A.O. Smith Water Heater Error Code 4

Error Code 4 means the flame sensor is detecting a weaker-than-acceptable current from the burner flame. The water heater uses a flame rectification process to confirm the burner is lit, and when that signal is too low, the unit faults and shuts down. This is a safety feature to prevent unconfirmed or unsafe burner operation. The issue is typically related to the sensor rod condition, its position in the flame, or the quality of the burner flame itself.

The error itself is a protective response, not a dangerous condition. The control board shuts the burner down because it cannot confidently confirm the flame is burning correctly. This prevents uncontrolled gas combustion. However, you should not repeatedly attempt to restart the unit without diagnosing the root cause, as persistent ignition attempts with an unresolved issue can create unsafe conditions.

If the fix is simply cleaning the flame sensor rod, the cost is minimal and can often be done without professional help. If the flame sensor rod itself needs replacement, parts typically cost in the range of $15 to $50 depending on the model. A control board replacement, if required, can range from $100 to $300 or more for parts alone. Adding professional labor, a service call to diagnose and repair this error generally runs between $100 and $350 depending on your location and what component ultimately needs attention.

Start by checking that the burner flame is steady and fully surrounding the tip of the flame sensor rod. If the flame looks normal, turn off power to the unit and clean the sensor rod with fine steel wool or emery cloth to remove any oxidation or carbon deposits. Also inspect the wiring connector at the sensor and control board for corrosion or looseness. If cleaning and connection checks do not resolve the error, the sensor rod or control board may need to be replaced.

A reset may temporarily clear the code and allow the unit to attempt ignition again, but if the underlying cause is not corrected, Code 4 will return. Resetting without addressing the flame sensor condition, flame quality, or wiring issue is not a long-term fix. Use a reset only as part of the diagnostic process after you have made a corrective adjustment, to confirm whether the issue has been resolved.