Bosch water heater displays E4 and will not run; unit may click but no sustained operation

Safety first: Turn OFF power and close water and gas valves. Allow the unit to cool completely before touching internal components.

• Check water flow: Ensure all inlet shutoff valves are fully open. Clean the cold water inlet filter screen on the bottom of the heater (if equipped) by closing the inlet valve, removing the filter, rinsing debris, and reinstalling.
• Inspect outlet temperature sensor: Locate the hot water outlet NTC sensor on the heat exchanger outlet pipe. Check for loose wiring or corrosion at the connector. If the sensor body is cracked or deformed, replace it.
• Descale the heat exchanger: If you have hard water, mineral buildup can cause overheating. Connect a descaling pump and hoses to the service valves and circulate a descaling solution (such as food-grade vinegar or manufacturer-approved chemical) for 30–45 minutes. Flush thoroughly with clean water afterward.
• Reset high-limit (if manual reset type): Some models have a manual reset high-limit thermostat on the heat exchanger. Press the reset button if present. If it trips repeatedly, the root cause (flow or scaling) must be corrected.
• Persistent E4: If the error returns after cleaning and descaling, the outlet NTC sensor or high-limit thermostat may be faulty and should be replaced by a technician.