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Worcester Bosch Boiler Issues: Errors and Fault Codes

When your Worcester Bosch boiler starts to play up, it can be difficult to know whether the fault is simple to fix or whether it’s a serious issue that requires urgent attention from an engineer. The majority of the time, your Worcester Bosch boiler will shut down or display a unique fault code or an error message to let you know that something isn’t right. Fault codes are designed to make it easy for your engineer to identify the possible cause of a fault, and you may be asked for the code when you call an engineer for assistance.

Your Worcester Bosch boiler manual should list the fault codes and error codes that could appear on your boiler. If you have misplaced your manual and your boiler is displaying a fault code, you can search through our tables below to find out what the error message means.

Worcester Bosch Boiler Fault Codes by Type

Fault DescriptionCodes That Will Appear On Your Boiler
Blocking ErrorThe boiler will stop operating but no fault code will appear. You can access the cause code by pressing the spanner/return button. The condition will clear once the fault clears. For instance, if the boiler is waiting for an area to cool, the error will clear automatically once the associated area has cooled down.
Locking ErrorThe boiler will flash a warning triangle and a fault code. It will also show a static cause code. You’ll need to press the boiler reset button to clear the code, but only after the boiler has been fixed.
Fault ErrorThe boiler will display an alphanumeric fault code to indicate the fault group.
Cause ErrorA three-digit number that can be accessed via the info menu by pressing the spanner/return button during a blocking error or when in normal operating status. The cause code should flash on the display during a locking error.
Maintenance CodeA maintenance code is displayed with the letter H followed by a number. The fault won’t stop the boiler from working but it tells you that the boiler needs to be looked at by a qualified engineer.

Worcester Bosch Boiler Locking Error Codes

Fault CodeCause CodePossible Cause
9A362Incorrect Heat Control Module (HCM) has been fitted
9U233Code plug is faulty or not connected
B7257Need to replace control board
C6215Fan running too fast
C6216Fan running too slow
C7214Fan not running at all
C7217Fan not running or the airflow is blocked
D1240Sensor wet or damaged – the boiler will usually block for 300 seconds before the fault code is displayed
D1241Sensor is disconnected or damaged  – the boiler will usually block for 300 seconds before the fault code is displayed
D1286Boiler overheating
E2222Flow sensor shorted or damaged
E2223Flow sensor damaged or disconnected
E5218 or 332Primary flow sensor overheated
E9219Main heat exchanger overheated
E9220Maximum temperature sensor on main heat exchanger has failed
E9221Maximum temperature sensor on main heat exchanger not recognised
E9224Flue gas thermostat overheated
EA227Failure of four ignition attempts – Need to wait at least 30 seconds before another attempt is made
EA229Established flame has been extinguished – Could be caused by a blocked sump or strong wind into the flue
EA234Gas valve coil disconnected
EA261No flame established during the first 4 seconds of ignition period
EA338Pump not detecting water – check pressure gauge and re-pressurise to 1 bar

Worcester Bosch Boiler Blocking Fault Codes

Fault CodeCause CodePossible Cause
A1281Low or no water in the system  – check pressure gauge and re-pressurise to 1 bar
C1264Fan stopped working
EA227No flame detected – boiler will block during 4 attempts before going to a locking fault
EF349Boiler operating at minimum burner load and there is a greater flow and return temperature than 18°C
No code212 or 213Airlock or reduced water content – check pressure gauge and re-pressurise to 1 bar
No code260Sensor disconnected or no water content – check pressure gauge and re-pressurise to 1 bar

Worcester Bosch Greenstar CDi Classic Combi Fault Codes

Fault CodeDescription
A7Hot water NTC sensor defective
A8Break in communication to FX controls
b1Code plug not detected
C6Fan speed too low
E2CH flow NTC sensor defective
E9Safety temperature limiter in CH flow has tripped
EAFlame not detected. If safe, check that the gas is on in the house OR check for a frozen condensate pipe. Could be a blocked flue outlet – contact a Gas Safe engineer
F0Internal error
F7Flame detected even though boiler is switched off
FAFlame detected after the gas has been shut off
FdReset button pressed by mistake

Worcester Bosch Greenstar CDi Highflow Combi Fault Codes

Fault CodeDescription
A7Hot water NTC sensor defective
A8Break in communication to weather compensating clip-in control of Greenstar Comfort II RF
B1Code plug not detected
C6Fan speed too low
D5Condensate pipe fault
E2CH flow NTC sensor defective
E9Safety temperature limiter in CH flow has tripped
EAFlame not detected. If safe, check that the gas is on in the house OR check for a frozen condensate pipe. Could be a blocked flue outlet – contact a Gas Safe engineer
F0Internal error
F7Flame detected even though boiler is switched off
FAFlame detected after gas shut off
FdReset button pressed by mistake
SEService required
U1, U2 or U3Boiler flow temperature is too high and the boiler will shut down until it has cooled down

