Worcester Bosch C6 fault — what it means and what to do

What does the Worcester Bosch C6 fault mean?

A C6 fault on a Worcester Bosch Greenstar boiler means the fan is running at the wrong speed — usually too slow — or the boiler has lost the fan's speed signal, so it locks out and won't fire. Worcester's own engineer's service booklet describes it as a fan speed fault.

The fan pushes flue gases out and draws combustion air in, so it's safety-critical. When the boiler can't confirm the fan is spinning at the right speed, it stops everything — no heating and no hot water — until the cause is found and cleared.

This isn't a job you fix yourself beyond one reset and a quick look at the outside flue. It needs a Gas Safe registered engineer. The good news: it's safe to switch the boiler off and leave it off overnight while you wait — the boiler has already shut itself down, so nothing's running unsafely.

Codes vary a little between Greenstar models, so check your manual for the exact wording. If you smell gas or your CO alarm sounds, that's a separate emergency — call the National Gas Emergency line on 0800 111 999. C6 on its own is not a gas emergency.

C6 at a glance — the key facts

What causes the C6 fault? (Is it always the fan, or something cheaper?)

Here's the reassuring bit: C6 is not always a dead fan. The boiler is reporting wrong airflow or a wrong fan-speed reading — and several things can cause that, some of them far cheaper than a new fan.

Worcester's own diagnostic ladder for a fan speed fault works through the cheap, simple causes first before condemning the fan: a loose fan cable connector, then the mains voltage, then the fan wiring, then the fan itself, and finally the control board (PCB). A good engineer follows the same order.

In the real world, the commonest trigger — especially in autumn and winter — is a blocked or restricted flue terminal: leaves, snow, ice, cobwebs, a wasp nest, even a football resting against the outside grille. The fan spins fine but can't move enough air, so the boiler reads it as too slow. That's also the cheapest fix.

Causes, roughly most to least common

A blocked or restricted flue or a clogged venturi (the plastic "throat" the fan draws through) are the usual culprits. Loose or corroded wiring at the fan connector comes next, then moisture or condensate issues around the fan. A genuinely worn-out fan or a faulty control board are real but less common — and the board is the rarest of all.

The trust hook: on a newer boiler, the fan probably isn't the problem

Worcester fans are pretty robust. Bearings typically don't start wearing out until around 8–12 years, so a true fan failure on a boiler under about 10 years old is relatively uncommon. If your Greenstar is on the younger side, suspect the cheaper causes first — a blocked flue or a loose connector — and don't let anyone condemn the fan before they've ruled those out.

C6 causes and symptoms

How do I fix the C6 fault?

There are only two things it's safe for you to try yourself. Everything beyond this is a Gas Safe registered engineer's job.

What you can safely do yourself

First, reset the boiler once. Press and hold the reset button (often marked with a spanner) for about five seconds. If the boiler fires and runs normally, it may have been a one-off glitch.

Second, if you can do it safely from ground level — no ladders, no climbing — take a look at the flue terminal outside and clear any obvious leaves, snow, ice or debris resting against it. A simple blockage there is the most common cause and the cheapest to put right.

That's it. Do not open the boiler, touch the fan, or poke at any wiring — C6 is tied directly to combustion and flue safety, and after a fan is replaced the gas valve has to be re-set up with a flue gas analyser. That's not a DIY job.

Don't keep resetting it

Reset the boiler only once. If C6 comes straight back, or clears and then returns, stop. Repeatedly hitting reset doesn't fix the underlying airflow or fan-speed problem — it just relatches the lockout and can mask a real fault. At that point, book a Gas Safe registered engineer.

When to call a Gas Safe registered engineer

Call an engineer if the reset doesn't clear it, if it keeps coming back, or if you hear a humming or droning noise from the boiler. An engineer will check the flue, venturi, wiring and fan signal before deciding whether the fan or board actually needs replacing — so you're not paying for parts you don't need. Smart Plan is a service plan, not insurance, and one of our Gas Safe registered engineers can come and fix it for you.

Is the C6 fault dangerous?

No — C6 itself is not a 999 or gas emergency. The fault is the boiler doing its job: it spotted the fan wasn't running properly and shut itself down so it can't burn gas with poor airflow. While it's locked out, it isn't running unsafely.

That means it's perfectly fine to switch the boiler off and leave it off overnight until an engineer can attend. You'll lose heating and hot water in the meantime, but that's a far better outcome than force-resetting a boiler that's trying to protect you.

One weather note: in a hard frost, don't leave the boiler fully off for days on end, because the condensate pipe can freeze. "Safe to leave off overnight while you wait" is the right framing — not "leave it off for a fortnight."

