Buderus Boilers Error Codes
Complete list of fault codes and troubleshooting guides. Use the search below to find your specific error.
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A11
Boiler displays A11 and will not ignite; fan may run briefly
Ionisation / Flame Detection Fault
A21
Boiler shows A21; burner stops and may not restart
Flow Sensor / Flow Switch Fault (DHW or system flow)
A3
Boiler displays A3 and shuts down; may be accompanied by overheating or rapid temperature rise
Safety Temperature Limiter (STB) Tripped / Overheat Protection
A4
Boiler shows A4 and will not start; may occur after power interruption or service work
Low Water Pressure / Water Shortage
A41
Boiler displays A41; may run at reduced output or shut down
Supply Voltage Out of Range
A5
Boiler displays A5; burner stops and may restart intermittently
Flue Gas Temperature Sensor Fault / Overtemperature
A51
Boiler shows A51; fan may run but burner does not ignite
Air Pressure Switch / Fan Speed Fault
A6
Boiler shows A6; may run at reduced output or shut down
Outside Temperature Sensor Fault (Weather Compensation)
A61
Boiler displays A61; burner shuts down unexpectedly during operation
Flame Loss During Operation
A7
Boiler does not ignite; display shows A7 shortly after a call for heat or hot water
Flow Temperature Sensor Fault
A70
Boiler shows A70; heating curve or room temperature control not functioning correctly
Room Thermostat / Room Sensor Communication Fault
A8
Boiler locks out with A8; burner does not start or shuts down quickly after ignition attempt
Return Temperature Sensor Fault
A9
Boiler shows A9 and stops; may occur during operation or at startup
DHW Temperature Sensor Fault
A90
Boiler displays A90; communication with external control or cascade manager fails
BUS Communication Fault (e.g., Logamatic, cascade control)
E2
Boiler shows E2 and locks out; no ignition
Ignition Failure
E3
Boiler displays E3; burner shuts down due to overheating
Primary Flow Overtemperature
E9
Boiler shows E9; often associated with low water content or dry firing
Water Shortage / Dry-Fire Protection
H1
Boiler displays H1; burner does not start, fan may run
Fan Fault / Fan Speed Too Low
H4
Boiler shows H4; burner shuts down, often with flue-related issues
Flue Gas Sensor Overtemperature / Blocked Flue
H6
Boiler displays H6; burner output limited or boiler shuts down
Return Temperature Too High / Poor Heat Dissipation
H7
Boiler shows H7; DHW performance reduced or intermittent
DHW Flow Rate Too Low / DHW Sensor or Flow Fault
U9
Boiler displays U9; control functions limited or locked
Parameter / Configuration Error
Symptom
Boiler cycles on and off rapidly without reaching set temperature
Short Cycling
Symptom
Boiler display is dim, flickering, or unreadable
Display / User Interface Fault
Symptom
Boiler display is on but there is no response to heating or hot water demand
Control / Demand Signal Fault
Symptom
Boiler emits a burning smell or plastic smell during operation
Overheating Electrical or Plastic Component
Symptom
Boiler frequently locks out and requires manual reset, with varying error codes or none displayed
Intermittent Lockout / Unstable Operation
Symptom
Boiler frequently loses pressure and needs topping up
Chronic Pressure Loss
Symptom
Boiler is very noisy (whistling, roaring, or droning) when firing
Combustion Noise / Resonance
Symptom
Boiler leaking water from underneath the casing
Internal Water Leak
Symptom
Boiler makes loud banging or knocking noises (kettling) during operation
Heat Exchanger Kettling / Local Boiling
Symptom
Boiler room or flue area shows signs of soot or black staining
Incomplete Combustion / Sooting
Symptom
Boiler runs but hot water is only lukewarm
Insufficient DHW Output
Symptom
Boiler runs but radiators stay cold or only lukewarm
Poor Circulation / Heating Circuit Flow Problem
Symptom
Boiler shuts down when a particular zone or underfloor circuit operates
Hydraulic Imbalance / Local Flow Restriction
Symptom
Boiler will not start at all; no display or lights
No Power / Dead Boiler
Symptom
Condensate water backs up or boiler shuts down in freezing weather
Blocked or Frozen Condensate Drain
Symptom
Domestic hot water temperature fluctuates between hot and cold
Unstable DHW Temperature Control
Symptom
Radiators get hot even when there is no call for heating
Unwanted Heating / Valve Bypass
| Error Code | Symptom | Fault Type | Guide |
|---|---|---|---|
| A11 |
Boiler displays A11 and will not ignite; fan may run briefly
|
Ionisation / Flame Detection Fault | View Details |
| A21 |
Boiler shows A21; burner stops and may not restart
|
Flow Sensor / Flow Switch Fault (DHW or system flow) | View Details |
| A3 |
Boiler displays A3 and shuts down; may be accompanied by overheating or rapid temperature rise
