Stiebel Eltron electronic tankless water heater will not heat; display shows E01 and unit does not activate when hot water tap is opened.

Safety first: Turn off the circuit breaker for the heater and close the cold water supply valve before opening the unit. Confirm power is off with a non-contact voltage tester.

• Check inlet filter and strainers: Shut off water, remove the cold water inlet filter screen on the heater, and clean any debris or scale. Rinse thoroughly and reinstall.
• Inspect flow sensor: Remove the front cover. Locate the flow sensor assembly on the cold water inlet (usually a plastic housing with wires). Check for visible scale or debris. If accessible, gently remove the sensor/turbine and clean with white vinegar and a soft brush. Do not damage the turbine wheel.
• Verify wiring: Ensure the flow sensor connector is fully seated on the main control board and that no wires are pinched or broken. Reseat the connector.
• Reset the unit: Restore water, open a hot tap to purge air, then restore power at the breaker. Clear the error by turning the unit off and on (or using the reset procedure in the manual). Test for proper activation when a hot tap is opened.
• If E01 persists: Replace the flow sensor assembly with an OEM Stiebel Eltron part compatible with your model. If wiring or board damage is suspected, contact a qualified technician.