Bosch Boilers Error Codes
Complete list of fault codes and troubleshooting guides. Use the search below to find your specific error.
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A7
Boiler runs but hot water temperature is unstable or incorrect; DHW temperature on display does not match actual outlet temperature.
DHW Temperature Sensor (NTC) Fault
A8
Boiler indicates an issue with the hot water sensor or control; DHW may not reach set temperature or may fluctuate.
DHW Sensor / Control Parameter Fault
A9
Boiler reports an internal control or sensor configuration fault; may limit hot water or heating output.
Control Parameter / Sensor Configuration Fault
C1
Boiler fan runs at incorrect speed or does not respond properly; may cause ignition failure or noisy operation.
Fan Speed Control Fault
C4
Boiler displays a flow temperature sensor fault; heating may shut down or run at incorrect temperature.
Flow Temperature NTC Sensor Fault
C6
Boiler indicates fan speed signal error; may lock out with no ignition.
Fan Speed Feedback Fault
C7
Boiler reports flue gas sensor or flue temperature fault; may shut down to prevent overheating.
Flue Gas Temperature Sensor Fault
C8
Boiler indicates gas valve or gas flow control fault; ignition may fail or flame may be unstable.
Gas Valve Control Fault
D1
Boiler shows a flow temperature sensor or sensor circuit fault; heating may not start or may overheat.
Flow NTC Sensor Circuit Fault
D3
Boiler indicates return temperature sensor fault; may cause poor modulation or lockout.
Return NTC Sensor Fault
D4
Boiler shows a DHW sensor circuit fault; hot water may be disabled.
DHW NTC Sensor Circuit Fault
E9
Boiler overheats and locks out; may display high temperature and shut down quickly after starting.
Primary Heat Exchanger Overheat / Safety Temperature Limiter Trip
EA
Boiler attempts to ignite but fails; you may hear clicking but no flame, then the boiler locks out with EA.
Ignition Failure / No Flame Detected
F0
Boiler shows a general sensor fault; may relate to multiple NTCs or internal sensors.
General Sensor Fault
F1
Boiler indicates a flow temperature sensor fault; heating may not operate correctly.
Flow NTC Fault
F2
Boiler indicates a return temperature sensor fault; may cause poor modulation or lockout.
Return NTC Fault
F3
Boiler shows a flue gas temperature sensor fault; may shut down to protect the appliance.
Flue Gas NTC Fault
F4
Boiler indicates a DHW temperature sensor fault; hot water may be disabled or unstable.
DHW NTC Fault
F5
Boiler reports a flue gas sensor or air pressure related fault; may not ignite.
Flue Gas / Air Pressure Fault
F7
Boiler detects flame when there should be none (false flame); may lock out.
False Flame Detection / Ionisation Fault
FA
Boiler reports a flue gas recirculation or flue system fault; may shut down shortly after ignition.
Flue Gas Recirculation / Flue System Fault
FD
Boiler indicates a coding plug or parameter module fault; may not operate.
Coding Plug / Parameter Module Fault
H07
Boiler indicates a low system pressure condition; heating may not start.
Low Water Pressure Fault
H11
Boiler indicates a circulation fault; flow temperature rises too quickly or there is insufficient flow.
Poor Circulation / Pump Fault
H13
Boiler indicates a flue gas temperature too high; may shut down during operation.
Excessive Flue Gas Temperature
H15
Boiler indicates a flow/return temperature differential fault; temperature difference is outside normal range.
Flow/Return Temperature Differential Fault
H17
Boiler indicates a DHW flow sensor or flow switch fault; hot water may not start when a tap is opened.
DHW Flow Detection Fault
H19
Boiler indicates a DHW temperature control fault; hot water may be too hot or too cold.
DHW Temperature Control Fault
H27
Boiler indicates a communication fault with an external controller (e.g. Bosch room controller or EMS bus).
Bus Communication Fault
Symptom
Boiler display is blank or unreadable, but the boiler may still operate partially.
