A.O. Smith Water Heater Error Code 721

Error code 721 indicates false flame detection, meaning the water heater's control system is sensing a flame when the burner should be off. This is a serious safety fault that requires immediate action. You should shut off all gas and power to the unit and contact A.O. Smith's Technical Service Department before attempting any repair.

A simple power reset is unlikely to permanently clear code 721 because the underlying fault, whether a shorted sensor, faulty wiring, or control board issue, will still be present when power is restored. Even if the unit temporarily restarts, the code will return. The root cause must be identified and repaired before the system will operate safely and reliably.

Repair costs vary depending on the failed component. Flame sensor replacement is typically the least expensive fix, often in the range of $100 to $250 including parts and labor. A faulty gas valve or control board replacement will cost more, generally ranging from $250 to $600 or higher depending on the model and local labor rates. A service technician will need to diagnose the specific cause before an accurate estimate can be provided.

No, it is not safe to continue operating the unit while code 721 is active. This error can indicate an uncontrolled flame, a stuck-open gas valve, or a critical electrical fault. Shut off the gas supply and disconnect power to the water heater immediately and do not use it until the cause has been diagnosed and corrected by a qualified technician.

A.O. Smith specifically directs users to contact their Technical Service Department as the first step for this code, which reflects the seriousness of a false flame detection fault. While a knowledgeable technician can inspect the flame sensor, wiring harness, and gas valve, this is not a repair recommended for inexperienced DIYers due to the gas and electrical safety risks involved.