Solution for error code 991
• Check the gas type of the water heater.
• Inspect the environment around the water heater. Determine how long the unit has been installed.
• Check the altitude/elevation of the area of where the water heater is installed.
• Check if there is any blockage in the intake air and/or exhaust.
• Check whether there is enough distance between the concentric terminals.
• 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.