The National Gas Emergency line, 0800 111 999, is only for a smell of gas or a CO alarm — not for a C6 code. If you ever do smell gas, call them, open windows, turn off the gas at the meter, don't touch electrical switches, and leave the property.

How much does it cost to repair a C6 fault?

As a rough industry guide for 2025/26, fixing a C6 typically runs somewhere between £150 and £400 fitted, but it depends entirely on the cause — and these are typical market ranges, not Smart Plan's prices.

Clearing a blocked flue is the cheap end (often £90–£140, or nothing if you can safely reach it). A venturi clean as part of a service is usually £180–£250. A new fan assembly — the most common paid C6 repair — is often £280–£420 fitted. A full fan-and-venturi kit can run £380–£480. A replacement control board, which is rare, can be £400–£620 fitted.

Repair or replace? The judgement on a 10-year-plus boiler

A one-off fan on an otherwise healthy 10-year-old boiler is usually worth doing. But if your boiler is 12 years or older and this is its second major repair — or it's facing a £400–£620 board — it's worth thinking about replacement instead. An older boiler runs less efficiently, and parts get harder to source, so a big repair on a tired unit can be money better spent toward a new one.

Worth a quick phone call first

If your boiler might still be in warranty — or has only just come out of it — phone Worcester Bosch directly on 0330 123 9559 before you pay anyone. Homeowners have had repairs done free, or goodwill repairs just out of warranty, simply by asking. And don't take the boiler's case off yourself: a DIY case removal can void your Worcester guarantee.

C6 vs C7 — which fan fault do I have?

C6 and C7 are easy to mix up because they're both fan faults, but there's a simple way to tell them apart.

C6 means the fan is running at the wrong speed — usually too slow, or the boiler can't read its speed properly. The fan is still turning, but not how the boiler expects. C7 (often shown as C7 214) means the fan isn't running at all — it's stopped, silent and dead.

The rule of thumb: if the fan is still spinning but the speed looks wrong, you've got a C6. If the fan is completely dead, that's a C7. Both are diagnosed the same way — flue and wiring first, then the fan — and both need a Gas Safe registered engineer, but C7 more often points to a fully failed fan or a complete loss of power to it.

Worth knowing too: other codes can look similar but point to a different part, so always confirm the exact code with your engineer. On newer displays, the 3-digit number after C6 narrows down the sub-fault — for example C6 215 is fan too slow and C6 216 is fan too fast — so ask your engineer to confirm the sub-code before you authorise an expensive part.

Safe checks before you call anyone out

The only things worth trying yourself with a C6. If they don't clear it, it's an engineer's job — don't go any further.

• Reset the boiler once — hold the reset button (often a spanner symbol) for about five seconds. Just once.
• If C6 returns or comes back later, stop resetting. It won't fix the fault and can hide a real problem.
• From ground level only — no ladders — check the outside flue terminal for leaves, snow, ice or a nest, and clear anything obvious resting against it.
• Listen for a humming or droning noise from the boiler, which can point to worn fan bearings — useful to mention to your engineer.
• Switch the boiler off and leave it off overnight while you wait; it's already locked itself out, so nothing's running unsafely.
• Don't open the boiler, touch the fan, or disturb any wiring — C6 is a combustion-safety fault and that's strictly a Gas Safe registered engineer's job.
• If you ever smell gas or your CO alarm sounds, that's a separate emergency — call 0800 111 999, open windows, turn off the gas at the meter, and leave the property.

When C6 won't clear — how Smart Plan helps

If you've reset once, checked the flue, and C6 is still showing, the next step is a Gas Safe registered engineer. That's where we come in — and you've got two easy ways to sort it.

Need it fixed now?

Book a one-off repair and we'll come and fix it for you. One of our Gas Safe registered engineers diagnoses the fault properly — ruling out the cheap causes before condemning the fan — and gets your heating back on.

Want cover for next time?

Take out an ongoing Smart Plan boiler module and you're sorted when the next code appears. Parts and labour are included up to your cover limit, and cover is modular — pick only what you want, don't pay for what you don't use.

Boiler cover runs up to £500 per year if your boiler's under 7 years old, or up to £200 if it's older. A £95 call-out fee applies. We've looked after over 15,000 customers, we've been trading since 2014 as UK Boiler Company Ltd, and we're rated on Trustpilot. When a fault code won't budge, we'll come and fix it for you.

Worcester Bosch C6 fault FAQs

C6 still showing? We'll come and fix it for you.

Book a one-off repair or set up an ongoing Smart Plan boiler module. A service plan, not insurance — parts and labour included up to your cover limit, with a £95 call-out fee.

Helpful links

• All Worcester Bosch fault codes
• No heating? Book a repair
• Boiler & central heating cover
• More boiler guides on the blog
• Check an engineer on the Gas Safe Register