Worcester Bosch C6 Error Code Causes & Fixes

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 on Worcester boilers in UK homes, from 1930s semis in Birmingham to new-build flats in Bristol and Victorian terraces in Manchester.

Reading up on Worcester Bosch boiler features explained is useful background if you’re trying to understand how the fan and flue system fit into the wider boiler, and comparing Worcester Bosch boiler performance ratings is worth doing if repeated C6 faults have you weighing up whether repair or replacement makes more sense for an older unit.

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 means the fan speed is 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 to 500 rpm, the boiler locks out on C6 for safety.

For the wider picture beyond this specific code, Worcester Bosch boiler error codes covers the full range of Worcester lockout codes if a different fault is showing alongside or instead of C6. Since C6 sometimes gets confused with a related air pressure switch fault, Worcester Bosch EA229 error code solutions is worth a look too if resetting doesn’t clear things up.

The Real Causes Seen Every Day in British Homes

1. Blocked or restricted flue terminal outside: leaves, snow, ice, footballs, or wasp nests in the horizontal flue, the most common cause in autumn and winter.
2. Fan actually failing: bearings worn after 8 to 12 years, or the motor is seizing, very common on Greenstar i and CDi models from 2010 to 2016.
3. Venturi blocked with dust or spider webs: the little plastic throat inside the fan assembly gets clogged. The fan spins but can’t pull enough air.
4. PCB not reading fan speed correctly: a faulty hall sensor on the fan, or a dry solder joint on the board.
5. Condensate or rainwater running back into the fan: happens when the flue isn’t pitched correctly outwards, common on older installations.
6. 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:

1. Reset the boiler: press and hold the spanner or reset button for 5 seconds. If it fires up and runs perfectly, it was a temporary glitch.
2. 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.
3. 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, and 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 or 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 registered at the Gas Safe Register.

Typical 2026 Repair Costs for Worcester C6 Fault

• Clear blocked flue only: £90 to £140 call-out
• New fan assembly (most common fix): £280 to £420 fitted (parts £160 to £220)
• Venturi clean plus fan service: £180 to £250
• New main PCB (rare): £420 to £620
• Full fan, venturi, and gasket kit (common on 10-year-old boilers): £380 to £480

Many Worcester engineers now quote a fixed “C6 fan fault” price because they carry out this repair so often.

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, which reduces back-pressure.
• If your boiler is 12 or more years old and on its second fan, start planning a replacement. A new Greenstar 4000 or 8000 will be far cheaper to run anyway.

Related Fault Codes on Other Boiler Brands

C6 is specific to Worcester Bosch, but similar fan and lockout faults show up under different codes on other makes.

If you have a Biasi boiler showing a comparable fault, Biasi boiler error code troubleshooting covers the equivalent ignition and lockout codes for that brand.

Alpha owners dealing with something similar can check alpha boiler fault codes explained for the equivalent codes on that range.

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 to 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 clearing on reset unless the bearings are already worn.

My boiler is only 5 years old, why is C6 happening already?

Usually a blocked venturi (spiders love them) or a manufacturing batch issue with early 4000-series fans, both of which are still under warranty.

C6 or 227/EA, which is it?

C6 means fan speed is wrong. 227/EA means the air pressure switch isn’t closing, a different fault with similar symptoms.

Does the C6 fault affect the Greenstar 8000 Style or only older models?

It affects the full Greenstar range, including newer 8000-series models, since the fan monitoring logic is common across the platform. That said, newer fans tend to be more reliable and fail less often than the older CDi and i-series units.

Is C6 the same as a C1 or C7 fault on Worcester Bosch boilers?

No. C1 typically relates to a sensor fault, and C7 to a pump-related issue, while C6 is specifically about fan speed. It’s worth confirming exactly which code is showing, since Worcester’s C-series codes cover several distinct fault types with different causes and fixes.

Staring at C6 right now? Note down the exact model, such as Greenstar 30i, CDi Classic 34, or 4000 30kW, before calling an engineer. It helps them give an accurate quote over the phone rather than a rough estimate.