properly modulate the burner. The turbine-type flow sensor may be jammed by debris. Remove and clean the flow sensor — check for mineral deposits or sediment. Spin the turbine manually to verify it moves freely. Replace the flow sensor if cleaning does not resolve the issue.

How do I fix error code 65 on my A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater?

The most successful fix is "Replace the Water Flow Sensor Assembly" (medium difficulty, ~60 minutes). Estimated DIY cost: $50-$70. HomeMD has 2 community-ranked fixes for this error.

Is error code 65 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 65 myself?

Yes — the top-ranked community fix ("Replace the Water Flow Sensor Assembly") is rated medium difficulty and takes about 60 minutes. Tools needed: Adjustable wrench, Teflon plumber’s tape, Non-contact voltage tester.

How much does it cost to repair A.O. Smith GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater error 65?

DIY cost: $50-$70. Professional repair typically costs $180-$250. 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 65 error?

Error code 65 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)
31 — Inlet Water Temperature Sensor Failure (2 fixes)
32 — Outlet Water Temperature Sensor Failure (2 fixes)

Which other A.O. Smith models have the 65 error?

Error code 65 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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