Display / User Interface Fault
Symptom
Boiler frequently cycles on and off with short run times, even when heating is needed.
Short Cycling
Symptom
Boiler frequently loses pressure; you must keep topping up the system.
Chronic Pressure Loss
Symptom
Boiler has power (display on) but does not fire for central heating; radiators stay cold.
No Central Heating Demand / Control Circuit Fault
Symptom
Boiler heats radiators but does not provide hot water on a combi model.
No Domestic Hot Water on Combi Boiler
Symptom
Boiler is noisy during operation; banging, kettling, or whistling sounds from the unit.
Boiler Kettling / Noisy Heat Exchanger
Symptom
Boiler leaks water from underneath or from internal components.
Internal Water Leak
Symptom
Boiler locks out intermittently, especially in cold weather, and the condensate pipe outside may be icy.
Frozen or Blocked Condensate Pipe
Symptom
Boiler or flue area smells of fumes or exhaust; occupants may feel unwell or notice condensation/soot.
Flue Gas Spillage / Combustion Fault
Symptom
Boiler or surrounding area smells of gas when the boiler is operating or shortly after.
Gas Leak / Incomplete Combustion
Symptom
Boiler runs but room temperature overshoots or undershoots setpoint significantly.
Poor Temperature Control / Thermostat Issues
Symptom
Boiler will not start at all; no lights or display, completely dead.
No Power Supply to Boiler
Symptom
Radiators heat unevenly; some are hot, others remain cold or only warm at the top.
Poor System Circulation / Air in Radiators
| Error Code | Symptom | Fault Type | Guide |
|---|---|---|---|
| A7 |
Boiler runs but hot water temperature is unstable or incorrect; DHW temperature on display does not match actual outlet temperature.
|
DHW Temperature Sensor (NTC) Fault | View Details |
| A8 |
Boiler indicates an issue with the hot water sensor or control; DHW may not reach set temperature or may fluctuate.
|
DHW Sensor / Control Parameter Fault | View Details |
| A9 |
Boiler reports an internal control or sensor configuration fault; may limit hot water or heating output.
|
Control Parameter / Sensor Configuration Fault | View Details |
| C1 |
Boiler fan runs at incorrect speed or does not respond properly; may cause ignition failure or noisy operation.
|
Fan Speed Control Fault | View Details |
| C4 |
Boiler displays a flow temperature sensor fault; heating may shut down or run at incorrect temperature.
|
Flow Temperature NTC Sensor Fault | View Details |
| C6 |
Boiler indicates fan speed signal error; may lock out with no ignition.
|
Fan Speed Feedback Fault | View Details |
| C7 |
Boiler reports flue gas sensor or flue temperature fault; may shut down to prevent overheating.
|
Flue Gas Temperature Sensor Fault | View Details |
| C8 |
Boiler indicates gas valve or gas flow control fault; ignition may fail or flame may be unstable.
|
Gas Valve Control Fault | View Details |
| D1 |
Boiler shows a flow temperature sensor or sensor circuit fault; heating may not start or may overheat.
|
Flow NTC Sensor Circuit Fault | View Details |
| D3 |
Boiler indicates return temperature sensor fault; may cause poor modulation or lockout.
|
Return NTC Sensor Fault | View Details |
| D4 |
Boiler shows a DHW sensor circuit fault; hot water may be disabled.
|
DHW NTC Sensor Circuit Fault | View Details |
| E9 |
Boiler overheats and locks out; may display high temperature and shut down quickly after starting.
|
Primary Heat Exchanger Overheat / Safety Temperature Limiter Trip | View Details |
| EA |
Boiler attempts to ignite but fails; you may hear clicking but no flame, then the boiler locks out with EA.
|
Ignition Failure / No Flame Detected | View Details |
| F0 |
Boiler shows a general sensor fault; may relate to multiple NTCs or internal sensors.
|
General Sensor Fault | View Details |
| F1 |
Boiler indicates a flow temperature sensor fault; heating may not operate correctly.
