Junkers Furnaces Error Codes

A2

A2 displayed, furnace stops, sometimes accompanied by gurgling noises

Low Water Pressure / Water Shortage

A3

Furnace locks out shortly after start, burner does not ignite, 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

Furnace shuts down with A7, burner stops, hot water and heating unavailable

Flow Temperature Sensor Fault (NTC)

A8

Display shows A8, furnace does not start heating cycle

Return Temperature Sensor Fault (NTC)

A9

Furnace shows A9 and stops, heating and hot water unavailable

DHW Outlet Temperature Sensor Fault (Domestic Hot Water NTC)

C1

Furnace displays C1, fan runs but burner does not ignite or shuts down quickly

Fan Speed / Airflow Fault

C6

Error C6, furnace attempts to start but shuts down with fan-related fault

Fan Speed Too Low / Fan Feedback Error

C7

Display shows C7, furnace will not start, fan does not run

Fan Does Not Start / Fan Circuit Fault

E2

Furnace shows 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 / Ionisation Circuit Error

E9

Furnace locks out with E9, overheating indicated, unit very hot to touch

Overheat Thermostat / Safety Temperature Limiter Tripped

EA

Error EA, furnace attempts ignition but no flame is established, multiple retries then lockout

No Flame Detected / Gas Supply or Ignition Fault

F0

Display shows F0, furnace will not operate

General Control Board Fault / Internal Error

F1

Furnace shows F1, no response to thermostat demand

External Control / Thermostat Communication Fault

F7

Error F7, furnace does not start, sometimes after power outage

Flame Detected When There Should Be None

H1

Display shows H1, furnace continues to run at reduced output or locks out

Flow Temperature Too High / Sensor Overrange

H3

H3 displayed, furnace cycles frequently and may shut down on high temperature

Return Temperature Too High / Poor Heat Dissipation

L0

Display shows L0, furnace does not respond, screen may be dim

Low Mains Voltage / Power Supply Fault

P0

P0 displayed, furnace in lockout, 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, runs hot then cold during shower

Unstable DHW Temperature Control

Symptom

Furnace display shows normal operation but radiators overheat, room becomes too hot

Overheating Rooms / Control Overshoot

Symptom

Furnace display works but no response to room thermostat, heating does not start

Control Signal Not Received

Symptom

Furnace does not start at all, no display, no response

No Power / Dead Appliance

Symptom

Furnace emits smell of burning plastic or electrical smell

Overheating Electrical Component

Symptom

Furnace emits smell of gas near appliance, even when not running

Gas Leak at Appliance

Symptom

Furnace frequently locks out during windy conditions, sometimes with flue-related codes

Wind-Induced Flue Disturbance

Symptom

Furnace frequently loses pressure, user must top up system often

Chronic Pressure Loss / Expansion Vessel Fault

Symptom

Furnace frequently trips external RCD or blows house fuse

Electrical Short / Earth Leakage

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 provides heating but no domestic hot water

DHW Priority / Diverter Valve Fault

Symptom

Furnace runs but flue terminal emits visible soot or dark smoke

Incomplete Combustion / Sooting

Symptom

Furnace starts but shuts down after a few seconds, no error code displayed

Short Cycling / Unstable Combustion

Symptom

Furnace vibrates excessively or produces humming/rumbling noise during operation

Mechanical Vibration / Fan or Pump Imbalance

Symptom

Radiators only partially warm, furnace runs continuously but home does not reach set temperature

Insufficient Heating Output / Circulation Problem