AO Smith 111: Ignition Failure

Your AO Smith tankless unit is calling for hot water, cycling its igniter, and giving up — code 111 on the display or remote means it never got a flame established in the first place.

Applies to: AO Smith tankless/condensing water heaters (mid- and high-efficiency indoor and outdoor models)

What it means

Ignition Failure — the unit could not establish a flame on start-up. Numeric codes like this one are specific to AO Smith's tankless and condensing lineup, which uses a digital display or wired remote instead of a blink pattern, and 111 is consistently the most-reported code on that platform because getting a flame lit depends on several things — gas supply, power, and clean ignition components — all lining up correctly at the same moment.

Most common causes

Inadequate gas supply or inlet pressure
Hi-limit switch not functioning correctly
Broken, loose, or sooted igniter or flame rod wiring
Dirty burner or manifold nozzles

What you can safely check yourself

Verify the gas supply is on and check the manifold pressure sticker/rating plate matches your gas type
Check for a tripped breaker or loss of power to the unit

When to call a professional

There's no visible spark or you don't hear the gas valve's normal "clunk" sound during an ignition attempt — this needs wiring or gas valve diagnosis.

Gas supply lines and gas control valves should only be worked on by a licensed plumber or HVAC technician. Anything beyond a breaker check should be left to a qualified electrician or plumber.

What the fix costs

A diagnostic visit for a no-ignition fault on a tankless unit typically runs $150–$600 depending on region and what's found, per Angi's and Fixr's water heater repair cost guides. A cleaned burner or reseated igniter wiring is on the lower end of that range; a full gas control valve replacement runs closer to $150–$450 installed. These are national estimates, not a quote — get a written estimate from a local licensed plumber before authorizing repair work.

Related codes

All AO Smith codes

Every AO Smith water heater fault code — the full list on one page. Ignition failure is one of several causes behind a broader symptom — see no hot water from a tankless water heater for the full picture.

Safety. If you smell gas at any point, stop, do not attempt to restart the unit, ventilate the area, and call your gas utility or a licensed technician immediately. Don't keep cycling power to a unit that repeatedly fails to ignite — repeated attempts don't fix a hardware fault and can let unburned gas accumulate.

Sources

AO Smith tankless water heater error code guide (manufacturer documentation)

This site is an independent reference, not a substitute for a licensed plumber or electrician. Gas water heaters can leak carbon monoxide, leak gas, or catch fire if serviced incorrectly. Anything involving the gas line, gas control valve, venting, or main electrical wiring should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.