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AO Smith tankless water heater error codes are model-specific, but common issues include: Code 991 (or 101) indicating air intake or ignition problems, which can be caused by clogged vents, lack of combustion air, or incorrect dip switch settings. Code 111 signifies an ignition failure, often due to a dirty igniter or flame sensor, while Code 121 indicates a loss of flame during operation. You can troubleshoot by checking gas supply, cleaning air filters, verifying vent clearances, resetting the unit, and consulting the specific model's manual for detailed instructions. Common A O Smith Error Codes Code 991 (or 101): This code suggests an issue with the air intake or a failure to ignite. Troubleshooting Steps: Check for blockages in the air intake or exhaust vents (like snow or debris), ensure proper vent termination clearances, and verify the dip switch settings are correct for the installation altitude. Code 111: This indicates an ignition failure. Troubleshooting Steps: Ensure the gas supply is open, clean the igniter and flame sensor, and reset the unit. Code 121: This signifies a loss of flame while the unit is heating. Troubleshooting Steps: Check the gas supply and pressure, inspect the flame rod for dirt or damage, and verify there are no issues with the heat exchanger or gas nozzles. This video explains how to troubleshoot AO Smith tankless water heater error code 991: 1m General Troubleshooting Steps 1. Check the Gas Supply: Confirm that the gas valve to the water heater is fully open. 2. Reset the Unit: Turn off the power and gas supply to the water heater for about five minutes to perform a hard reset. 3. Inspect Venting and Air Intake: Look for any blockages or obstructions in the air intake and exhaust vents, as this is a common cause of combustion issues. 4. Inspect Components: Gently clean the igniter and flame sensor with a soft cloth, as dirt can prevent proper operation. 5. Consult the Manual: For a definitive list of codes and detailed troubleshooting steps specific to your model, refer to the installation and operation manual for your AO Smith tankless water heater. 6. Contact a Professional: If the error persists after these steps, contact a certified technician to diagnose the problem further. You can also contact tankless water . ca they are experienced and licensed to repair and install all types of A O Smith water heaters in Canada.  Below is a small list of towns and cities they do service and repair in.  If you need help with a A O Smith tankless water heaters in Canada go to https://tanklesswater.ca/ Thery are well known for being the best tankless water heater repair people in canada. They do repair and service in all these areas of Ontario Canada. Find AO Smith  tankless water heater repair service in Ontario areas including Toronto, Scarborough, Etobicoke, Mississauga , Oakville, Stouffville, North York, Scarbourough, Burlington, Ontario Canada. Cities near Whitchurch-Stouffville, Ontario: Markham, Canada Aurora, Canada Uxbridge, Canada Richmond Hill, Canada Pickering, Canada Newmarket, Canada Scarborough, Canada Ajax, Canada Whitby, Canada Vaughan, Canada Port Perry, Canada Bradford, Canada Oshawa, Canada Toronto, Canada Sutton, Canada A O Smith Tankless water heater parts repair service in certain areas.