Quick answer: The C6 fault code (often shown as C6 215 or C6 216) means your Worcester Bosch boiler has detected the fan running outside its safe speed range, either too fast or too slow. The fan controls how air and flue gases move through the combustion chamber, so the boiler locks out rather than risk burning without the right airflow. This one almost always needs a Gas Safe registered engineer.
Written by the engineering team at Edinburgh Boiler Company, Scotland’s number one boiler installer. C6 is a fault we see a spike in every autumn, when boilers come back into full use after a quiet summer.
What does C6 error code mean on a Worcester boiler?
C6 means the fan speed doesn’t match what the control board asked for. The PCB tells the fan to run at a specific rate, then checks the feedback signal coming back from it. If the actual speed sits outside the acceptable window, the boiler stops.
That check matters because everything about safe combustion depends on airflow. The fan draws air in for the burner and pushes flue gases out through the flue. Too slow, and you get incomplete combustion and gases that don’t clear properly. Too fast, and the air-to-gas ratio is wrong in the other direction. Neither is acceptable, so the boiler refuses to run.
| What C6 is | What C6 is not |
|---|---|
| A combustion airflow fault | A water pressure or circulation fault |
| A lockout on a genuine safety interlock | A software glitch to be reset away |
| Usually the fan itself or a flue restriction | Something to diagnose with the casing off |
Is C6 the same as C7?
No, and the difference tells you a lot about what’s failed. Both are fan faults, but they describe different conditions.
| Code | What the boiler sees | Most likely cause |
|---|---|---|
| C6 215 / C6 216 | Fan running, but at the wrong speed | Worn bearings, restricted flue, debris in the fan or venturi |
| C7 214 / C7 217 | Fan not running at all | Failed fan motor, no power to the fan, wiring fault |
If yours reads C6, the fan is at least turning. That leaves room for the fault to be a restriction rather than a dead component, which is often the cheaper outcome. If it reads C7, the fan has stopped completely and you’re usually looking at a replacement.
What causes a C6 fault code of a Worcester Bosch boiler?
| Cause | How common | What’s happening |
|---|---|---|
| Worn fan motor or bearings | Very common | The motor can’t hold its commanded speed as bearings dry out |
| Blocked or restricted flue terminal | Common | Leaves, nesting material, snow or debris increase resistance |
| Debris in the fan or venturi | Common | Dust, soot or insulation fragments unbalance the impeller |
| Wiring or connector fault to the fan | Occasional | Corroded plug or damaged loom disrupts the speed signal |
| Air pressure switch problems | Occasional | On older models, an APS fault presents alongside fan symptoms |
| Condensate back-pressure | Less common | A blocked trap or pipe raises resistance in the system |
| PCB fault | Less common | Board misreads the feedback signal or sends the wrong command |
Bearings are the standout. A boiler fan spins for thousands of hours a year, and Scottish heating seasons are long. Bearing wear is gradual, which is why so many C6 faults are preceded by months of a boiler that sounded slightly louder or more rattly than it used to. If you noticed a change in noise before the lockout, tell the engineer. It’s useful.
Why do Scottish homes see more flue-related C6 faults?
Three reasons, and they’re all weather and geography.
Coastal salt. Scotland has an enormous coastline relative to its size, and a lot of housing sits within a few miles of it. Salt-laden air corrodes flue terminals and eats into fan housings faster than inland air does. Properties in places like Portobello, Musselburgh, North Berwick and along the Fife and Ayrshire coasts see this routinely.
Wind and driven debris. Sustained high winds push leaves, moss and nesting material into flue terminals that would stay clear in a sheltered location. Exposed rural properties and upper-floor tenement flues both suffer.
Snow and ice. A horizontal flue terminal at low level can get partially blocked by drifted snow. It doesn’t take much restriction to move the fan outside its speed window.
Add to that the summer effect. Scottish heating seasons are long, but there’s still a quiet period. A fan with tired bearings often runs through summer at low demand without complaint, then fails within a fortnight of the heating coming back on properly in October.
What can I safely do myself?
Very little, and that’s the honest answer. Two things:
1. Reset the boiler once. Hold the reset button for three to five seconds. If it clears and stays clear, note the date, because a C6 that returns within days is a component on the way out rather than a one-off.
2. Look at the flue terminal from the ground. Find where the flue comes out of the wall or roof and check for anything obvious. Leaves packed in, a bird’s nest, ivy grown across it, drifted snow. If there’s a clear obstruction you can reach safely from standing height, clearing it is reasonable.
