A.O. Smith Water Heater Error Code 991

Error code 991 indicates a persistent abnormal combustion condition in the burner system. The heater has detected that combustion is not occurring within normal parameters and has faulted out as a safety measure. This code specifically notes that manifold gas pressure may need adjustment as part of the resolution. Diagnostics should follow the same steps outlined for error code 101.

No. When the 991 code is active, the heater has shut down the burner as a precaution against abnormal combustion. Attempting to force operation without correcting the fault could pose a risk of incomplete combustion or carbon monoxide production. Leave the unit off and contact a licensed technician to inspect and repair the system before resuming normal use.

Repair costs vary depending on the root cause. A service call to diagnose and adjust manifold pressure typically ranges from $100 to $250. If a flame sensor replacement is needed, parts and labor generally run $150 to $350. A gas valve replacement, which is a more involved repair, can cost anywhere from $300 to $600 or more depending on the model and local labor rates. Getting a written estimate after diagnosis is the best approach before authorizing repairs.

A manual reset may temporarily clear the fault and allow the heater to attempt operation again, but if the underlying combustion problem is not corrected, the 991 code will return. Resetting without diagnosing and fixing the root cause, such as incorrect manifold pressure or a restricted vent, is not a permanent solution. The fault is persistent by nature, meaning the control board has already attempted to recover on its own before displaying 991.

Begin by following the diagnostic steps for error code 101, as the 991 code shares that troubleshooting pathway. Check for venting obstructions, inspect the flame sensor, and verify the gas supply is adequate. If those steps do not resolve the issue, have a licensed technician measure the manifold gas pressure with a manometer and adjust it as needed at the gas valve. Component replacement such as a new gas valve or flame sensor may also be required depending on what the diagnostics reveal.