Junkers Boilers Error Codes

A2

Boiler does not start; display shows A2 and burner symbol does not appear

No flame detection on start-up

A3

Boiler locks out with A3; fan runs briefly then stops, burner does not ignite

Flue gas temperature sensor fault / overtemperature

A7

Boiler shows 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 shows A9; heating flow temperature reading is implausible or boiler overheats quickly

Flow temperature sensor (NTC) fault

C1

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

Fan speed / air pressure fault

C6

Boiler shows C6; fan runs erratically or at incorrect speed

Fan speed control / feedback error

C7

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

Fan not running

D3

Boiler shows D3; burner stops and 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 shows 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 shows E9 or low-pressure symbol; boiler does not start

Low system water pressure

EA

Boiler shows EA; ignition failure or flame lost during operation

Ignition / flame loss fault

F0

Boiler shows 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 shows 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 shows P2; hot water priority or pump overrun active

DHW priority / pump overrun mode

Symptom

Boiler condensate pipe dripping heavily or gurgling noises from condensate trap

Condensate drainage issue

Symptom

Boiler display works but no response to any buttons

Keypad / user interface failure

Symptom

Boiler emits smell of burning plastic or electrical overheating

Overheating electrical component

Symptom

Boiler emits 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 but some radiators remain cold at the top

Air trapped in radiators

Symptom

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

Boiler not responding to room thermostat

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; 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 boiler appears to run and hot water works

No central heating circulation