A.O. Smith water heater thermopile
Use for low millivolt flash-code faults after cleaning the pilot and checking draft.
$25-$80
Check price
A.O. Smith Water Heater Error Codes: Complete Guide
A.O. Smith manufactures one of the widest lines of water heating equipment in North America, including the ProLine, Vertex (condensing), Voltex (heat pump), and Effex (high-efficiency gas) series. Error codes vary by model — this guide covers all major A.O. Smith fault codes.
A.O. Smith Tankless Error Codes (ATI/ATH Series)
| Code | Description | Common Cause |
|---|---|---|
| E01 | Ignition failure | Gas supply, igniter, flame sensor |
| E02 | Flame signal lost during operation | Gas pressure, flame sensor |
| E03 | Overheat / high temperature limit | Scale, low flow, high setpoint |
| E04 | Cold water inlet sensor fault | Sensor failed or wiring |
| E05 | Hot water outlet sensor fault | Sensor failed or wiring |
| E06 | Flue gas temperature sensor fault | Flue sensor fault |
| E07 | Low water flow | Dirty inlet filter, low pressure |
| E08 | Combustion fan fault | Fan motor or speed sensor |
| E09 | Gas valve fault | Gas valve or wiring |
| E10 | Overheat — high limit trip | Second-level overheat protection |
| E11 | Scale detector alert | Scale buildup in heat exchanger |
A.O. Smith ProLine Gas Storage — Flash Codes
Standard ProLine and Craftmaster gas storage heaters use LED flash codes:
| Flash Count | Fault | Action |
|---|---|---|
| 1 flash / 3 sec | Normal — pilot lit | No action needed |
| 2 flashes | Thermopile voltage < 350 mV | Clean thermopile, check draft |
| 3 flashes | Pilot out | Relight — check gas supply |
| 4 flashes | Ignition failure | Check spark, gas, thermocouple |
| 5 flashes | Sensor fault | Temperature sensor issue |
| 6 flashes | Thermal switch open | Scale or high temperature trip |
| 7 flashes | Gas valve fault | Gas valve or wiring issue |
A.O. Smith Voltex Heat Pump Error Codes
| Code | Description | Action |
|---|---|---|
| E01 | Low refrigerant / compressor fault | Check refrigerant and compressor |
| E02 | High refrigerant pressure | Check condenser coil and fan |
| E03 | Fan motor fault | Check fan motor and capacitor |
| E04 | Ambient temperature out of range | Operating range: 37–120°F |
| E05 | Compressor overload | Check amps and refrigerant |
| E06 | Control board fault | Replace control board |
| HP | Heat pump mode active | Normal display |
A.O. Smith Vertex (Condensing Gas Storage) Faults
| Fault | Description | Action |
|---|---|---|
| E1 | Ignition failure | Gas, igniter, flame sensor |
| E2 | Overheat | Scale buildup — descale unit |
| E3 | Flue blocked / combustion air fault | Check vent and combustion air |
| E4 | Blower fault | Check combustion blower |
| E5 | Gas valve fault | Check valve and gas supply |
Most Common A.O. Smith Faults
E03 — Overheat (Tankless)
Scale buildup is the leading cause in hard water areas:
- Perform descaling with white vinegar or commercial descaler
- Clean inlet filter screen
- Lower setpoint — keep below 120°F for safe operation
- Check minimum flow rate requirement (0.5 GPM for most models)
2 Flashes — Low Thermopile Voltage (Gas Storage)
- Clean the thermopile with fine steel wool
- Verify the pilot flame surrounds the thermopile tip
- Check for draft problems — wind or negative pressure in the space
- Replace thermopile if voltage stays below 350 mV
E01 — Ignition Failure (Tankless)
- Verify gas pressure at the unit (3.5 in. w.c. natural gas)
- Check igniter spark — should be visible in dark conditions
- Clean flame sensor
- Check for blocked burner ports
A.O. Smith Parts Reference
| Part | Notes |
|---|---|
| Thermopile | Match lead length — most ProLine models use 36” |
| Temperature sensors | NTC thermistor — model-specific |
| Condensate trap | Check and clean on Vertex condensing models |
| Pressure relief valve | Replace if weeping — match BTU rating |
| Anode rod | Replace every 3–5 years — extends tank life |
Need Replacement Parts?
Before ordering, match the water-heating part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
Pro tip: A.O. Smith and State Water Heaters are the same products with different labels — parts and service procedures are identical across both brands.