E10 error and the display shows extremely high or zero hardness readings (on models with built-in sensor).

Safety first: Turn off power before disconnecting any sensor wiring. Water contact with electronics can cause damage.

• Check cable: Inspect the hardness sensor cable from the Clearwater probe to the control board for cuts, abrasions, or loose connectors.
• Clean sensor: If accessible, remove the hardness probe from its port and soak the tip in a manufacturer-approved cleaner or a mild vinegar solution to remove iron and scale. Rinse thoroughly before reinstalling.
• Recalibrate: Use the controller’s calibration menu to recalibrate the sensor using a known hardness test kit reading from your tap water.
• Bypass sensor (if allowed): Some Clearwater models allow switching to time/volume-based regeneration only. Enable this mode if the sensor is unreliable until it can be replaced.

If E10 persists after cleaning and recalibration, replace the Clearwater hardness sensor probe.