AO Smith Water Heater Not Heating: Diagnosis and Fixes

Common Symptoms

• No hot water at any faucet — cold water from every tap
• Lukewarm water that never reaches the thermostat setpoint
• Pilot light extinguishes immediately when releasing the gas control knob
• iCOMM or Remote Monitor app showing E01, E02, E05, E06, or E07 fault code
• Running out of hot water after the first 10–15 gallons (lower element failure)
• Rotten-egg smell or discolored hot water from corroded anode rod

Most Likely Causes

1. 1

   Snap-In Thermocouple Failure — Gas Signature Series (Most Common)

   AO Smith Signature series gas water heaters use a snap-in thermocouple that inserts into a retention clip on the pilot assembly — different from the threaded thermocouple on some other brands. The thermocouple tip must be positioned correctly in the pilot flame to generate sufficient millivolt output. A degraded thermocouple produces less than 17mV (functional range: 25–35mV), causing the gas valve to close when the pilot button is released. AO Smith 100109415 is the OEM replacement thermocouple for Signature and ProLine series gas models.

2. 2

   Failed Upper Heating Element — Electric Models (E06 Code)

   AO Smith electric water heaters (Signature 50, Signature Premier, ProLine electric) use an upper element that heats newly added cold water at the top of the tank. E06 (upper element open circuit) or a complete loss of hot water indicates the upper element has burned out. The formula for expected resistance on a 4500W 240V element: R = V²/W = 240²/4500 ≈ 12.8Ω — test with a multimeter, OL means replace. AO Smith 0012498 is the OEM replacement element for most Signature electric models.

3. 3

   Failed Lower Heating Element — Electric Models (E07 Code)

   E07 (lower element open circuit) means the lower element — responsible for recovery heating after the upper element satisfies — has failed. The symptom is normal hot water for the first 10–15 gallons, then cold. The lower element is submerged in the sediment layer at the tank bottom and experiences more thermal cycling than the upper element, making it more likely to fail first. Same resistance test as the upper element: 12–16Ω functional, OL = replace.

4. 4

   Dry Fire / No Water — E05 Code

   E05 (dry fire detected) means the heating element was energized without water present — typically caused by powering on the unit before the tank is fully filled, or a failure of the cold water inlet check valve allowing the tank to partially drain. A dry-fired element is usually destroyed (burned open) and must be replaced. Before replacing, ensure the tank is completely filled by opening a hot water tap and confirming steady water flow before restoring power.

5. 5

   Inlet or Outlet Temperature Sensor Fault — E01, E02

   E01 (inlet temperature sensor fault) and E02 (outlet temperature sensor fault) indicate an open or shorted NTC thermistor. These sensors monitor the temperature of cold water entering and hot water leaving the tank. A failed sensor prevents accurate temperature regulation, potentially causing the unit to underheat or overheat. On AO Smith Signature Premier models with iCOMM connectivity, sensor faults generate immediate app notifications.

6. 6

   Accelerated Anode Rod Corrosion — High-Mineral Water Areas

   AO Smith Signature series units are known to experience faster-than-average anode rod depletion in homes with high-mineral-content or aggressive water chemistry. A depleted anode rod (corroded to the steel core wire) causes accelerated tank corrosion, rust-colored hot water, rotten-egg odor, and eventually tank failure. In hard water areas, AO Smith recommends inspecting the anode every 2 years and replacing it when more than half the magnesium has corroded away. Consider upgrading to a stainless steel or zinc/aluminum anode to extend intervals.

