Clearing The "101" And "991" Error Code - A.O. Smith Water Heater 520 Direct Vent Indoor Service Manual

12-3. Clearing the "101" and "991" error code

The "101" and "991" error codes signify imperfect (abnormal) combustion, caused by insufficient intake
air and/or obstructions in the exhaust.
A. If the "101" and "991" error code occurs, please check the following:
1. What is the gas-type of the water heater: liquid propane or natural gas?
2. How long has the water heater been installed and been in use?
3. Where is the water heater installed: indoor, outdoor, attic, etc?
4. What is the altitude/elevation of the area of installation?
5. Are there any obstructions in the intake air and/or exhaust?
Damper sticking.
Vent Flaps installed on the Termination.
Snow build up around Termination.
Installed in a closet. (No Ventilation or lack of combustion air)
6. If installed in laundry room, check the burner and heat exchanger. Dust and lint may deposit in
burner and heat exchanger.
7. If installed in restaurant, check the burner and fan motor. Grease and dirt may deposit in burner
and fan motor.
8. Check if the manifold pressure of the water heater is too high.
B. How to clear the "101" and "991" error code (after resolving the main root cause of the
error):
On the computer board, press and hold the "INCREASE" and "DECREASE" buttons simultaneously for
at least 3 seconds until the red light turns on. Turn off the 120 VAC by Power ON-OFF switch and
turn back on. The "101" and "991" error code should then be cleared.
"Increase" button
520 Indoor / 520 Outdoor Service Manual
"Decrease" button
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CONFIDENTIAL
Red light
Ver. 1.00