That’s it. Do not get on a ladder, do not go on a roof, and do not remove the boiler casing. The fan sits directly in the combustion airflow path, and anything involving it has to be signed off with combustion analysis afterwards.
When should I call a Gas Safe engineer?
Almost always, and immediately if any of these apply:
- The code returns after one reset.
- You’ve heard the boiler getting noisier over recent weeks.
- The flue terminal is blocked but you can’t reach it safely.
- There’s black staining or sooting around the flue outlet.
- Anyone in the house has been getting headaches or feeling unwell when the heating is on.
That last one is not a boiler code question. If you suspect combustion products are entering the property, leave, get fresh air, and call 0800 111 999. Make sure you have a working carbon monoxide alarm regardless.
What will the engineer do?
- Read the fault history from the boiler, which shows how often C6 has occurred and under what conditions.
- Inspect and test the fan, checking current draw, speed feedback and bearing condition by hand.
- Check the flue and air intake along their full length, including any internal sections, for blockage, sagging or joint failure.
- Inspect the venturi and burner for debris build-up.
- Test the wiring harness and connectors between the PCB and the fan.
- Run a full combustion analysis and record the readings.
The flue inspection matters more than people expect. A flue that has sagged in a loft or a joint that has separated behind a ceiling can cause fan speed faults without anything visible from outside.
How much does it cost to fix a C6 fault in Scotland?
Most C6 repairs land between £150 and £500. Guide prices for Scotland in 2026, parts fitted.
| Fix | Guide cost | Time on site | Notes |
|---|---|---|---|
| Diagnostic visit | £70–£130 | 45–90 mins | Usually deducted from the repair |
| Flue terminal clearing | £90–£180 | 30–60 mins | More if access equipment is needed |
| Flue terminal replacement | £180–£350 | 1–2 hours | Common on coastal properties |
| Fan clean and venturi clean | £140–£240 | 1–1.5 hours | Sometimes buys another season, not a permanent fix |
| Fan replacement | £280–£480 | 1.5–2.5 hours | The most common paid outcome |
| Wiring harness repair | £150–£280 | 1–2 hours | |
| Air pressure switch | £180–£280 | 1–1.5 hours | Older models only |
| PCB replacement | £350–£650 | 2–3 hours | Least common cause |
Access is the biggest variable in Scotland. A flue terminating high on a tenement gable or on a steep slate roof needs a tower or a roofer, and that can add substantially. Any quote should state clearly whether access is included.
Will my Worcester guarantee cover a C6 repair?
If the boiler is inside its guarantee period and your annual services are up to date, a failed fan is a boiler component and should be covered. Worcester Bosch guarantees run up to 10 or 12 years on Greenstar models when installed by an accredited installer and registered correctly.
A blocked flue terminal is a different matter. That’s usually treated as maintenance rather than a component failure, so expect to pay for clearing a nest or removing debris regardless of guarantee status. And a missed annual service invalidates most manufacturer guarantees, so dig out your service records before you call.
How do I stop C6 coming back?
- Book the annual service. An engineer can hear and feel a fan starting to go long before it locks out.
- Keep the flue terminal clear. Check it visually a couple of times a year, especially after autumn leaf fall and after any storm.
- Cut back vegetation growing near the flue outlet. Ivy is the usual culprit.
- Watch for terminal corrosion if you’re near the coast. Mention it at your service and get it replaced before it starts restricting flow.
- Act on new noises. A boiler that’s started humming, whining or rattling is giving you weeks of warning. Use them.
Is it worth repairing, or replacing the boiler?
Repair if the boiler is under ten years old. A fan replacement on a seven-year-old Greenstar is straightforward, good value, and buys you years.
Start comparing against a replacement quote once the boiler is past twelve, particularly if the fan quote is at the top of the range or the flue needs work at the same time. At that point you’re spending several hundred pounds on one component of an appliance where the gas valve, PCB and heat exchanger are all the same age.
The bottom line
C6 is a fan speed fault, and it sits on the combustion safety side of the boiler. Reset once, check the flue terminal from ground level, then book a Gas Safe registered engineer. Scottish coastal salt, wind-blown debris and long heating seasons mean we see this more than most of the UK, and the fault very often gives you audible warning before it locks out.
Need it looked at? Edinburgh Boiler Company covers Edinburgh, the Lothians and central Scotland with Gas Safe registered engineers, full combustion testing on every repair, and written pricing before work starts.
This guide is general advice. Refer to your specific Worcester Bosch model’s user manual for reset instructions, and never remove the boiler casing or work at height to inspect a flue. Gas work must be carried out by a Gas Safe registered engineer.