Quick DIY Checks

1. 1Step 1 — Check iCOMM / Remote Monitor for fault codes: if your AO Smith Signature Premier or Voltex unit has iCOMM Wi-Fi connectivity, open the AO Smith Connected Water Heater app and check the fault history. Active codes: E01 = inlet sensor, E02 = outlet sensor, E05 = dry fire / no water, E06 = upper element open, E07 = lower element open. For non-connected units, look at the gas control valve display or control board LED for blink patterns. Identifying the fault code first saves time — E06 means focus on the upper element, E07 means focus on the lower element. For gas models without a display, proceed directly to pilot and thermocouple inspection.
2. 2Step 2 — Gas models: test the snap-in thermocouple: light the pilot following the label instructions (turn to PILOT, press and hold, click igniter, hold 60 seconds after pilot lights). Slowly release. If the flame dies immediately, the thermocouple output is insufficient. Test with a multimeter in millivolt mode: probe the thermocouple body (ground) and the terminal that connects to the gas valve while the pilot is lit and warm. AO Smith Signature series thermocouples should output 25–35mV. Below 17mV = replace. The snap-in thermocouple on Signature series units pulls out of the pilot assembly retention clip — no threads to remove, just a clip release. AO Smith 100109415 is the OEM replacement. Also inspect the pilot tube orifice for spider webs or debris — clean with a thin wire or compressed air if the pilot flame appears yellow or weak.
3. 3Step 3 — Electric models: test upper and lower heating elements: turn off BOTH poles of the water heater circuit breaker. Remove the upper access panel and fold back the fiberglass insulation. Use a non-contact voltage tester to confirm zero voltage before touching wiring. Disconnect both wire leads from the upper element. Set multimeter to resistance (Ω) and probe across both element terminals: functional 4500W 240V element reads 12–16Ω (12.8Ω nominal: R = 240²/4500). OL = burned out, replace AO Smith 0012498. A reading of 0–2Ω indicates a shorted element — replace and inspect the breaker before restoring power. Repeat at the lower access panel for the lower element. The upper element handles initial heating and complete loss of hot water; the lower element handles recovery and running-out-quickly symptoms.

1. 4Step 4 — Flush sediment at 6-month intervals and check the TCO reset: connect a garden hose to the bottom drain valve and route to a safe drain. Close the cold water inlet valve on top of the unit. Open a hot water tap inside to break vacuum. Open the drain valve and flush 2–3 gallons — in hard water areas, flush until water runs clear (may take 5–10 minutes). Cloudy water with white particles confirms calcium sediment on the lower element. While flushing, check the upper thermostat reset button (TCO): it is the red button on the upper thermostat behind the upper access panel. If it is tripped (popped out), press firmly until it clicks, restore breaker power, and wait 45 minutes before re-testing. If the TCO trips again, a shorted element or stuck thermostat is causing an overheat condition — test elements per Step 3.
2. 5Step 5 — Inspect and replace the anode rod: locate the anode rod hex fitting on top of the tank (may be under insulation, a cover, or the same port as the hot water outlet on some models). Use a 1-1/16 inch socket wrench. Turn off the cold water supply and relieve pressure at a hot tap before removing. Inspect the rod: if it has corroded to the core wire or is coated in heavy nodules, replace immediately. In high-mineral-content water areas (common in the western U.S.), AO Smith recommends a stainless steel or magnesium anode over standard aluminum. Camco 11562 magnesium anode is compatible with most AO Smith tanks — confirm thread size (3/4-inch NPT is most common).
3. 6Step 6 — Test the TPR valve and perform a final safety inspection: with the unit at operating temperature and cold water supply on, lift the TPR lever briefly (1–2 seconds). Water should flow from the discharge pipe, then stop when you release the lever. A TPR valve that drips continuously after being exercised, or won't open when lifted, must be replaced — Watts 240A or an equivalent rated for your tank's temperature and pressure settings. Also confirm the discharge pipe is oriented correctly (pointing down, terminating within 6 inches of the floor, never capped or plugged). Restore full operation and monitor for 24 hours to confirm the fault is resolved.

Repair vs Replace

Recommended Tools & Parts

• AO Smith 100109415 Thermocouple

  OEM replacement snap-in thermocouple for AO Smith Signature and ProLine series gas water heaters. Generates millivolt signal to hold the gas valve open. Replace when output drops below 17mV. Snap-in retention clip fits Signature series pilot assembly without threading.

  $15–$25

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• AO Smith 0012498 Heating Element

  OEM replacement 4500W 240V screw-in heating element for AO Smith Signature and ProLine electric water heaters. Test existing element before replacing (12–16Ω = good, OL = burned out). Requires 1.5-inch element socket wrench — drain tank before replacing screw-in elements.

  $20–$35

  Buy on Amazon →

• Camco 11562 Anode Rod

  Magnesium anode rod compatible with most AO Smith tank water heaters. In high-mineral-content water areas, consider a stainless steel or zinc/aluminum anode for longer service life. Replace when corroded to the steel core wire — inspect annually in hard water regions.

  $20–$35

  Buy on Amazon →

• Fluke 117 Multimeter

  Professional digital multimeter for testing heating elements (resistance), thermocouple output (millivolts), and thermostat continuity. The Fluke 117 is a reliable choice for water heater diagnosis and general home appliance repair.

  $80–$120

  Buy on Amazon →

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

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