|
Flow NTC Fault | View Details |
| F2 |
Boiler indicates a return temperature sensor fault; may cause poor modulation or lockout.
|
Return NTC Fault | View Details |
| F3 |
Boiler shows a flue gas temperature sensor fault; may shut down to protect the appliance.
|
Flue Gas NTC Fault | View Details |
| F4 |
Boiler indicates a DHW temperature sensor fault; hot water may be disabled or unstable.
|
DHW NTC Fault | View Details |
| F5 |
Boiler reports a flue gas sensor or air pressure related fault; may not ignite.
|
Flue Gas / Air Pressure Fault | View Details |
| F7 |
Boiler detects flame when there should be none (false flame); may lock out.
|
False Flame Detection / Ionisation Fault | View Details |
| FA |
Boiler reports a flue gas recirculation or flue system fault; may shut down shortly after ignition.
|
Flue Gas Recirculation / Flue System Fault | View Details |
| FD |
Boiler indicates a coding plug or parameter module fault; may not operate.
|
Coding Plug / Parameter Module Fault | View Details |
| H07 |
Boiler indicates a low system pressure condition; heating may not start.
|
Low Water Pressure Fault | View Details |
| H11 |
Boiler indicates a circulation fault; flow temperature rises too quickly or there is insufficient flow.
|
Poor Circulation / Pump Fault | View Details |
| H13 |
Boiler indicates a flue gas temperature too high; may shut down during operation.
|
Excessive Flue Gas Temperature | View Details |
| H15 |
Boiler indicates a flow/return temperature differential fault; temperature difference is outside normal range.
|
Flow/Return Temperature Differential Fault | View Details |
| H17 |
Boiler indicates a DHW flow sensor or flow switch fault; hot water may not start when a tap is opened.
|
DHW Flow Detection Fault | View Details |
| H19 |
Boiler indicates a DHW temperature control fault; hot water may be too hot or too cold.
|
DHW Temperature Control Fault | View Details |
| H27 |
Boiler indicates a communication fault with an external controller (e.g. Bosch room controller or EMS bus).
|
Bus Communication Fault | View Details |
| – |
Boiler display is blank or unreadable, but the boiler may still operate partially.
|
Display / User Interface Fault | View Details |
| – |
Boiler frequently cycles on and off with short run times, even when heating is needed.
|
Short Cycling | View Details |
| – |
Boiler frequently loses pressure; you must keep topping up the system.
|
Chronic Pressure Loss | View Details |
| – |
Boiler has power (display on) but does not fire for central heating; radiators stay cold.
|
No Central Heating Demand / Control Circuit Fault | View Details |
| – |
Boiler heats radiators but does not provide hot water on a combi model.
|
No Domestic Hot Water on Combi Boiler | View Details |
| – |
Boiler is noisy during operation; banging, kettling, or whistling sounds from the unit.
|
Boiler Kettling / Noisy Heat Exchanger | View Details |
| – |
Boiler leaks water from underneath or from internal components.
|
Internal Water Leak | View Details |
| – |
Boiler locks out intermittently, especially in cold weather, and the condensate pipe outside may be icy.
|
Frozen or Blocked Condensate Pipe | View Details |
| – |
Boiler or flue area smells of fumes or exhaust; occupants may feel unwell or notice condensation/soot.
|
Flue Gas Spillage / Combustion Fault | View Details |
| – |
Boiler or surrounding area smells of gas when the boiler is operating or shortly after.
|
Gas Leak / Incomplete Combustion | View Details |
| – |
Boiler runs but room temperature overshoots or undershoots setpoint significantly.
|
Poor Temperature Control / Thermostat Issues | View Details |
| – |
Boiler will not start at all; no lights or display, completely dead.
|
No Power Supply to Boiler | View Details |
| – |
Radiators heat unevenly; some are hot, others remain cold or only warm at the top.
|
Poor System Circulation / Air in Radiators | View Details |
No matching codes found
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