A.O. Smith Error Code E1: Thermistor Failure

Severity
Warning
Type
digital display
Estimated repair cost: $100 — $250 (parts + labor)
Warning: Your A.O. Smith water heater has detected a problem with one of its temperature sensors and has shut down to prevent potential overheating or other issues. You will not have hot water until this is resolved.

How to Fix A.O. Smith Code E1

  1. Step 1: Check the water heater's display for any other error codes. Note them down, as they may provide additional clues.
  2. Step 2: Reset the water heater by turning off the power at the breaker for 30 seconds, then turning it back on. This may clear a temporary error.
  3. Step 3: If the E1 error persists after resetting, contact a qualified HVAC technician to diagnose and repair the issue.

How to Reset A.O. Smith Code E1

  1. Turn off the unit using the power button on the front panel.
  2. Turn off the gas supply valve.
  3. Turn off the circuit breaker supplying power to the unit. Wait 30 seconds.
  4. Restore power at the breaker, then open the gas valve.
  5. 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 E1 Mean?

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.

Common Causes

  • Faulty thermistor (inlet or outlet)
  • Loose or corroded wiring connections to the thermistor(s)
  • Damaged wiring harness leading to the thermistor(s)
  • Control board failure

Diagnostic Steps

If... Check Expected
E1 error is present Measure the resistance of the inlet thermistor at the control board connector Resistance should match the thermistor's temperature/resistance chart (available in the service manual). Typically around 10k ohms at room temperature.
Inlet thermistor resistance is out of range Measure the resistance of the outlet thermistor at the control board connector Resistance should match the thermistor's temperature/resistance chart. Typically around 10k ohms at room temperature.
Both thermistors appear good Check the wiring harness for continuity between the thermistor connector and the control board connector Should have continuity (low resistance).
Wiring is good and thermistors appear functional Check voltage to the board Appropriate voltage to the board

Applicable A.O. Smith Models

Replacement Parts

Part OEM Number Est. Cost Where to Buy
Inlet Thermistor 9005329005 — Find Part
Outlet Thermistor 9005329005 — Find Part
Control Board — — —

Related Codes on Same Models

Other codes that affect the same A.O. Smith model families as E1.

Code Description Severity
E06 Flammable Vapor Detection Critical
E2 Dry Fire Protection Warning
E3 Sensor Fault 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