A. O. Smith SF Error Code: Air filter is dirty
Updated October 2026 · Sourced from A. O. Smith documentation
🧭 Check a few things first
The guide says the air filter is dirty.
Is it safe to keep running? Check the steps below first. Call a qualified person if the code returns after the filter is clean.
SF at a glance
| Code / how it shows | Meaning | Applies to |
|---|---|---|
| SF Shown on the water heater display | Air filter is dirty | A. O. Smith Hybrid Electric Heat Pump water heaters (Voltex), Installation Instructions and Use & Care Guide 100269085 |
Check this first
Only steps a homeowner can do safely. Do not open electrical panels or the burner area.
- Turn off power at the circuit breaker, as the guide says.
- Clean the air filter following the Air Filter Maintenance section of the guide, then restore power.
🛑 Stop and call a licensed pro if
- Call a qualified person if the code returns after the filter is clean.
- You smell gas. Leave the house, do not touch switches, and call your gas utility from outside.
- Codes for flame failure, overheating, venting or the control board that return after one reset.
- Water leaking from the heater, the relief valve discharging, or scorching around the burner area.
- A carbon monoxide alarm sounds. Get everyone outside and call 911 or the fire department.
Safe first steps for any A. O. Smith code
- Write down the exact code shown on the controller or the number of status light flashes.
- Turn the water heater power off and back on (or press the reset button the manual describes) once. If the code comes back, stop resetting.
- Make sure the gas supply valve to the heater is open and other gas appliances in the home are working.
- Make sure the cold water supply valve is fully open and hot water taps are not partly closed.
- Look at the outside vent terminal for snow, ice, leaves or anything blocking it (look only).
Source: A. O. Smith Hybrid Electric Heat Pump Water Heater Use and Care Guide 100269085, Diagnostic Codes, p. 20. Codes are series-specific, so your unit's own manual is the final word. Reviewed October 2026.
Ryan Giles · Red Seal Certified Plumber · Ask a plumber · Ask an HVAC technician
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