A.O. Smith Error Code E3: Sensor Fault
How to Fix A.O. Smith Code E3
- Step 1: Check the display for any other error codes that might give more specific information about the sensor at fault.
- Step 2: Try resetting the water heater by turning off the power at the breaker for a few minutes, then turning it back on. This might clear a temporary fault.
- Step 3: If the error persists, contact a qualified HVAC technician or plumber to diagnose and repair the issue. Do not attempt to disassemble the unit yourself.
How to Reset A.O. Smith Code E3
- Turn off the unit using the power button on the front panel.
- Turn off the gas supply valve.
- Turn off the circuit breaker supplying power to the unit. Wait 30 seconds.
- Restore power at the breaker, then open the gas valve.
- Turn the unit back on. If the error returns, the underlying issue needs repair.
Resetting clears the code from the display but does not fix the underlying fault. If the code returns after reset, proceed to the diagnostic steps above.
What Does A.O. Smith Error Code E3 Mean?
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.
Common Causes
- Faulty thermistor (temperature sensor) – most common failure.
- Loose or corroded wiring connections to the thermistor.
- Damaged wiring harness leading to the control board.
- Control board malfunction (less common, but possible).
Diagnostic Steps
| If... | Check | Expected |
|---|---|---|
| E3 error is present | Check thermistor resistance with a multimeter | Compare reading to the temperature/resistance chart in the A.O. Smith service manual. Should be within specified range for the measured temperature. |
| Thermistor resistance is out of range | Inspect wiring harness and connections to the thermistor for damage or corrosion | Wiring and connections are clean and secure. |
| Wiring and connections appear good | Check voltage at the control board terminals for the thermistor circuit | Voltage should be within the range specified in the A.O. Smith service manual. |
| Voltage is incorrect or absent | Inspect the control board for signs of damage (burn marks, corrosion) | Control board appears undamaged. |
Applicable A.O. Smith Models
Replacement Parts
| Part | OEM Number | Est. Cost | Where to Buy |
|---|---|---|---|
| Thermistor (various types) | — | — | — |
| Wiring harness | — | — | — |
| Control board | — | — | — |
Related Codes on Same Models
Other codes that affect the same A.O. Smith model families as E3.
| Code | Description | Severity |
|---|---|---|
| E06 | Flammable Vapor Detection | Critical |
| E1 | Thermistor Failure | Warning |
| E2 | Dry Fire Protection | Warning |
| E4 | High Temperature Cutoff | Critical |
| E5 | Communication Error | Warning |
| ECO | Emergency Cutoff | Critical |
| LOC | Controls Locked | Warning |
| LWR | Lower Element Fault | Warning |
| UPR | Upper Element Fault | Warning |