Bosch tankless water heater shows E3 and shuts down during operation

Safety first: Turn OFF power and gas. Do not operate the heater if you suspect vent blockage or exhaust leakage; risk of carbon monoxide exposure.

• Inspect venting: From the heater to the termination, check that the vent pipe is the correct diameter and material specified by Bosch. Look for crushed sections, sagging, bird nests, or debris at the termination cap. Clear any obstructions.
• Check vent length and elbows: Compare your installation to the maximum equivalent length allowed in the installation manual. Excessive length or too many elbows can cause high flue temperatures; consult a professional if your venting exceeds specifications.
• Examine flue gas sensor: Locate the flue gas temperature sensor (usually clipped to the exhaust outlet or heat exchanger). Ensure it is firmly attached and wiring is intact. If the sensor is cracked, deformed, or has damaged wiring, it should be replaced.
• Descale heat exchanger: Heavy scale buildup can cause localized overheating. If you have hard water and no softener, consider performing a descaling procedure using a pump and descaling solution per Bosch instructions.
• Test operation: Restore power and gas, then run hot water. If E3 recurs, discontinue use and contact a qualified technician to evaluate venting design and sensor operation.