Worcester Bosch C6 Fault Code: Fan Fault Explained
The C6 code is a fan fault. It means the boiler's control board has detected that the fan is either not running, running too slowly, or running too fast for safe operation. The fan's job is to push combustion gases out through the flue, so if it isn't spinning at the correct speed the boiler shuts down (locks out) to prevent dangerous fumes from building up. You may also see a sub-code: C6 215 means fan speed too low (the most common variant), C6 216 means fan speed too high, and C6 224 is another fan-related variant. Because this fault touches on combustion and flue safety, the boiler will not relight until the underlying problem is resolved.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the Worcester Bosch C6 fault code mean?
The C6 code is a fan fault. It means the boiler's control board has detected that the fan is either not running, running too slowly, or running too fast for safe operation. The fan's job is to push combustion gases out through the flue, so if it isn't spinning at the correct speed the boiler shuts down (locks out) to prevent dangerous fumes from building up. You may also see a sub-code: C6 215 means fan speed too low (the most common variant), C6 216 means fan speed too high, and C6 224 is another fan-related variant. Because this fault touches on combustion and flue safety, the boiler will not relight until the underlying problem is resolved.
Common causes
- Worn or failed fan Common
Over time the fan's bearings or shaft wear out, causing it to spin slowly, stiffly, or stop altogether. The board then reads a fan speed that's too low (often shown as C6 215). More likely on boilers over ten years old, but it can happen sooner.
- Blocked or restricted air intake or flue Common
Debris, nesting, or obstructions in the air intake or flue restrict airflow. This can make the fan struggle and report an incorrect speed, triggering the fault.
- Wiring faults or moisture ingress Sometimes
Loose, corroded, or damaged connections between the fan and the PCB, or moisture and condensate getting into the wiring, can interrupt the fan signal and cause a C6 lockout.
- Faulty PCB (control board) Rare
The control board may fail to detect the fan's true speed or fail to power it correctly. Often caused by ageing or water damage leading to burnt connections or a cracked board. This is the worst-case and least economical cause to repair.
How to fix it
- Reset the boiler DIY safe
Press and hold the reset button as described in your boiler's user guide. A one-off glitch can sometimes clear with a reset. Do not reset more than 2-3 times - repeated lockouts mean a genuine fault that needs an engineer.
- Check for obvious external flue or intake obstructions DIY safe
From outside, visually check that the flue terminal isn't blocked by leaves, debris, or nesting. Do not dismantle anything - just look. If you can see and safely clear loose external debris, do so, then reset once.
- Have the fan and its wiring inspected and tested Gas Safe engineer
A Gas Safe engineer will check the fan speed, the wiring and connections to the PCB, and look for moisture or corrosion. This requires opening the boiler casing and working on internal components - it is not a homeowner task.
- Replace the fan or repair wiring as needed Gas Safe engineer
If the fan is worn or has failed, or if wiring is damaged, the engineer will replace the affected part and carry out combustion and flue safety checks afterwards.
- Replace the PCB if diagnosis points to the control board Gas Safe engineer
If the fan and wiring test fine, the PCB may be at fault. The engineer will assess whether replacement is economical given the boiler's age before proceeding.
- Call a Gas Safe registered engineer Gas Safe engineer
The C6 fault relates directly to combustion and flue safety. If a reset and external intake check don't resolve it, book a Gas Safe registered engineer to diagnose and repair it safely.
Parts you may need
- Replacement fan · from £175
- Fan wiring harness / connector · from £40
- Control board (PCB) · from £350
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£500, depending on the underlying cause.
Frequently asked questions
Can I fix a Worcester Bosch C6 fault myself?
No. Beyond a one-off reset and checking the external flue terminal for obvious blockages, the C6 fault must be diagnosed by a Gas Safe registered engineer. It involves combustion and flue safety, internal wiring, and the fan or control board - all of which require qualified, legal repair.
What does the sub-code C6 215 mean?
C6 215 means the fan speed is too low - the most common variant of the C6 fault. It usually points to a worn or failing fan, a restricted air intake, or a wiring problem preventing the fan from reaching the correct speed.
How much does it cost to fix a C6 fault?
A typical fan-related repair costs roughly £150 to £400, including parts and labour. If the control board (PCB) needs replacing, costs can rise to £200-£500 or more. On an older boiler, a failed PCB may make replacing the boiler more economical than repairing it.
Is it safe to keep using my boiler with a C6 fault?
The boiler will normally lock itself out for safety, so it won't run while the fault is active. Don't keep resetting it repeatedly to force it on - the fan removes toxic combustion gases, so running with a fan fault is potentially dangerous. Get it inspected by a Gas Safe engineer.