Display shows E4 and unit will not heat; may appear immediately when a hot tap is opened.

Safety: Turn OFF electrical power at the breaker and close the cold water supply valve.

Flow sensor checks:

• Remove the front cover of the Eemax heater.
• Locate the flow sensor assembly on the cold water inlet. It may be a turbine-style sensor or magnetic flow switch with wires leading to the control board.
• Disconnect the flow sensor connector and inspect for corrosion or bent pins. Clean and reseat.
• Remove the flow sensor body (have a towel ready for residual water). Inspect the turbine or internal mechanism for scale, debris, or damage.
• Clean the sensor gently with white vinegar or a descaling solution and a soft brush. Ensure the turbine spins freely.
• Verify the arrow on the flow sensor body matches the direction of water flow (from inlet to heater). Correct orientation if installed backwards.
• Reinstall the sensor with new O-rings if worn, tighten fittings, and slowly open the water supply to check for leaks.
• Restore power and open a hot water tap to confirm that flow is detected and E4 is cleared.

If the code persists, replace the Eemax flow sensor assembly. If a new sensor does not resolve the issue, the control board flow input may be defective.