Worcester Bosch boilers use a structured fault‑code system to identify issues and help users understand why the boiler may not be operating normally. These codes generally fall into two main categories: locking errors, which shut the boiler down for safety and require a manual reset once the underlying issue is resolved, and blocking errors, which temporarily pause operation but may clear automatically once conditions return to normal. Together, these fault types offer a clear indication of whether the boiler is experiencing a critical fault or a temporary interruption.
Our aim is to provide a simple, reliable overview of the most common Worcester Bosch locking and blocking errors to help you quickly identify what each code means and what steps you can take next. By understanding the difference between these fault types and recognising the symptoms they represent, you’ll be able to troubleshoot more confidently and know when professional assistance may be required.
Worcester Bosch
Locking Error Accompanied by a flashing fault code and a warning triangle. Press the reset button to resolve.ied by a flashing fault code and a warning triangle. Press the reset button to resolve.
Block Error No data will be shown for this kind of error and your boiler will cease to function. You will be able to find the cause code (three digit number) by pressing the spanner / return button.
Fault Error An alpha numeric code that will indicate the error by showing the fault group (note: these vary by make and model).
Maintenance Code Begins with a H. This warning informs you that the issue that will not stop the boiler working, but will need the attention of an engineer.
Common Locking Errors
9A Incorrect HCM fitted
9U Problem with the code plug (faulty or not connected)
B7 Replace control board
C6 Running too fast/slow
C7 Fan not running/airflow blocked
D1 Sensor wet/damaged/overheated (boiler will block for 300 seconds)
E2 Flow sensor shorted/damaged/disconnected
E5 Primary flow sensor overheated
E9 Heat exchanger overheated/failed/not-recognised/flue gas thermostat overheated/li>>
EA Failure of 4 ignition attempts/established flame extinguished/gas valve coil disconnected OR pump not detecting water
Common Blocking Errors
A1 Little or no water in the system (check pressure gauge and repressurise to 1 bar)
C1 Fan stopped
EA No flame detected (you have 4 attempts to unblock boiler before a locking fault)
EF Boiler operating at minimum burner load
No Code Air lock OR reduced water content (check pressure gauge and repressurise to 1 bar)
260 Sensor disconnected OR no water content (check pressure gauge and repressurise to 1 bar)
Valliant Fault Error Codes
F.49 eBUS fault: short circuit/overload OR two power supplies with different polarities on the eBUS
F.52 Mass flow sensor not connected/plug not connected OR incorrectly connected
F.53 Mass flow sensor filter below venturi filter cap/blocked OR gas flow pressure too low
F.54 Gas pressure fault (in combination with F.28/F.29) little to no gas supply pressure OR gas valve closed
F.56 Mass flow sensor regulation (gas valve defective, cable harness to gas valve defective)
F.57 Fault during comfort safety mode (ignition electrode highly corroded)
F.61 Valve regulation (short circuit/gas valve defective/electronic defective
F.62 Gas valve switch-off delay (delayed shutdown of gas valve/delayed extinguishing of flame signal/gas valve leaking/electronics defective)
F.63 EEPROM error (electronics defective)
F.64 Electronics/NTC fault (supply or return NTC short circuited/electronics defective)
F.65 Electronic temperature fault (electronics overheating due to external influences/ electronics defective)
F.67 Electronics/flame fault (implausible flame signal/electronics defective)
F.68 Unstable flame signal fault (air in gas, gas flow pressure too low, wrong air ratio, etc)
F.70 Invalid device specific number (DSN) spare part case: Display and PCB replaced at same time and DSN not set, wrong or missing output range coding resistor
F.71 Flow NTC fault: flow temperature sensor signalling constant value/flow temperature sensor incorrectly positioned at supply pipe/flow temperature sensor defective
F.72 Flow/return NTC fault; flow/return NTC temperature difference too great → flow and/or return temperature sensor defective
F.73 Water pressure sensor signal in the wrong range (too low). Interruption/short circuit of water pressure sensor, interruption/short circuit to GND in supply line to water pressure sensor or water pressure sensor defective
F.74 Water pressure sensor signal outside correct range (too low). Cable to water pressure sensor has short-circuited at 5V/24 V or internal fault in water pressure sensor
F.75 Fault, no pressure change detection when starting pump. Water pressure sensor and/or pump defective, air in heating installation, too low water pressure in boiler; check adjustable bypass, connect external expansion vessel to return
F.76 Overheating protection on primary heat exchanger has responded. Cable or cable connections for safety fuse in primary heat exchanger or primary heat exchanger defective
F.77 Flue non-return flap/condensate pump fault. No response from flue non-return flap or condensate pump defective
F.78 Interruption to DHW outlet sensor at external controller. UK link box is connected, but hot water NTC not bridged
F.83 Flow and/or return temperature sensor temperature change fault. When the burner starts, the temperature change registered at flow and/or return temperature sensor is non-existent or too small. Not enough water in the boiler. Flow and/or return temperature sensor not in correct position at pipe
F.84 Fault: Flow/return temperature sensor difference implausible. Flow and return temperature sensors return implausible values. Flow and return temperature sensors have been inverted. Flow and return temperature sensors have not been correctly fitted
F.85 Fault: Flow and return temperature sensors incorrectly fitted. Flow and/or return temperature sensors have been fitted to the same pipe/wrong pipe
F.92 Coding resistor fault. The coding resistor on the PCB does not match the entered gas family. Check the resistor, repeat the gas family check and enter the correct gas family
F.93 Gas group fault. Combustion quality outside the permitted range: Wrong gas restrictor, recirculation, wrong gas family, internal pressure measuring point in venturi blocked (do not use lubricant on O-ring in venturi.). Gas family check prematurely terminated
Ideal Boiler Fault Codes
C0 BCC Activation Fault
C2 BCC Fault
F1 Low Water Pressure
F2 Flame Loss
F3 Fan Fault
F4 Flow Thermistor
F5 Return Thermistor
F6 Outside Sensor Failures
F9 Unconfigured PCB
L1 Flow Temperature Overheat or No Water Flow
L2 Flame Loss
L5 5 Boiler resets in 15 minutes
L6 False Flame Lockout
