If your Worcester Bosch boiler is flashing C6 (sometimes alternating with a blue light locked on, or showing “C6” followed by a fan symbol), your heating and hot water have just stopped. Whether you have a Greenstar i, Si, CDi Classic, 4000, 8000, or an older 28i Junior, the C6 code is Worcester’s way of saying “The fan has stopped or is running at the wrong speed”.
This is one of the most frequent winter faults we see on Worcester boilers in UK homes – from 1930s semis in Birmingham to new-build flats in Bristol and Victorian terraces in Manchester.
The good news? It’s rarely an emergency danger. The bad news? It’s almost never a DIY fix and you’ll need a Gas Safe engineer.
What Exactly Does C6 Mean on a Worcester Bosch Boiler?
C6 = Fan speed too low or incorrect
The boiler constantly monitors the fan that pushes harmful flue gases safely out of your home. If the speed drops below the expected range (usually by more than 400–500 rpm), the boiler locks out on C6 for safety.
The Real Causes We See Every Single Day in British Homes
- Blocked or restricted flue terminal outside Leaves, snow, ice, footballs, or wasp nests in the horizontal flue (most common cause in autumn/winter).
- Fan actually failing Bearings worn after 8–12 years, or the motor is seizing (very common on Greenstar i and CDi models 2010–2016).
- Venturi blocked with dust/spider webs The little plastic throat inside the fan assembly gets clogged – the fan spins but can’t pull enough air.
- PCB not reading fan speed correctly Faulty hall sensor on the fan or a dry solder joint on the board.
- Condensate or rainwater running back into the fan Happens when the flue isn’t pitched correctly outwards (common on older installations).
- Air pressure switch (APS) fault Sometimes misdiagnosed as C6 when it’s actually a related 227 or EA code.
Safe DIY Checks You’re Allowed to Do Right Now
These are the only things you can legally and safely try:
- Reset the boiler Press and hold the spanner/reset button for 5 seconds. If it fires up and runs perfectly, it was a temporary glitch.
- Go outside and check the flue terminal Clear any leaves, snow, ice, or debris from the outside grille (only if it’s easily and safely reachable from ground level – no ladders!).
- Check the boiler isn’t in a really dusty cupboard Give the front a gentle hoover if it’s filthy.
That’s it. Do NOT remove the case, do NOT try to spin the fan by hand, do NOT poke anything into the flue.
When You MUST Call a Gas Safe Engineer (Which Is Almost Always with C6)
Everything else involves removing the sealed combustion chamber cover and working near gas-carrying parts. Only a registered engineer can:
- Remove and clean/replace the fan assembly
- Clean the venturi properly
- Test and replace the PCB if needed
- Check flue pitch and seals
- Carry out combustion analysis after repair
Always verify they’re Gas Safe at www.gassaferegister.co.uk
Typical 2025 Repair Costs for Worcester C6 Fault
- Clear blocked flue only: £90–£140 call-out
- New fan assembly (most common fix): £280–£420 fitted (parts £160–£220)
- Venturi clean + fan service: £180–£250
- New main PCB (rare): £420–£620
- Full fan + venturi + gasket kit (common on 10-year-old boilers): £380–£480
Many Worcester engineers now quote a fixed “C6 fan fault” price because they do so many.
How to Stop C6 Happening Again
- Annual service – the engineer cleans the fan and venturi every year
- Keep the outside flue terminal clear (trim back bushes in autumn)
- Fit a plume management kit if your flue points towards a neighbour or patio (reduces back-pressure)
- If your boiler is 12+ years old and on its second fan, start planning a replacement – a new Greenstar 4000 or 8000 will be far cheaper to run anyway.
Frequently Asked Questions About Worcester C6 Code
Will resetting fix C6 permanently?
Only if it was a one-off blockage or glitch. If it returns within minutes or hours, the fan or venturi is failing.
Is C6 covered by Worcester warranty?
Yes – if your boiler is still in its 7–12 year warranty AND has a full annual service history by a Worcester Accredited Installer.
Why does C6 happen in high wind?
Strong gusts can stall the fan momentarily – usually clears on reset unless the bearings are already worn.
My boiler is only 5 years old – why C6 already?
Usually a blocked venturi (spiders love them) or a manufacturing batch issue with early 4000-series fans – still under warranty.
C6 or 227/EA – which is it?
C6 = fan speed wrong. 227/EA = air pressure switch not closing (different fault, similar symptoms).
Staring at C6 right now? Tell us your exact model (Greenstar 30i, CDi Classic 34, 4000 30kW etc.) and we’ll tell you the most likely fix and ballpark cost in your area.
Stay warm,
The Boilers2Go Team
With years of experience in boilers and home improvements in general, Myles Robinson is a passionate advocate for making home heating simpler, safer, and more affordable for households across the UK. With years of experience in the heating industry, Myles combines hands-on knowledge with a focus on customer-first service, helping homeowners choose the right boiler, understand their systems, and keep their heating running efficiently all year round. His articles break down complex topics into clear, practical advice you can trust.