A.O. Smith Water Heater Error Code 391

Error code 391 indicates an air-fuel ratio rod failure. The AFR rod is a combustion sensing component that monitors the burner flame, and this code means the control board is not receiving a valid signal from it. The heater shuts down the burner when this fault is detected to prevent unsafe operation. Diagnosis starts with checking the wiring connections to the rod for burns, cuts, or looseness.

The heater is designed to shut down the burner when error 391 is detected, which reduces the immediate risk of unsafe combustion. However, you should not attempt to bypass the fault or force the unit to operate while this code is active. Leave the unit off and have it diagnosed before restoring normal operation.

Repair costs vary depending on which component has failed. If only a wiring repair or connector replacement is needed, costs tend to be on the lower end, typically in the range of $75 to $200 including labor. Replacing the AFR rod itself may cost between $150 and $350 depending on parts and labor rates in your area. If the control board requires replacement, costs can range from $300 to $600 or more.

A knowledgeable DIYer can start by inspecting the wiring connections to the AFR rod for visible damage or loose connectors, which is the first recommended diagnostic step. If the wiring appears intact, replacing the AFR rod itself is a more involved repair that requires comfort working around gas combustion components. Any repairs involving the gas valve, control board, or internal combustion system are best left to a licensed technician.

A manual reset may temporarily clear the fault and allow the unit to attempt ignition again, but if the underlying problem with the AFR rod or its wiring has not been resolved, the error will return. Resetting without addressing the root cause is not a fix and may result in repeated shutdowns. The fault should be properly diagnosed and repaired before relying on the unit.