Error Code	Symptom	Fault Type	Guide
A2	Bosch water heater displays A2; unit may continue to run but with limited performance	Fan Speed / Combustion Air Flow Warning	View Details
A3	Bosch tankless water heater shows A3; unit may derate or shut down intermittently	Water Flow Rate / Modulation Warning	View Details
C1	Bosch water heater displays C1; unit may lock out or require reset	Communication Error Between Control Boards	View Details
C6	Bosch tankless water heater shows C6; fan speed or combustion control issue	Fan Speed Calibration / Combustion Control Error	View Details
E0	Bosch tankless water heater powers on but does not ignite; display shows E0	Parameter / Configuration Error	View Details
E1	Bosch tankless water heater shows E1 and will not ignite; burner never lights	Ignition Failure / Flame Not Established	View Details
E2	Bosch water heater displays E2 and shuts down shortly after ignition attempt	Flame Loss / Flame Signal Error	View Details
E3	Bosch tankless water heater shows E3 and shuts down during operation	Exhaust / Flue Gas Temperature Overheat	View Details
E4	Bosch water heater displays E4 and will not run; unit may click but no sustained operation	High Limit Temperature Cutoff / Overheat	View Details
E5	Bosch tankless heater shows E5 and shuts down; may start then quickly stop	Water Flow Sensor / Flow Switch Error	View Details
E6	Bosch water heater displays E6; unit may overheat or shut down intermittently	Inlet / Outlet Temperature Sensor Discrepancy	View Details
E7	Bosch tankless water heater shows E7 and will not operate	Fan / Combustion Air Blower Error	View Details
E8	Bosch water heater displays E8; unit may start then shut down quickly	Gas Valve / Gas Proportional Valve Error	View Details
E9	Bosch tankless water heater shows E9 and will not operate	Exhaust / Flue Gas Sensor Failure	View Details
EA	Bosch condensing tankless water heater shows EA and shuts down	Condensate Drain / Condensate Trap Error	View Details
H07	Bosch electric tankless water heater shows H07 and does not heat water	Outlet Temperature Sensor Fault (Electric Model)	View Details
H11	Bosch electric water heater displays H11; unit does not energize heating elements	Flow Sensor / Flow Switch Fault (Electric Model)	View Details
H27	Bosch electric tankless heater shows H27; unit may shut down or not heat properly	Heating Element Overtemperature / Dry-Fire Protection	View Details
–	Bosch electric tankless water heater runs but water is only slightly warm	Partial Heating Element Failure	View Details
–	Bosch gas water heater produces a strong gas smell near the unit	Gas Leak / Combustion Gas Leakage	View Details
–	Bosch tankless water heater does not start at all; no lights or display	No Power / Dead Unit	View Details
–	Bosch tankless water heater ignites but water temperature fluctuates; hot-cold-hot cycling during shower	Temperature Instability / Modulation Control Issue	View Details
–	Bosch tankless water heater makes loud banging or knocking noises when operating	Water Hammer / Combustion or Scale Noise	View Details
–	Bosch tankless water heater vibrates excessively or rattles during operation	Mechanical Vibration / Mounting Issue	View Details
–	Bosch water heater causes carbon monoxide alarm to sound when operating	Flue Gas Spillage / Incomplete Combustion	View Details
–	Bosch water heater display or control panel is unresponsive, but unit still heats water	User Interface / Keypad Failure	View Details
–	Bosch water heater emits a burning smell or visible smoke from the unit	Overheating / Electrical or Combustion Fault	View Details
–	Bosch water heater frequently cycles on and off with small hot water draws (e.g., handwashing)	Short Cycling / Low-Flow Operation Issue	View Details
–	Bosch water heater has power but does not ignite when hot water is opened; no error code displayed	Insufficient Flow to Activate / Flow Detection Failure	View Details
–	Bosch water heater leaks from the pressure relief valve discharge pipe	Excessive Pressure / Thermal Expansion	View Details
–	Bosch water heater leaks water from the bottom of the unit	Internal Water Leak	View Details
–	Bosch water heater produces lukewarm water only; cannot reach set temperature	Insufficient Heating Capacity / Derated Operation	View Details
–	Bosch water heater runs but then shuts off after a few minutes, requiring manual restart	Overheat or Safety Lockout Without Displayed Code	View Details
–	Bosch water heater shows condensation or water droplets on or inside the casing	Condensation / Moisture Intrusion	View Details
–	Bosch water heater takes a very long time to deliver hot water to fixtures	Long Hot Water Delivery Time / Plumbing Layout Issue	View Details
–	Bosch water heater trips the circuit breaker when it starts	Electrical Overload / Short Circuit	View Details
–	Bosch water heater's hot water output smells like rotten eggs (sulfur odor)	Bacterial Contamination / Anode Reaction (Storage-Type Bosch Heaters)	View Details

Bosch Water Heaters Error Codes

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