A.O. Smith E3 Error Code — What It Means
On A.O. Smith electric tankless water heaters, the E03 or E3 error code signals a thermistor-related fault in the inlet temperature circuit. The control board has detected an open or failed temperature sensor (T-In thermistor) condition. This prevents the unit from measuring incoming water temperature accurately, so the heater shuts down to protect itself.
The manufacturer troubleshooting manual directs technicians to inspect the thermistor wiring and replace the thermistor if the error persists after reconnecting. In addition, incoming water colder than 40°F can trigger this fault until the system and supply warm above that threshold. This is not a problem with the upper thermostat or high-limit switch; those components belong to tank-style heaters and do not apply to tankless models.
Before You Replace Anything
Many online posts incorrectly describe E3 as an upper thermostat problem, which applies only to tank heaters. On A.O. Smith tankless models, E3 always points to the thermistor circuit, so check and reseat the T-In connector before ordering any part.
Common Causes
- Loose or corroded thermistor wiring (~50%) The T-In sensor connector has backed out, collected corrosion, or lost continuity at the plug, causing the board to read an open circuit.
- Failed T-In thermistor (~30%) The temperature sensor itself has opened internally or drifted out of range, and the control board can no longer read a valid resistance.
- Incoming water below 40°F (~15%) The manufacturer manual specifically notes that water colder than 40°F can trigger this fault until the heater and supply warm above that threshold.
- Damaged thermistor harness (~5%) Wiring between the sensor and the board has been nicked, pinched, or chewed by rodents, breaking continuity in the T-In circuit.
Quick Diagnosis
Answer these to narrow it down fast.
Does the error clear after you disconnect and firmly reconnect the T-In thermistor plug?
No: The thermistor or its wiring has likely failed; proceed to measure resistance or replace the sensor.
Is your incoming cold-water supply extremely cold (well water in winter, for example)?
No: Temperature is not the issue; focus on the sensor and its wiring.
After replacing the T-In thermistor, does the E3 code return immediately?
No: The old thermistor was the problem; the heater should now operate normally.
Step-by-Step Fix
- Shut off electrical power at the circuit breaker before opening the heater cover or touching any wiring.
- Remove the front panel to access the control board and thermistor connectors on the inlet water line.
- Locate the T-In thermistor (inlet temperature sensor) and inspect its connector for corrosion, moisture, or a loose fit.
- Disconnect and reconnect the thermistor plug firmly, making sure it clicks or seats completely.
- Restore power and run a hot-water faucet to see if the heater fires and the E3 code clears.
- If the code returns, turn off power again, disconnect the thermistor, and replace it with a new A.O. Smith-approved sensor.
- If incoming water is below 40°F, turn off power and water supply, open a drain valve to empty the unit, and wait for warmer inlet conditions before restarting.
- If the error persists after replacing the thermistor and verifying warm inlet water, contact A.O. Smith technical support for further board-level diagnostics.
Parts Often Needed
| Part | Notes |
|---|---|
| A.O. Smith T-In thermistor (inlet temperature sensor) | Amazon | Match the part number to your exact tankless model; generic thermistors may have the wrong resistance curve. |
| Thermistor wiring harness | Amazon | Order only if the wire itself is damaged; most cases need only the sensor. |
When to Call a Pro
Call a licensed plumber or electrician if you are uncomfortable working around 240 V wiring, if the thermistor connector is difficult to access, or if the error returns after you have reseated and replaced the sensor. A technician can measure thermistor resistance with a multimeter, verify board voltage, and contact A.O. Smith support for warranty or board-replacement guidance. Professional service is also wise if your incoming water is fed by a well or other system that may have debris or air in the line, because those upstream issues can cause secondary faults that mask the true thermistor problem.
Rough cost: DIY runs about $40–80 in parts, 20–40 min. A pro service call runs about $150–250.