Bosch water heater displays E6; unit may overheat or shut down intermittently

Safety first: Turn OFF power and gas. Allow the heater to cool.

• Inspect NTC sensors: Locate both the cold water inlet and hot water outlet temperature sensors on the heat exchanger piping. Check that each sensor is firmly clamped and that wiring connectors are secure.
• Check sensor resistance: With power off, disconnect each sensor and measure resistance with a multimeter. Compare readings to the Bosch service chart for your model at room temperature. Replace any sensor that is out of specification.
• Descale heat exchanger: If the outlet temperature rises too quickly due to restricted flow, the control may detect an abnormal differential. Perform a descaling procedure if you have hard water or visible scale.
• Control board evaluation: If both sensors test good and wiring is intact, the main control board may be misreading temperatures and should be evaluated and replaced by a professional.