AO Smith variable-speed pool pump shows “ERR4” and will not run; sometimes appears after a thunderstorm or power surge.

Safety Warning: Internal drive components store high voltage even after power is removed. Do not open sealed electronic modules unless you are qualified.

• Power cycle: Turn OFF the breaker for at least 5 minutes to allow internal capacitors to discharge. Turn power back ON and see if ERR4 clears.
• Inspect for moisture: Check the control keypad and drive housing for signs of water ingress, condensation, or corrosion. Ensure the pump is not positioned where sprinklers or roof runoff hit the electronics.
• Check external wiring: Confirm that line and ground connections are tight and that there are no shorts or damaged conduit feeding the pump.
• Surge protection: If ERR4 appeared after a storm, consider installing a surge protector on the pool equipment subpanel to prevent future damage.
• Replace drive/control module: If the error persists and the motor is mechanically sound, the AO Smith control board/drive module is likely defective and must be replaced as an assembly according to manufacturer instructions.