BoilerFaultCodes A
FAIL POINT A - Is the facia on/off switch turned on?
Worcester Bosch 24/28/35 CDi MkII Fault codes & diagnostics
26 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.
Combi Natural Gas 24-35 kW 2019-present
Download the Worcester Bosch 24/28/35 CDi MkII manual
PDF
The fault code is already on your display
Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →
Severity at a glance
Each cell = one fault code. Hover to identify. 5 Emergency
4 High
17 Unrated
Just want the most common Worcester Bosch 24/28/35 CDi MkII faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes
All 26 documented codes
Tap any card for details
B
FAIL POINT B - Has fuse F2 blown?
C
FAIL POINT C - Green diode is damaged
D
FAIL POINT D - With CH control knob fully clockwise, does the boiler ignite and appear to run normally in central heating mode?
E
FAIL POINT E - With a hot water tap fully open, does the boiler ignite and appear to run normally in hot water mode?
E00 716
Automatic Air Vent
E00-711 Your boiler is struggling to transfer heat between its internal systems, which means it cannot reliably provide hot water or heating.
Your boiler is struggling to transfer heat between its internal systems, which means it cannot reliably provide hot water or heating.
Parts commonly replaced
- Plate-to-plate heat exchanger
- Diverter valve
E00-716
Fan Assembly
E00-720 Your boiler is failing to ignite the flame needed to produce heat and hot water.
Your boiler is failing to ignite the flame needed to produce heat and hot water.
Parts commonly replaced
- Ignition electrode
- Gas valve
- Printed Circuit Board (PCB)
E00-721 Your boiler's internal gas system is not running correctly, meaning the unit cannot safely ignite to provide heating or hot water.
Your boiler's internal gas system is not running correctly, meaning the unit cannot safely ignite to provide heating or hot water.
Parts commonly replaced
- Gas valve
- Burner manifold assembly
- Ignition electrode
Showing 11–20 of 26
E00-730 Your boiler is not detecting the correct airflow needed to operate safely, so it has shut itself down to prevent any issues.
Your boiler is not detecting the correct airflow needed to operate safely, so it has shut itself down to prevent any issues.
Parts commonly replaced
- Air pressure switch
- Fan assembly
- Venturi/tubing
E00-737 Your boiler has detected an issue with the fuel supply or combustion process, meaning it has safely shut itself down to prevent unsafe operation.
Your boiler has detected an issue with the fuel supply or combustion process, meaning it has safely shut itself down to prevent unsafe operation.
Parts commonly replaced
- Gas valve
- PCB
- Flame sensing electrode
E00-738 Your boiler has detected an issue with the fuel delivery system specifically related to the propane gas supply, preventing it from igniting safely.
Your boiler has detected an issue with the fuel delivery system specifically related to the propane gas supply, preventing it from igniting safely.
Parts commonly replaced
- Injector
- Gas Valve
- Burner Seal
E00-741 Your boiler has identified a problem with how the gas-to-air mixture is being delivered to the burner, which indicates an issue with your fuel supply settings.
Your boiler has identified a problem with how the gas-to-air mixture is being delivered to the burner, which indicates an issue with your fuel supply settings.
Parts commonly replaced
- Gas valve
- Burner injector assembly
- PCB
E01 612
Flame Sensor + Harness
E80-450 Your boiler is unable to regulate the gas supply correctly, which is preventing it from firing up to provide your heating and hot water.
Your boiler is unable to regulate the gas supply correctly, which is preventing it from firing up to provide your heating and hot water.
Parts commonly replaced
- Gas Valve
- Wiring Harness
- Gas Valve Solenoid
E80-451 Your boiler has detected an issue with its internal gas supply system which is preventing it from heating your home safely.
Your boiler has detected an issue with its internal gas supply system which is preventing it from heating your home safely.
Parts commonly replaced
- Gas Valve
- Wiring Harness
- PCB
F
FAIL POINT F - Is the gas valve mode switch in the normal position / Is the boiler in a very cold environment (less than 5°C)?
G
FAIL POINT G - Is the boiler fully water pressurised and is all air vented?
H
FAIL POINT H - Is the gas supply connected and at the correct inlet and outlet pressures?
Showing 21–26 of 26
I
FAIL POINT I - Is the multiway connector at board position ST16 pushed fully home on to the board?
J
FAIL POINT J - POWER ON, CH DEMAND, HW DEMAND, BURNER ON (Green/Red/Red/Red flashing)
K
Other faults not covered by above FAIL POINT K
taehrevO yramirP
Primary Overheat
tluaF rosneS
Sensor Fault
tuokcoL renruB
Burner Lockout
Status & diagnostic codes 1 operating states & engineer diagnostics
Not faults — these are normal operating states. Tap to expand.
1 operating states & engineer diagnostics
Not faults — these are normal operating states. Tap to expand.
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