AO Smith Water Heater Error Codes: E1, E2, E3, E4, E5 — Causes & Reset Steps

Common Symptoms

• Error code flashing or displayed on the AO Smith LED or digital panel
• No hot water or insufficient hot water despite the unit being powered on
• Water heater repeatedly shuts off and won't stay running
• Control panel showing 'Err' followed by a number
• Pilot won't light or gas valve won't open on gas models
• Water heater tripping the high-limit reset button

Most Likely Causes

1. 1

   E1 — Upper Thermostat / High-Limit Fault

   E1 on AO Smith electric water heaters indicates the upper thermostat has tripped the high-limit safety cutout, meaning tank water exceeded ~170°F. Causes: thermostat stuck in the 'on' position continuously heating the tank, a failed high-limit sensor, loose or corroded thermostat contact with the tank wall, or sediment buildup at the bottom of the tank forcing longer heating cycles that overheat the upper portion. E1 is the most common AO Smith electric error and is usually cleared by pressing the red reset button behind the upper element access panel — but the underlying cause must be addressed or the fault will recur.

2. 2

   E2 — Lower Thermostat Fault

   E2 indicates the lower thermostat has detected an out-of-range temperature condition or has failed. On AO Smith Signature and ProLine electric models, the lower thermostat controls the lower heating element and monitors the lower-tank water temperature. A failed lower thermostat can cause the lower element to run continuously (leading to E1 when heat migrates up) or prevent the lower element from activating at all (resulting in the upper element carrying full heating load, which increases cycling time and energy use). The lower thermostat is located behind the lower element access panel.

3. 3

   E3 — Temperature Sensor Fault

   E3 on AO Smith Signature and Voltex models indicates the tank temperature sensor (thermistor) has an open or shorted circuit. The thermistor is a small probe that monitors water temperature and reports it to the digital control module. A failed thermistor prevents accurate temperature control — the unit may overheat, underheat, or refuse to operate. Causes: a disconnected or corroded thermistor connector, a failed thermistor probe, or water intrusion into the sensor port. On Voltex hybrid models, E3 can also indicate an ambient air temperature sensor fault on the heat pump module.

4. 4

   E4 — Dry Fire / Heating Element Burned Dry

   E4 indicates the water heater detected a 'dry fire' condition — the heating element was energized without water surrounding it. This occurs when the tank is not fully filled before the unit is powered on (common after installation, maintenance drains, or after the home water supply was shut off), or when the dip tube is cracked and allows an air pocket to form around the upper element. AO Smith Signature and ProLine electric models include a dry-fire protection circuit that cuts power and stores E4. The element may be damaged — test element resistance before resetting.

5. 5

   E5 — Flue / Exhaust Temperature Fault (Gas Models)

   E5 appears on AO Smith gas water heaters with power vent or direct vent systems and indicates the flue exhaust temperature sensor detected temperatures outside the acceptable range. Causes: a blocked vent pipe (bird nest, debris, ice in cold climates), a failed draft inducer fan motor on power-vent models, a cracked or loose vent connector allowing hot gases to recirculate, or a failed flue temperature sensor. An E5 fault shuts down the gas valve for safety — the unit will not attempt to fire until the fault is cleared.

6. 6

   Err Codes — Gas Control Valve Communication Errors

   On AO Smith gas models with electronic gas control valves (including the Honeywell-branded control fitted to many AO Smith ProLine gas models), 'Err' or 'Error' codes displayed as a series of LED flashes indicate gas valve electronic faults. The number of flashes before a pause is the code: 2 flashes = thermopile voltage low (pilot is lit but voltage is below the threshold to open the main gas valve); 4 flashes = high-limit switch tripped (water overheated, requires reset); 5 flashes = sensor failure (temperature sensor on the gas control); 7 flashes = gas control valve electronics failed (valve replacement required). These codes are displayed on the LED window of the gas control valve, not on a separate display.

Quick DIY Checks

1. 1Step 1 — Identify your AO Smith model type and locate the error display: AO Smith electric models (Signature ESRT, ProLine electric) display E-codes on a small LED window on the upper control board behind the top access panel. AO Smith Voltex hybrid heat pump models display codes on the front-panel LCD. Gas models display LED flash codes on the Honeywell gas control valve window (the small clear window on the gas valve body). Before resetting any code, write down the exact code displayed — it tells you precisely which component needs attention. Resetting without noting the code means you lose the diagnosis if the fault recurs.
2. 2Step 2 — Reset E1 (upper high-limit trip) on electric models: turn off the 240V circuit breaker. Remove the upper element access panel (two screws) and pull back the insulation. Locate the red reset button on the upper thermostat — it's a small circular button at the center of the thermostat body. Press it firmly until you feel a click. Replace the insulation and access panel, restore power, and test for hot water. IMPORTANT: If E1 recurs within 1–2 heating cycles, the thermostat has failed and is allowing the tank to overheat — replace the upper thermostat ($10–$25, 30-minute job). Do not repeatedly reset E1 without investigating the underlying cause.
3. 3Step 3 — Diagnose and reset E2 (lower thermostat fault): turn off the circuit breaker. Remove the lower access panel. Inspect the lower thermostat — check that it's firmly seated flat against the tank wall (the thermostat must make direct metal-to-metal contact with the tank to read temperature accurately). Check the wiring connections. With a multimeter set to continuity, test the lower thermostat: it should read closed (continuity) at room temperature. An OL reading at room temperature means the thermostat has failed open and needs replacement. Replacement lower thermostat kits for AO Smith typically include both upper and lower thermostats.

