A.O. Smith HVAC Error Codes: Complete Diagnostic List
10 diagnostic codes documented · Official site ↗
🔴 Critical — Shut Down Immediately
Flammable Vapor Detection
The E06 error code on your A.O. Smith water heater indicates that the unit's flammable vapor sensor has detected the presence of potentially explosive gases. This is a serious safety concern. The water heater is designed to shut down automatically to prevent ignition of these vapors. This sensor is located near the base of the unit to detect heavier-than-air gases like propane or natural gas. It's important to address this issue immediately to prevent a fire or explosion. The unit will remain locked out until the issue is resolved and the system is reset.
High Temperature Cutoff
The A.O. Smith E4 error code indicates that the high temperature limit switch has tripped. This switch is a critical safety device designed to shut down the water heater if the water temperature exceeds a pre-set limit, typically around 190-200°F. When this happens, the unit locks out to prevent further heating and potential damage or even a dangerous situation like scalding or tank rupture. The error code will appear on the digital display, and the water heater will not operate until the issue is resolved and the system is reset. It's important to address this issue promptly, as repeated overheating can damage the tank and other components, shortening the lifespan of your water heater. Ignoring this code can lead to unsafe water temperatures or even a complete system failure.
Emergency Cutoff
The A.O. Smith water heater's 'ECO' or Emergency Cutoff code signifies that the high limit switch has tripped. This switch is a crucial safety device designed to shut down the water heater if the water temperature exceeds a pre-set limit, typically around 190-200°F. When this happens, the heater stops working to prevent dangerous conditions like scalding hot water or, in extreme cases, a tank rupture due to excessive pressure and temperature. Ignoring this warning can be very dangerous. This error is not something to take lightly. It means the system detected a problem that could cause harm. The most common reasons for this are overheating due to a faulty thermostat, a malfunctioning heating element that won't shut off, or a blockage in the tank that prevents proper water circulation. Because this is a safety issue, it's important to get it checked out by a qualified technician.
🟡 Warning — Schedule Service
Thermistor Failure
The E1 error code on your A.O. Smith tankless water heater indicates a problem with one of its thermistors. Thermistors are temperature-sensitive resistors that send temperature readings to the unit's control board. The control board uses these readings to regulate the burner and ensure safe and efficient operation. An E1 error specifically points to a failure or out-of-range reading from either the inlet or outlet thermistor. This could be due to a faulty sensor, wiring issues, or a problem with the control board itself. Because the unit cannot accurately monitor water temperature, it shuts down to prevent potential overheating or damage.
Dry Fire Protection
The E2 error code on your A.O. Smith tankless water heater indicates a 'Dry Fire Protection' lockout. This safety feature is triggered when the unit senses the burner is firing without adequate water flow to cool it. The heater's control board monitors temperature sensors, and if the temperature rises too rapidly or exceeds a set limit, it assumes a dry-fire condition and shuts down the unit to prevent damage. This is important because running a tankless water heater without sufficient water flow can cause the heat exchanger to overheat and potentially crack or rupture. A cracked heat exchanger can lead to water leaks and, in some cases, carbon monoxide production if combustion gases are not properly vented. The E2 error is a protective measure to prevent these issues. The unit will remain locked out until the problem is resolved and the error is cleared.
Sensor Fault
The E3 error code on your A.O. Smith tankless water heater indicates a problem with one of its temperature sensors. These sensors are vital for monitoring water temperature and ensuring the unit operates safely and efficiently. The heater's control board relies on accurate readings from these sensors to regulate the burner and prevent overheating. When the control board detects a faulty sensor signal (either an open circuit, short circuit, or out-of-range reading), it shuts down the unit and displays the E3 error code. This shutdown is a safety measure to prevent potential damage to the unit or, in rare cases, unsafe operating conditions. The specific sensor causing the issue needs to be identified and replaced to restore normal operation. Because this error locks out the water heater, you won't have hot water until the problem is resolved.
Communication Error
The E5 error code on your A.O. Smith tankless water heater indicates a communication problem between the main control board and another component, most often the remote controller or the display panel. Think of it like a dropped call – the different parts of the unit can't talk to each other. This communication breakdown prevents the water heater from operating safely and efficiently, so it shuts down. It's important to address this promptly to restore hot water and prevent potential damage to the unit. This error could stem from a loose wire, a faulty communication cable, or a problem with one of the control boards themselves. Sometimes, power fluctuations can also cause this error. Because multiple components could be at fault, diagnosing the root cause requires some electrical testing. It's not something you can easily fix without the right tools and knowledge.
Controls Locked
The 'LOC' error code on your A.O. Smith water heater indicates that the control panel has been locked. This feature is designed to prevent unauthorized changes to the water heater's settings. When locked, the unit will typically not respond to button presses or temperature adjustments, effectively disabling its normal functionality. This lockout can occur for several reasons, including accidental activation of the lock feature, a software glitch, or as a safety measure triggered by an underlying issue within the water heater. Because the unit is locked, you won't be able to adjust the temperature or access diagnostic information until it's unlocked. This usually requires specific knowledge of A.O. Smith control systems.
Lower Element Fault
The 'LWR' error code on your A.O. Smith water heater indicates a problem with the lower heating element circuit. The control board monitors the electrical current flowing through the heating elements. If it detects a short circuit, open circuit, or other electrical anomaly, it will display this error and shut down the element to prevent further damage or potential fire hazard. This usually means the element has failed, or there's a wiring issue. Because the water heater relies on both elements to properly heat water, a fault in one will prevent the unit from operating correctly. The lower element is especially important as it heats the larger volume of water at the bottom of the tank.
Upper Element Fault
The 'UPR' error code on your A.O. Smith water heater indicates a problem with the upper heating element. This element is responsible for heating the top portion of the tank, providing you with hot water. The control board has detected either a short circuit, an open circuit, or some other electrical anomaly with this element. Because of this fault, the water heater has likely shut down the heating process to prevent further damage to the element or other components. This means you'll be without hot water until the issue is addressed. It's important to resolve this promptly to avoid potential issues like sediment buildup in the tank due to inconsistent heating.