A. O. Smith LC Error Code: Scale build-up in heat exchanger
Updated October 2026 · Sourced from A. O. Smith documentation
The manual says an LC code (shown as 00 in the maintenance history) means the unit is beginning to lime up and must be flushed. Failure to flush can damage the heat exchanger.
Is it safe to keep running? Check the steps below first. The manual gives a flush procedure using pumps and vinegar or citric acid. Call a licensed pro if you are not comfortable doing the flush. After flushing, reset by turning power off and back on.
LC at a glance
| Code / how it shows | Meaning | Applies to |
|---|---|---|
| LC Error code flashes on the temperature controller display | Scale build-up in heat exchanger | A. O. Smith 305, 505 and 705 direct vent indoor tankless water heaters |
Check this first
Only steps a homeowner can do safely. Do not open electrical panels or the burner area.
- Check the Water Quality section of the manual to see if your water needs treating.
- Do not ignore the code, since the manual says damage from lime build-up is not covered by the warranty.
🛑 Stop and call a licensed pro if
- The manual gives a flush procedure using pumps and vinegar or citric acid. Call a licensed pro if you are not comfortable doing the flush. After flushing, reset by turning power off and back on.
- 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 Direct Vent Tankless Water Heater Operation and Installation Manual (305, 505, 705 Indoor), Error Code Table, p. 8. Codes are series-specific, so your unit's own manual is the final word. Reviewed October 2026.
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