Junkers
Junkers Furnaces Error Codes
Complete list of fault codes and troubleshooting guides. Use the search below to find your specific error.
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A2
A2 displayed, furnace stops, sometimes accompanied by gurgling noises.
Low Water Pressure / Water Shortage
A3
The furnace locks out shortly after starting; the burner does not ignite, and the display shows A3.
Ignition Failure / No Flame Detection
A4
A4 displayed, furnace does not start heating circuit.
Flow Switch / Flow Sensor Fault (Heating Circuit)
A5
A5 displayed, no domestic hot water even when tap is opened.
DHW Flow Sensor / Turbine Fault
A6
A6 displayed, furnace may provide intermittent hot water or heating.
Outdoor Temperature Sensor Fault (if fitted)
A7
The furnace shuts down with error A7; the burner stops, and hot water and heating are unavailable.
Flow Temperature Sensor Fault (NTC)
A8
The display shows A8, and the furnace does not start the heating cycle.
Return Temperature Sensor Fault (NTC)
A9
The furnace shows A9 and stops; heating and hot water are unavailable.
DHW Outlet Temperature Sensor Fault (Domestic Hot Water NTC)
C1
The furnace displays C1; the fan runs, but the burner does not ignite or shuts down quickly.
Fan Speed / Airflow Fault
C6
Error C6: The furnace attempts to start but shuts down with a fan-related fault.
Fan Speed Too Low / Fan Feedback Error
C7
The display shows C7; the furnace will not start, and the fan does not run.
Fan Does Not Start / Fan Circuit Fault
E2
Furnace displays E2, burner stops, sometimes after running for a short period.
Flame Signal Lost During Operation
E5
Error E5, furnace stops, sometimes accompanied by loud ignition or combustion noise.
Flame Detection / Ionization Circuit Error
E9
The furnace locks out with E9, indicating overheating, and the unit is very hot to the touch.
Overheat Thermostat / Safety Temperature Limiter Tripped
EA
Error EA: The furnace attempts ignition, but no flame is established, leading to multiple retries and then lockout.
No Flame Detected / Gas Supply or Ignition Fault
F0
Display shows F0; the furnace will not operate.
General Control Board Fault / Internal Error
F1
Furnace displays F1, with no response to thermostat demand.
External Control / Thermostat Communication Fault
F7
Error F7, furnace does not start, sometimes after a power outage.
Flame Detected When There Should Be None
H1
Display shows H1; the furnace continues to run at reduced output or locks out.
Flow Temperature Too High / Sensor Overrange
H3
H3 displayed; the furnace cycles frequently and may shut down due to high temperature.
Return Temperature Too High / Poor Heat Dissipation
L0
Display shows L0; the furnace does not respond, and the screen may be dim.
Low Mains Voltage / Power Supply Fault
P0
P0 displayed; the furnace is in lockout, with no heating or hot water.
Gas Pressure / Gas Supply Fault
P1
Error P1, furnace modulates down or locks out during high demand.
Maximum Flow Temperature Exceeded
P2
P2 displayed, furnace output limited, may not reach set temperature.
Minimum Flow Rate Not Achieved
S1
S1 displayed, furnace in standby, does not fire even though heat is requested.
Chimney / Flue Sensor or Draft Fault (on models with flue sensor)
U9
U9 displayed intermittently, furnace may continue to operate with reduced performance.
Communication Fault Between User Interface and Main PCB
Symptom
Domestic hot water temperature fluctuates, running hot then cold during a shower.
Unstable DHW Temperature Control
Symptom
Furnace does not start at all, no display, and no response.
No Power / Dead Appliance
Symptom
Furnace leaking water from bottom of casing or condensate connections.
Condensate Leak / Internal Water Leak
Symptom
Furnace makes loud banging or knocking noises when starting or stopping (kettling).
Boiling / Kettling in Primary Heat Exchanger
Symptom
Furnace starts but shuts down after a few seconds; no error code is displayed.
Short Cycling / Unstable Combustion
Symptom
Furnace vibrates excessively or produces humming/rumbling noise during operation.
Mechanical Vibration / Fan or Pump Imbalance
Symptom
Radiators are only partially warm; the furnace runs continuously, but the home does not reach the set temperature.
Insufficient Heating Output / Circulation Problem
Symptom
The furnace display shows normal operation, but radiators overheat, and the room becomes too hot.
Overheating Rooms / Control Overshoot
Symptom
The furnace display works, but there is no response to the room thermostat, and heating does not start.
Control Signal Not Received
Symptom
The furnace emits a smell of burning plastic or an electrical smell.
Overheating Electrical Component
Symptom
The furnace emits a smell of gas near the appliance, even when not running.
Gas Leak at Appliance
Symptom
The furnace frequently locks out during windy conditions, sometimes with flue-related codes.
Wind-Induced Flue Disturbance
Symptom
The furnace frequently loses pressure, requiring the user to top up the system often.
Chronic Pressure Loss / Expansion Vessel Fault
Symptom
The furnace frequently trips the external RCD or blows the house fuse.
Electrical Short / Earth Leakage
Symptom
The furnace provides heating but no domestic hot water.
DHW Priority / Diverter Valve Fault
Symptom
The furnace runs, but the flue terminal emits visible soot or dark smoke.
