Control display shows Err1 and unit will not start a regeneration cycle

Safety first: Unplug the softener or switch off the dedicated breaker before opening the control valve cover. Close the inlet and outlet bypass valves to isolate the softener from the plumbing system.

Step-by-step checks:

• 1. Verify water flow and plumbing: Open the bypass slightly and run a nearby faucet. Confirm that water is actually flowing through the softener. If there is no flow, check the Marlo bypass valve for correct position and any closed shutoff valves.
• 2. Inspect the turbine/flow sensor assembly: Remove the control valve cover on the Marlo head. Locate the turbine or flow sensor module (usually mounted in the outlet side of the valve body). Disconnect the sensor plug from the control board. Gently pull the turbine housing and inspect the turbine wheel for debris, scale, or broken vanes. Clean with a soft brush and water; do not use harsh chemicals.
• 3. Check wiring and connector: Inspect the flow sensor wiring harness from the turbine to the control board for cuts, pinches, or corrosion at the connector. Reseat the connector firmly. If wires are brittle or insulation is damaged, replace the harness.
• 4. Test sensor function (if you have a meter): With the unit unplugged, measure resistance across the sensor leads (for a Hall-effect or reed sensor, you should see a finite resistance, not open). If the sensor is open or shorted, it is defective.
• 5. Reset and test: Restore power, fully open the bypass, and initiate a manual regeneration. Watch the display to see if the Err1 clears and if the flow rate registers when water is running.

When to replace parts: If cleaning and reseating the turbine does not clear Err1, replace the Marlo turbine/flow sensor assembly. If a known-good sensor still does not register flow, the Marlo control board may be defective and should be replaced or serviced by a qualified technician.