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