AO Smith 991: Imperfect Combustion

The unit runs but the display flags 991, sometimes after a preceding 101 warning — AO Smith's way of saying the burner isn't getting a clean enough mix of air and gas to combust properly, even though it's still firing.

Applies to: AO Smith tankless/condensing water heaters

What it means

Imperfect Combustion — the unit detects poor-quality combustion, most often from insufficient combustion air or a venting problem (often preceded by a related "101" warning). Because this code is fundamentally about air, not fuel, the fix is usually somewhere in the intake/exhaust path or the installation settings rather than in the gas supply itself, which is what separates it from ignition and flame-loss codes.

Most common causes

Wrong gas type for the installation
Blocked air intake or exhaust vent
Incorrect vent length or clearances
Altitude DIP switch set incorrectly for your elevation
Dirt, grease, or dust buildup on the burner, fan, or heat exchanger

What you can safely check yourself

Check that intake and exhaust vent terminals outside are unobstructed
If you know your home's elevation, confirm the altitude DIP switch setting matches the installation manual

When to call a professional

The code returns after you've cleared any obvious vent blockage — a full combustion and venting inspection is needed.

Venting configuration, gas type verification, and DIP switch settings beyond what's documented for homeowner use should be handled by a licensed HVAC technician or plumber.

What the fix costs

Clearing a simple vent obstruction costs nothing beyond your own time. If a technician visit is needed to correct venting length, clearances, or a DIP switch/altitude setting, expect a diagnostic-and-adjustment visit in the $150–$300 range typical of Angi's water heater service call estimates; a full re-vent job, if the original installation was undersized, can run considerably higher and is closer to a mini-installation project than a repair.

Related codes

All AO Smith codes

Every AO Smith water heater fault code — the full list on one page. Not sure whether what you're seeing is really this code? Water heater error code flashing or blinking covers how to read a tankless display correctly first.

Safety. Poor combustion can mean incomplete burning and carbon monoxide production, not just an inconvenient shutdown. If this code keeps returning after you've cleared visible vent blockage, stop relying on the unit until a technician has verified the venting and combustion air supply — don't just keep resetting it.

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.