Junkers Boilers Error Codes

A2

The boiler does not start; the display shows A2, and the burner symbol does not appear.

No Flame Detection on Start-up

A3

The boiler locks out with A3; the fan runs briefly then stops, and the burner does not ignite.

Flue Gas Temperature Sensor Fault / Overtemperature

A7

Boiler displays A7; domestic hot water temperature is unstable or incorrect.

DHW temperature sensor (NTC) fault.

A8

Boiler displays A8 and does not respond to room thermostat commands.

Communication fault with room controller/sensor.

A9

Boiler displays A9; heating flow temperature reading is implausible or the boiler overheats quickly.

Flow temperature sensor (NTC) fault.

C1

Boiler displays C1; the fan starts but ignition does not proceed, or the boiler shuts down.

Fan speed/air pressure fault.

C6

Boiler displays C6; the fan runs erratically or at an incorrect speed.

Fan speed control/feedback error.

C7

Boiler displays C7; no fan operation is detected when there is a heat demand.

Fan not running.

D3

Boiler displays D3; the burner stops and the boiler remains off for a defined waiting time.

Anti-cycling/burner blocking time active.

E2

Boiler displays E2; flow temperature sensor error.

Flow NTC sensor open or short circuit.

E3

Boiler displays E3; return temperature sensor fault

Return NTC sensor open or short circuit

E5

Boiler displays E5; flue gas temperature too high or sensor fault

Flue gas overtemperature / flue NTC fault

E9

Boiler locks out with E9; overheating or dry-fire condition.

Safety temperature limiter (STB) tripped/overheat.

E9 (water pressure)

Boiler displays E9 or low-pressure symbol; boiler does not start

Low system water pressure

EA

Boiler displays EA; ignition failure or flame loss during operation.

Ignition/flame loss fault.

F0

Boiler displays F0; internal control fault

Control board hardware error

F1

Boiler displays F1; sensor supply or internal sensor error

Internal sensor / reference voltage fault

F7

Boiler displays F7; flame signal detected when burner should be off

False flame detection

H7

Boiler displays H7; service reminder or maintenance required

Service interval reached

P2

Boiler displays P2; hot water priority or pump overrun active

DHW priority / pump overrun mode

Symptom

Boiler condensate pipe dripping heavily or gurgling noises from the condensate trap.

Condensate drainage issue

Symptom

Boiler display works but no response to any buttons

Keypad / user interface failure

Symptom

Boiler emits a smell of burning plastic or electrical overheating.

Overheating electrical component

Symptom

Boiler emits a smell of gas near the casing.

Gas leak at boiler connections

Symptom

Boiler fan is very noisy during operation.

Worn or unbalanced fan

Symptom

Boiler frequently loses pressure and needs topping up.

Chronic pressure loss

Symptom

Boiler frequently trips the household circuit breaker or RCD

Electrical short or earth leakage

Symptom

Boiler ignition causes a loud bang (delayed ignition)

Delayed gas ignition

Symptom

Boiler leaking water from underneath the casing

Internal water leak

Symptom

Boiler makes loud banging or kettling noises during operation

Heat exchanger kettling / localised boiling

Symptom

Boiler runs continuously and overheats rooms even when the thermostat is turned down

Boiler not responding to room thermostat

Symptom

Boiler runs, but some radiators remain cold at the top.

Air trapped in radiators

Symptom

Boiler starts, then shuts down after a few seconds without showing a code.

Short cycling / premature shutdown

Symptom

Boiler will not start at all; the display is blank.

No power / dead boiler

Symptom

Domestic hot water temperature fluctuates between hot and cold.

Unstable DHW temperature control

Symptom

Radiators are cold, but the boiler appears to run and hot water works.

No central heating circulation