AO Smith Water Heater Error Codes Explained
AO Smith manufactures a wide range of residential water heaters — from traditional gas tank models to high-efficiency condensing tank units and tankless (on-demand) systems. Each product line uses a slightly different diagnostic system, but all AO Smith water heaters communicate faults through status lights, blink codes, or alphanumeric display codes. This guide covers the most common error codes across AO Smith's residential gas water heater product lines.
Reading AO Smith Error Codes by Product Type
Standard gas tank models (ProLine, Signature): Use a single LED status light on the gas control valve that blinks in patterns similar to Rheem (they share the same FVIR technology). Patterns of 2-7 flashes indicate specific fault conditions.
High-efficiency gas models (Vertex, ProLine Master): Use a digital display on the control unit showing alphanumeric error codes.
Tankless models (Signature 100/200, ProLine XE): Display alphanumeric error codes on the front panel LCD display. These are more detailed and unit-specific.
Common AO Smith Water Heater Error Codes
| Code / Blink Pattern | Meaning | What to Do |
|---|---|---|
| 1 flash (3-sec interval) | Normal operation | No action needed |
| 2 flashes | Thermopile low voltage / weak pilot flame | Relight pilot; clean or replace thermocouple |
| 3 flashes | Pilot outage — failed to maintain pilot | Relight pilot; replace thermocouple if needed |
| 4 flashes | High-limit (ECO) trip — overtemperature | Technician required — root cause diagnosis needed |
| 5 flashes | Temperature sensor failure | Technician required |
| 6 flashes | Flammable vapor sensor lockout | Remove vapors; relight; call tech if recurring |
| 7 flashes | Gas valve failure | Technician required — valve replacement |
| E05 (Vertex/digital) | Ignition failure | Check gas supply; one reset; call tech |
| E10 (Vertex/digital) | High flue temperature | Check vent for blockage; call technician |
| E20 (tankless) | Cold water inlet temperature sensor fault | Technician required |
| E30 (tankless) | Fan motor fault | Technician required |
| E110 (tankless) | Ignition failure — no flame detected | Check gas; clean igniter; call if recurring |
Pilot Light and Thermocouple Issues
The most common call Burkhardt technicians receive on AO Smith standard tank units is a pilot that won't stay lit (3-flash code) or a thermopile reading low voltage (2-flash code). In both cases, the root cause is usually the thermocouple or thermopile — the thin metal probe that sits in the pilot flame and generates a tiny electrical current to hold the gas valve open.
To check: relight the pilot and hold the pilot button for a full 60 seconds before releasing. If the pilot goes out when you release the button, the thermocouple/thermopile is not generating enough voltage. The part costs $15-$25, but replacing it requires disconnecting the gas supply line and the pilot assembly — a task that should be done by a licensed plumber or HVAC technician.
Scale Buildup on AO Smith Tankless Models
AO Smith tankless units in hard water areas (common in Southeastern Wisconsin) will accumulate scale (calcium carbonate) on the heat exchanger, eventually triggering E20 or E30 faults as flow is restricted and sensors are affected. AO Smith recommends annual descaling for tankless units in hard water areas using a citric acid or white vinegar flush. If scaling has progressed too far, the heat exchanger may need replacement.
When to Call a Professional
High-limit trips (4 flashes, E05 with recurring resets), flammable vapor lockouts that recur, gas valve faults, and all tankless-specific errors beyond basic vent clearing require professional diagnosis. AO Smith tankless units in particular require specific descaling procedures and combustion analysis that homeowners cannot perform safely without training and tools.
Burkhardt Heating, Cooling, Plumbing & Electric services AO Smith tank and tankless water heaters across Southeastern Wisconsin, with same-day appointments available for most service calls.
AO Smith Water Heater Service in Wisconsin
Water heater fault codes are rarely urgent — until the unit stops producing hot water entirely. Catching and resolving them early is almost always faster and less expensive than an emergency replacement call.
If you see any of these codes, call Burkhardt at 414-206-3049. Our NATE-certified technicians can diagnose and repair the issue — usually in a single visit.
Ready to schedule service? Call Burkhardt Heating, Cooling, Plumbing & Electric at 414-206-3049 or contact us online. Available 24/7 for emergencies.
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