Bosch tankless heater shows E5 and shuts down; may start then quickly stop

Safety first: Turn OFF electrical power and close the cold water inlet valve. Allow the unit to cool.

• Clean inlet filter: Locate the cold water inlet filter at the bottom of the unit. Place a towel or container underneath, remove the filter, and rinse out debris under running water. Reinstall securely.
• Service flow sensor: The flow sensor is typically installed on the cold water inlet manifold. Disconnect power and water, then remove the sensor per the service manual. Inspect the turbine or paddle wheel for scale or debris. Clean gently with a soft brush and descaling solution if needed.
• Check wiring: Inspect the harness from the flow sensor to the control board. Ensure connectors are fully seated and free of corrosion or damage.
• Test operation: Reassemble, open the water valve, check for leaks, then restore power. Open a hot water tap to verify that the unit detects flow and ignites.
• Replace if necessary: If E5 persists and the turbine does not spin freely, replace the water flow sensor assembly.