Boiler
C6
The C6 error code on a Worcester Bosch boiler indicates a fault with the fan speed — specifically that the fan is not reaching the required RPM (revolutions per minute) during operation. It’s a moderately serious fault that will likely cause your boiler to lock out and stop working, leaving you without heating or hot water. In this guide, you’ll learn exactly what triggers the C6 code, the most common causes on Worcester Bosch boilers, what you can safely do yourself, and when you need to call a Gas Safe registered engineer.
What does this error mean?
The C6 error code is Worcester Bosch’s way of telling you that the boiler’s fan is not spinning at the correct speed — or in some cases, not spinning at all. The fan (also called the flue fan or combustion fan) is a critical safety and operational component. Its job is to push waste combustion gases safely out through the flue and to draw fresh air in for burning gas. The boiler’s PCB (printed circuit board — essentially the boiler’s brain) constantly monitors the fan’s speed via a sensor called a Hall effect sensor or tachometer. If the fan speed falls outside the expected range during start-up or normal operation, the PCB triggers the C6 lockout to prevent unsafe combustion or a build-up of carbon monoxide. In short: your boiler has detected that it cannot safely manage airflow, so it has shut itself down as a precaution. This is the boiler doing its job — but the underlying fault still needs fixing.
Est. repair cost
£150–£350 all-in for a fan or sensor replacement by a Gas Safe engineer; £300–£600 if a PCB replacement is needed. Free if your boiler is within its Worcester Bosch guarantee period.
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Reset the boiler: Press and hold the reset button on your Worcester Bosch boiler (usually marked with a flame symbol or ‘Reset’) for 3–5 seconds, then release. Wait 2–3 minutes to see if the boiler fires up successfully. A one-off C6 can occasionally be caused by a temporary voltage fluctuation or brief sensor glitch, and a reset may clear it. If the error returns, move to the next step.
Step 2 — Check your boiler’s manual: Confirm the C6 code in your specific model’s manual (available free on the Worcester Bosch website at worcester-bosch.co.uk using your boiler’s serial number). Occasionally, fault code numbering varies slightly between older and newer models — verify you’re dealing with a fan speed fault before proceeding.
Step 3 — Check the external flue terminal: Go outside and visually inspect the flue outlet on the exterior wall of your home. Look for obvious blockages such as a bird nest, debris, leaves, or a damaged terminal cowl. Do NOT attempt to reach inside the flue. If you can see a blockage from a safe distance, you may be able to carefully remove it — but if the terminal is at height or requires ladders, do not attempt this yourself.
Step 4 — Listen for the fan: When you attempt a reset, stand near the boiler and listen carefully during start-up. You should hear the fan spin up (a whirring sound) before the burner ignites. If you hear nothing, or a grinding/rattling noise, this points strongly to a mechanical fan fault — stop here and call a Gas Safe engineer. Do not continue attempting resets if you suspect a mechanical fault.
Step 5 — Do not attempt further DIY: Beyond the steps above, all remaining diagnosis and repair — including accessing the fan, testing the Hall effect sensor, checking wiring, inspecting internal components, or replacing the PCB — must be carried out by a Gas Safe registered engineer. Worcester Bosch boilers are pressurised gas appliances and it is illegal and dangerous for an unqualified person to work on internal gas components. Do not remove any boiler casing panels.
📞 When to call a professional
For a C6 error code on a Worcester Bosch boiler, you should call a Gas Safe registered engineer if the boiler does not clear the fault after a single reset attempt — which will be the case for the vast majority of homeowners. This is not a fault you can diagnose or repair yourself. When you call, tell the engineer: ‘My Worcester Bosch boiler is showing a C6 error code, which I understand relates to a fan speed fault. I’ve attempted a reset and the fault has returned.’ This will help them arrive prepared with the likely parts needed — particularly a replacement fan assembly or Hall effect sensor, which are common fixes for this code. If your boiler is under the Worcester Bosch guarantee (they offer industry-leading 8–12 year guarantees on many Greenstar models), call Worcester Bosch directly on 0330 123 9339 before booking an independent engineer, as the repair may be fully covered at no cost to you.
⚖️ Repair or replace?
If your Worcester Bosch boiler is under 10 years old and has been regularly serviced, repairing a C6 fault is almost always the right choice. Worcester Bosch is the UK’s most popular boiler brand, and parts — including fan assemblies, capacitors, and PCBs — are widely available from merchants like Plumbase, Wolseley, and online suppliers. Repair is straightforward for an experienced engineer. If your boiler is over 12–15 years old, it’s worth having an honest conversation with your engineer about value for money. A replacement fan assembly for Worcester Bosch Greenstar models typically costs £80–£180 in parts alone, and labour adds £100–£200 depending on your region and the engineer’s rate. If the fault turns out to be a PCB failure, parts costs rise to £200–£400, and at that point — on an older boiler — a new Worcester Bosch Greenstar or Viessmann boiler (installed cost typically £2,000–£3,500) may make more financial sense, especially given the improved efficiency and a fresh manufacturer guarantee.