Display shows “Err 09” and the unit reports a power or low-voltage fault.

Safety first: Turn off power at the outlet or breaker before inspecting the transformer or wiring.

Check external power:

• Confirm the outlet has proper voltage using a plug-in tester or multimeter.
• Avoid plugging the softener into a circuit shared with large motors (e.g., well pump, air compressor) that can cause voltage dips.

Test the transformer:

• Unplug the transformer from the control head.
• Use a multimeter to measure AC voltage at the transformer output; it should match the rating on the label (commonly 24 VAC).
• If voltage is low or zero, replace the transformer with the correct AO Smith part.

Inspect control head power input:

• Check the low-voltage connector at the control board for corrosion or looseness.
• Reseat the connector and look for any signs of heat damage on the board.

If a known-good transformer does not clear the error, the control board power circuitry is likely damaged and should be replaced.