A.O. Smith Error Code E4: High Temperature Cutoff
How to Fix A.O. Smith Code E4
- Step 1: Check the water temperature setting on the unit. Ensure it is set to a reasonable temperature (around 120°F is generally recommended).
- Step 2: Inspect the area around the water heater for any obstructions blocking airflow. Clear away any boxes, debris, or other items that might be restricting ventilation.
- Step 3: If the error persists after checking the temperature setting and airflow, contact a qualified HVAC technician to diagnose and repair the problem. Do not attempt to reset the unit repeatedly without addressing the underlying cause.
How to Reset A.O. Smith Code E4
- 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 E4 Mean?
The A.O. Smith E4 error code indicates that the high temperature limit switch has tripped. This switch is a critical safety device designed to shut down the water heater if the water temperature exceeds a pre-set limit, typically around 190-200°F. When this happens, the unit locks out to prevent further heating and potential damage or even a dangerous situation like scalding or tank rupture. The error code will appear on the digital display, and the water heater will not operate until the issue is resolved and the system is reset. It's important to address this issue promptly, as repeated overheating can damage the tank and other components, shortening the lifespan of your water heater. Ignoring this code can lead to unsafe water temperatures or even a complete system failure.
Common Causes
- Scale buildup on the tank's heat exchanger restricting water flow and causing localized overheating.
- Faulty high-temperature limit switch giving a false reading.
- Defective thermistor providing incorrect temperature readings to the control board.
- Gas valve malfunction causing excessive gas flow and overheating.
- Blocked flue or vent, causing combustion gases to back up and overheat the unit.
Diagnostic Steps
| If... | Check | Expected |
|---|---|---|
| E4 error code is present | Measure voltage at the high-limit switch with a multimeter | 0 VAC if tripped, line voltage if closed |
| High-limit switch is tripped (0 VAC) | Check resistance of the thermistor at the control board connector | Compare to temperature/resistance chart in the service manual. Should be within spec for the measured water temperature. |
| Thermistor resistance is out of spec | Inspect the heat exchanger for scale buildup using a borescope | Minimal scale buildup. Excessive scale indicates a need for descaling or flushing. |
| Scale buildup is minimal and thermistor is good | Check gas valve operation. Measure gas pressure at the manifold | Manifold pressure should be within the range specified on the unit's rating plate. |
Applicable A.O. Smith Models
Replacement Parts
| Part | OEM Number | Est. Cost | Where to Buy |
|---|---|---|---|
| High Temperature Limit Switch | — | — | — |
| Thermistor | — | — | — |
| Gas Valve | — | — | — |
Related Codes on Same Models
Other codes that affect the same A.O. Smith model families as E4.
| Code | Description | Severity |
|---|---|---|
| E06 | Flammable Vapor Detection | Critical |
| E1 | Thermistor Failure | Warning |
| E2 | Dry Fire Protection | Warning |
| E3 | Sensor Fault | Warning |
| E5 | Communication Error | Warning |
| ECO | Emergency Cutoff | Critical |
| LOC | Controls Locked | Warning |
| LWR | Lower Element Fault | Warning |
| UPR | Upper Element Fault | Warning |