Incomplete Combustion / Sooting
| Error Code | Symptom | Fault Type | Guide |
|---|---|---|---|
| A2 |
A2 displayed, furnace stops, sometimes accompanied by gurgling noises.
|
Low Water Pressure / Water Shortage | View Details |
| A3 |
The furnace locks out shortly after starting; the burner does not ignite, and the display shows A3.
|
Ignition Failure / No Flame Detection | View Details |
| A4 |
A4 displayed, furnace does not start heating circuit.
|
Flow Switch / Flow Sensor Fault (Heating Circuit) | View Details |
| A5 |
A5 displayed, no domestic hot water even when tap is opened.
|
DHW Flow Sensor / Turbine Fault | View Details |
| A6 |
A6 displayed, furnace may provide intermittent hot water or heating.
|
Outdoor Temperature Sensor Fault (if fitted) | View Details |
| A7 |
The furnace shuts down with error A7; the burner stops, and hot water and heating are unavailable.
|
Flow Temperature Sensor Fault (NTC) | View Details |
| A8 |
The display shows A8, and the furnace does not start the heating cycle.
|
Return Temperature Sensor Fault (NTC) | View Details |
| A9 |
The furnace shows A9 and stops; heating and hot water are unavailable.
|
DHW Outlet Temperature Sensor Fault (Domestic Hot Water NTC) | View Details |
| C1 |
The furnace displays C1; the fan runs, but the burner does not ignite or shuts down quickly.
|
Fan Speed / Airflow Fault | View Details |
| C6 |
Error C6: The furnace attempts to start but shuts down with a fan-related fault.
|
Fan Speed Too Low / Fan Feedback Error | View Details |
| C7 |
The display shows C7; the furnace will not start, and the fan does not run.
|
Fan Does Not Start / Fan Circuit Fault | View Details |
| E2 |
Furnace displays E2, burner stops, sometimes after running for a short period.
|
Flame Signal Lost During Operation | View Details |
| E5 |
Error E5, furnace stops, sometimes accompanied by loud ignition or combustion noise.
|
Flame Detection / Ionization Circuit Error | View Details |
| E9 |
The furnace locks out with E9, indicating overheating, and the unit is very hot to the touch.
|
Overheat Thermostat / Safety Temperature Limiter Tripped | View Details |
| EA |
Error EA: The furnace attempts ignition, but no flame is established, leading to multiple retries and then lockout.
|
No Flame Detected / Gas Supply or Ignition Fault | View Details |
| F0 |
Display shows F0; the furnace will not operate.
|
General Control Board Fault / Internal Error | View Details |
| F1 |
Furnace displays F1, with no response to thermostat demand.
|
External Control / Thermostat Communication Fault | View Details |
| F7 |
Error F7, furnace does not start, sometimes after a power outage.
|
Flame Detected When There Should Be None | View Details |
| H1 |
Display shows H1; the furnace continues to run at reduced output or locks out.
|
Flow Temperature Too High / Sensor Overrange | View Details |
| H3 |
H3 displayed; the furnace cycles frequently and may shut down due to high temperature.
|
Return Temperature Too High / Poor Heat Dissipation | View Details |
| L0 |
Display shows L0; the furnace does not respond, and the screen may be dim.
|
Low Mains Voltage / Power Supply Fault | View Details |
| P0 |
P0 displayed; the furnace is in lockout, with no heating or hot water.
|
Gas Pressure / Gas Supply Fault | View Details |
| P1 |
Error P1, furnace modulates down or locks out during high demand.
|
Maximum Flow Temperature Exceeded | View Details |
| P2 |
P2 displayed, furnace output limited, may not reach set temperature.
|
Minimum Flow Rate Not Achieved | View Details |
| S1 |
S1 displayed, furnace in standby, does not fire even though heat is requested.
|
Chimney / Flue Sensor or Draft Fault (on models with flue sensor) | View Details |
| U9 |
U9 displayed intermittently, furnace may continue to operate with reduced performance.
|
Communication Fault Between User Interface and Main PCB | View Details |
| – |
Domestic hot water temperature fluctuates, running hot then cold during a shower.
|
Unstable DHW Temperature Control | View Details |
| – |
Furnace does not start at all, no display, and no response.
|
No Power / Dead Appliance | View Details |
| – |
Furnace leaking water from bottom of casing or condensate connections.
|
Condensate Leak / Internal Water Leak | View Details |
| – |
Furnace makes loud banging or knocking noises when starting or stopping (kettling).
|
Boiling / Kettling in Primary Heat Exchanger | View Details |
| – |
Furnace starts but shuts down after a few seconds; no error code is displayed.
|
Short Cycling / Unstable Combustion | View Details |
| – |
Furnace vibrates excessively or produces humming/rumbling noise during operation.
|
Mechanical Vibration / Fan or Pump Imbalance | View Details |
| – |
Radiators are only partially warm; the furnace runs continuously, but the home does not reach the set temperature.
|
Insufficient Heating Output / Circulation Problem | View Details |
| – |
The furnace display shows normal operation, but radiators overheat, and the room becomes too hot.
|
Overheating Rooms / Control Overshoot | View Details |
| – |
The furnace display works, but there is no response to the room thermostat, and heating does not start.
|
Control Signal Not Received | View Details |
| – |
The furnace emits a smell of burning plastic or an electrical smell.
|
Overheating Electrical Component | View Details |
| – |
The furnace emits a smell of gas near the appliance, even when not running.
|
Gas Leak at Appliance | View Details |
| – |
The furnace frequently locks out during windy conditions, sometimes with flue-related codes.
|
Wind-Induced Flue Disturbance | View Details |
| – |
The furnace frequently loses pressure, requiring the user to top up the system often.
|
Chronic Pressure Loss / Expansion Vessel Fault | View Details |
| – |
The furnace frequently trips the external RCD or blows the house fuse.
|
Electrical Short / Earth Leakage | View Details |
| – |
The furnace provides heating but no domestic hot water.
|
DHW Priority / Diverter Valve Fault | View Details |
| – |
The furnace runs, but the flue terminal emits visible soot or dark smoke.
|
Incomplete Combustion / Sooting | View Details |
No matching codes found
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