Worcester Greenstar Heatslave II Oil Combi Fault Codes

Fault CodeCause CodeDescription
E9210Maximum temperature thermostat or air pressure witch activated. Reset overheat thermostat or air pressure switch before resetting boiler
E5218Flow temperature too high
E9219, 220 or 221Safety temperature too high, safety sensor short circuit or safety sensor open circuit
E2222, 223 or 233Flow sensor short circuit, flow sensor open circuit or heat control module code plug missing or defective
F0237 or 238Software relay error – Resistors between plus X4 pins 3 & 5 disconnected or water in the burner control box
F0239 or 259Watchdog relay error or A/D reference relay error – If error persists, change the control board
F0263Appliance powered off with locking fault present – press reset button to clear
D4271Temperature difference between flow and safety sensors exceeds the limit
E9276 or 277Flow temperature or safety temperature exceeds 95°C
F0278, 279 or 290Sensor test error, A/D convertor error or A/D convertor reference error
E5332Primary flow has exceeded 105°C – primary sensor overheated
9A360Incorrect HCM for boiler variant – replace with correct HCM
A7808DHW sensor defective – check and replace defect sensor as necessary
A0809Thermal store sensor defective – check and replace defect sensor as necessary
9F855 or 856Burner lockout or 5 burner lockouts repeated resets. Check the burner and fuel supply, and ensure oil supply is free of contaminants. After five 855 burner lockouts, code 856 is displayed.
H29N/AWarning triangle and house symbol displayed on controller – weather compensation activated but no weather sensor detected. Refer to section 5.11 on page 32 of manual
H27N/AWarning triangle symbol displayed on controller – DCW in sensor defect

Worcester Bosch Greenstar CDi Classic System Fault Codes

Fault CodeDescription
b1Code plug not detected
C6Fan speed too low
E9Safety temperature limiter in CH flow has tripped
EAFlame not detected. If safe, check that the gas is on in the house OR check for a frozen condensate pipe. Could be a blocked flue outlet – contact a Gas Safe engineer
F0Internal error
F7Flame detected even though boiler is switched off
FAFlame detected after gas shut off
FdReset button pressed by mistake

Worcester Bosch GB162 System Fault Codes

Fault CodeCause CodeDescription
N/A208Boiler is in chimney sweep mode
N/A200Boiler is in heating mode
N/A201Boiler is in domestic hot water mode
N/A202Boiler is waiting because there is a heat demand from the on/off or modulating control
N/A203Boiler is on standby
N/A204Boiler is waiting because the actual temperature is higher than the calculated or selected temperature
N/A212The temperature recorded by the flow temperature sensor or safety sensor is rising too fast
N/A226Diagnoses tool was connected and is now locked out
N/A260The flow temperature sensor is not detecting a rise in temperature after a burner start
N/A265Boiler is waiting in response to a heat demand – boiler regularly switches to partial load
N/A268Component test phase
N/A270Boiler is modulating upwards
N/A283Boiler is preparing for a burner start – fan and pump activated
N/A284Gas valve is switched on
N/A305Boiler is waiting for domestic hot water operation to finish
N/A235The KIM (boiler identification module) is too new for burner control unit
N/A360The fitted KIM is not compatible with the burner control unit
9A361The fitted burner control unit is not compatible with the KIM
9U233The burner control unit or KIM is faulty
A3317Short circuit in the flue gas temperature sensor contacts
B7257Burner control unit or KIM is faulty
C1264No control signal or power supply to fan
C4276Boiler switched off for a max of 2 minutes because it has been operating non-stop for 24 hours
C6215Fan speed too high
C6216Fan speed too low
C7214Fan doesn’t run during start phase
C7217Fan speed is irregular on start up
CE207Water pressure too low
CE266Pump test failed
D1240Interruption to contacts of boiler return temperature sensor
D3232External switching contact is open
D4213Temperature recorded by the flow temperature sensor or return temperature sensor is rising too fast
D4271Temperature differential between flow and safety temperature sensor is too high
D4286Boiler return temperature sensor has detected a return temperature higher than 105°C
E2222Short circuit in flow temperature sensor contacts
E2223Interruption to flow temperature sensor contacts
E5218Temperature sensed by flow temperature sensor is higher than 105°C
E9210Temperature measured by flue gas sensor is open and too high
E9219Safety temperature sensor has sensed a temperature higher than 105°C
E9220Contacts for the safety temperature sensor have shorted or the safety temperature sensor has detected a temperature that exceeds 105°C
E9224Interruption to the safety temperature sensor
E9276Temperature flow sensor has sensed a temperature higher than 95°C
E9277Safety temperature sensor has sensed a temperature higher than 95°C
E9285Boiler temperature sensor has sensed a temperature higher than 95°C
E9318Interruption to flue gas sensor contacts
EA227Insufficient ionisation current measured following ignition of burner
EA229Insufficient ionisation current detected by system during the burning phase
EA234Interruption to contact of the gas valve
EA261Faulty burner control unit
EA269Ignition device activated for too long
F0238Faulty burner control unit (KIM)
F0239Error with burner control unit (KIM)
F0242 to 253 and 255, 259, 263, 267 and 272Faulty burner control unit (KIM)
F0278Sensor test failed – check cabling and plug sensors
F0279Faulty burner control unit or KIM – may need to replace control unit
F0280Faulty burner control unit
F0287Error with burner control unit or KIM
F0290Faulty burner control unit or KIM
F7228Ionisation current measured before burner started
F7328Power failure occurred
FA306Ionisation current measured after burner went out
FD231Interruption to mains voltage during interlocking fault
EC256Burner control unit error
EH258Faulty burner control unit – check plug and wiring
H07 Water pressure too low
HrE Boiler keeps resetting
rE Boiler resets

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