Here you are Dorothy
AO Smith code 991
Solution for error
code 991
Check the gas type of the water heater. Inspect the environment around the water heater. ... Check if there is grease and/or dirt in the burner (Part #101) and the fan motor (Part #103), especially if the water heater has been installed in a contaminated area.
More help here, but you probably need a service call, especially if it is a gas water heater.
https://www.google.com/search?q=error+code+of+991
..
×