1. 4Step 4 — Address E3 (temperature sensor fault): with the unit powered off, locate the thermistor — on Signature electric models it's a probe inserted into a well on the side of the tank, accessible from the upper or lower control access area. Disconnect the thermistor connector and test resistance with a multimeter: at 70°F the reading should be approximately 10,000 ohms (10k Ω). A reading of 0 Ω indicates a shorted sensor; OL (open) indicates an open/failed sensor. Replace if out of spec. On Voltex hybrid models, check both the tank thermistor and the ambient air sensor on the heat pump intake — ice formation around the heat pump intake in cold basements (below 40°F) can also trigger E3.
2. 5Step 5 — Recover from E4 (dry fire) on electric models: turn off the circuit breaker. Open a hot water faucet in the house. Slowly open the cold water supply valve to the water heater and confirm water flows from the hot faucet (indicating the tank is filling). Once a steady stream of water flows from the hot tap without sputtering, the tank is fully filled and the heating element is submerged. Close the hot tap. With a multimeter set to resistance, test the heating element at its terminals (behind the access panels): 4500W elements should read 10–16 Ω; 5500W should read 9–13 Ω. An OL reading indicates the element burned out during the dry-fire event and must be replaced before restoring power. Once the element is confirmed good (or replaced), restore power — E4 clears on the next successful heating cycle.
3. 6Step 6 — Clear E5 (flue/vent fault) on gas power-vent models: set the gas control to PILOT to disable the burner. Inspect the entire vent pipe run: check the exterior vent termination cap for blockages (bird nests, debris, snow/ice in cold climates). Check all vent pipe joints inside the home for disconnections or cracks. On power-vent models, listen at the exhaust fan (located on top of the unit) — it should spin up before ignition; a seized or slow fan is a common E5 cause. Reset the fault by setting the gas control to OFF for 5 minutes, then restarting through the normal ignition sequence. If E5 returns immediately, the flue temperature sensor on the vent hood has failed and needs replacement.
4. 7Step 7 — Decode gas control LED flash codes (Err codes): stand in front of the water heater and observe the LED window on the Honeywell gas control valve (the box with the temperature dial). Count the flashes before the long pause: 2 flashes = thermopile voltage low: relight the pilot manually; if the pilot stays lit but 2-flash code persists, test thermopile output voltage (should be 650–850mV across the thermopile leads with the pilot lit; below 400mV = replace the thermopile). 4 flashes = high-limit tripped: set gas control to PILOT, wait for the tank to cool 30 minutes, then restart. 5 flashes = temperature sensor fault: the sensor in the gas control body has failed — the entire gas control valve must be replaced. 7 flashes = gas control valve electronics failed: replace the gas control valve.

Repair vs Replace

Recommended Tools & Parts

• AO Smith Thermostat Kit (Upper + Lower)

  Replacement upper and lower thermostat set for AO Smith Signature and ProLine electric water heaters. Fixes E1 and E2 error codes caused by thermostat high-limit trips or thermostat failure. Includes both thermostats to replace as a matched set.

  $15–$30

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• AO Smith Thermistor / Temperature Sensor

  Replacement tank temperature sensor (thermistor) for AO Smith Signature and Voltex hybrid models. Fixes E3 temperature sensor fault codes. Match to your specific model number for correct probe length and connector type.

  $20–$50

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• AO Smith Heating Element 4500W or 5500W

  Replacement heating element for AO Smith electric water heaters. Required after E4 dry-fire events that damage the element. Available in 4500W and 5500W — check the data label on your water heater for the correct wattage.

  $20–$45

  Buy on Amazon →

• Honeywell WV8840A Gas Control Valve

  Replacement gas control valve for AO Smith ProLine and Signature gas water heaters equipped with the Honeywell electronic gas valve. Required for 5-flash and 7-flash error codes indicating gas valve sensor or electronics failure. Match the BTU rating to your existing unit.

  $60–$150

  Buy on Amazon →

• Universal Water Heater Thermopile

  Replacement thermopile for AO Smith gas water heaters showing 2-flash low voltage codes. Compatible with Honeywell gas control valves. Test thermopile output with a multimeter before replacement to confirm low voltage.

  $20–$40

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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