|
Safety Temperature Limiter (STB) Tripped / Overheat Protection | View Details |
| A4 |
Boiler shows A4 and will not start; may occur after power interruption or service work
|
Low Water Pressure / Water Shortage | View Details |
| A41 |
Boiler displays A41; may run at reduced output or shut down
|
Supply Voltage Out of Range | View Details |
| A5 |
Boiler displays A5; burner stops and may restart intermittently
|
Flue Gas Temperature Sensor Fault / Overtemperature | View Details |
| A51 |
Boiler shows A51; fan may run but burner does not ignite
|
Air Pressure Switch / Fan Speed Fault | View Details |
| A6 |
Boiler shows A6; may run at reduced output or shut down
|
Outside Temperature Sensor Fault (Weather Compensation) | View Details |
| A61 |
Boiler displays A61; burner shuts down unexpectedly during operation
|
Flame Loss During Operation | View Details |
| A7 |
Boiler does not ignite; display shows A7 shortly after a call for heat or hot water
|
Flow Temperature Sensor Fault | View Details |
| A70 |
Boiler shows A70; heating curve or room temperature control not functioning correctly
|
Room Thermostat / Room Sensor Communication Fault | View Details |
| A8 |
Boiler locks out with A8; burner does not start or shuts down quickly after ignition attempt
|
Return Temperature Sensor Fault | View Details |
| A9 |
Boiler shows A9 and stops; may occur during operation or at startup
|
DHW Temperature Sensor Fault | View Details |
| A90 |
Boiler displays A90; communication with external control or cascade manager fails
|
BUS Communication Fault (e.g., Logamatic, cascade control) | View Details |
| E2 |
Boiler shows E2 and locks out; no ignition
|
Ignition Failure | View Details |
| E3 |
Boiler displays E3; burner shuts down due to overheating
|
Primary Flow Overtemperature | View Details |
| E9 |
Boiler shows E9; often associated with low water content or dry firing
|
Water Shortage / Dry-Fire Protection | View Details |
| H1 |
Boiler displays H1; burner does not start, fan may run
|
Fan Fault / Fan Speed Too Low | View Details |
| H4 |
Boiler shows H4; burner shuts down, often with flue-related issues
|
Flue Gas Sensor Overtemperature / Blocked Flue | View Details |
| H6 |
Boiler displays H6; burner output limited or boiler shuts down
|
Return Temperature Too High / Poor Heat Dissipation | View Details |
| H7 |
Boiler shows H7; DHW performance reduced or intermittent
|
DHW Flow Rate Too Low / DHW Sensor or Flow Fault | View Details |
| U9 |
Boiler displays U9; control functions limited or locked
|
Parameter / Configuration Error | View Details |
| – |
Boiler cycles on and off rapidly without reaching set temperature
|
Short Cycling | View Details |
| – |
Boiler display is dim, flickering, or unreadable
|
Display / User Interface Fault | View Details |
| – |
Boiler display is on but there is no response to heating or hot water demand
|
Control / Demand Signal Fault | View Details |
| – |
Boiler emits a burning smell or plastic smell during operation
|
Overheating Electrical or Plastic Component | View Details |
| – |
Boiler frequently locks out and requires manual reset, with varying error codes or none displayed
|
Intermittent Lockout / Unstable Operation | View Details |
| – |
Boiler frequently loses pressure and needs topping up
|
Chronic Pressure Loss | View Details |
| – |
Boiler is very noisy (whistling, roaring, or droning) when firing
|
Combustion Noise / Resonance | View Details |
| – |
Boiler leaking water from underneath the casing
|
Internal Water Leak | View Details |
| – |
Boiler makes loud banging or knocking noises (kettling) during operation
|
Heat Exchanger Kettling / Local Boiling | View Details |
| – |
Boiler room or flue area shows signs of soot or black staining
|
Incomplete Combustion / Sooting | View Details |
| – |
Boiler runs but hot water is only lukewarm
|
Insufficient DHW Output | View Details |
| – |
Boiler runs but radiators stay cold or only lukewarm
|
Poor Circulation / Heating Circuit Flow Problem | View Details |
| – |
Boiler shuts down when a particular zone or underfloor circuit operates
|
Hydraulic Imbalance / Local Flow Restriction | View Details |
| – |
Boiler will not start at all; no display or lights
|
No Power / Dead Boiler | View Details |
| – |
Condensate water backs up or boiler shuts down in freezing weather
|
Blocked or Frozen Condensate Drain | View Details |
| – |
Domestic hot water temperature fluctuates between hot and cold
|
Unstable DHW Temperature Control | View Details |
| – |
Radiators get hot even when there is no call for heating
|
Unwanted Heating / Valve Bypass | View Details |
No matching codes found
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