🚿Water Heater

A.O. Smith · ATI-540H-N 0.54 GPM 199,900 BTU High Efficiency Tankless Gas Water Heater

A.O. Smith ATI-540H-N 0.54 GPM 199,900 BTU High Efficiency Tankless Gas Water Heater 33 Error Fix: Heat Exchanger Temperature Sensor Failure

The heat exchanger temperature sensor has failed. This sensor monitors the exchanger surface temperature to prevent overheating and damage. Disconnect power and inspect the sensor and its wiring. The sensor is mounted directly on the heat exchanger body. Replace the sensor if resistance readings are out of range.

High severity2 fixes available✨ Free fix available

Quick Answer

Error 33 on the A.O. Smith ATI-540H-N 0.54 GPM 199,900 BTU High Efficiency Tankless Gas Water Heater means: Heat Exchanger Temperature Sensor Failure. The top-ranked community fix is “Test and Replace Heat Exchanger Temperature Sensor” (medium difficulty, ~60 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:

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: $180-$250.

Other A.O. Smith Models with Error 33

GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater

water_heater · 2 fixes

Other A.O. Smith ATI-540H-N 0.54 GPM 199,900 BTU High Efficiency Tankless 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 ATI-540H-N 0.54 GPM 199,900 BTU High Efficiency Tankless Gas Water Heater Error 33

What does error code 33 mean on a A.O. Smith ATI-540H-N 0.54 GPM 199,900 BTU High Efficiency Tankless Gas Water Heater?

Error code 33 on the A.O. Smith ATI-540H-N 0.54 GPM 199,900 BTU High Efficiency Tankless Gas Water Heater means: Heat Exchanger Temperature Sensor Failure. The heat exchanger temperature sensor has failed. This sensor monitors the exchanger surface temperature to prevent overheating and damage. Disconnect power and inspect the sensor and its wiring. The sensor is mounted directly on the heat exchanger body. Replace the sensor if resistance readings are out of range.

How do I fix error code 33 on my A.O. Smith ATI-540H-N 0.54 GPM 199,900 BTU High Efficiency Tankless Gas Water Heater?

The most successful fix is "Test and Replace Heat Exchanger Temperature Sensor" (medium difficulty, ~60 minutes). Estimated DIY cost: $20-$30. HomeMD has 2 community-ranked fixes for this error.

Is error code 33 on a A.O. Smith ATI-540H-N 0.54 GPM 199,900 BTU High Efficiency Tankless 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 ATI-540H-N 0.54 GPM 199,900 BTU High Efficiency Tankless Gas Water Heater error 33 myself?

Yes — the top-ranked community fix ("Test and Replace Heat Exchanger Temperature Sensor") is rated medium difficulty and takes about 60 minutes. Tools needed: T20 Torx screwdriver, Digital multimeter, Insulated gloves.

How much does it cost to repair A.O. Smith ATI-540H-N 0.54 GPM 199,900 BTU High Efficiency Tankless Gas Water Heater error 33?

DIY cost: $20-$30. 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 ATI-540H-N 0.54 GPM 199,900 BTU High Efficiency Tankless Gas Water Heater have?

The A.O. Smith ATI-540H-N 0.54 GPM 199,900 BTU High Efficiency Tankless 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 33 error?

Error code 33 also appears on 1 other A.O. Smith model: GT15-340-NI Commercial 340,000 BTU Commercial Natural 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 ATI-540H-N 0.54 GPM 199,900 BTU High Efficiency Tankless Gas Water Heater have?

The A.O. Smith ATI-540H-N 0.54 GPM 199,900 BTU High Efficiency Tankless 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 33 error?

Error code 33 also appears on 1 other A.O. Smith model:

GT15-340-NI Commercial 340,000 BTU Commercial Natural Gas Water Heater (water heater) · 2 fixes

Ad Space — banner

Replace ADSENSE_PUBLISHER_ID in AdUnit.tsx