Worcester Bosch C6 fault code: what it means and what to do

What does C6 mean on a Worcester Bosch boiler?

On a Worcester Bosch Greenstar boiler, the C6 fault code means the fan is not running at the expected speed — it has either stopped, slowed below the safe threshold, or the boiler can no longer read its speed signal correctly.

The fan's job is to draw air into the combustion chamber and push the flue gases safely outside. The boiler constantly checks the fan is spinning at the right rate before and during firing.

If that fan reading is wrong, the boiler does the safe thing: it refuses to ignite and shows C6. This is a protective lockout, not a sign the boiler is dangerous in itself — it has spotted a problem and shut down deliberately. In practice it usually means no hot water from the boiler and no heating until the fault is fixed.

C6 appears across the Greenstar range (including the popular Greenstar i and CDi models), and the core meaning — fan speed too low or not detected — is consistent. The exact display wording or any accompanying number can vary slightly between models, so always cross-check against your boiler's own manual where you can.

Common causes of C6

A C6 fault almost always traces back to airflow or the fan itself. The usual culprits are:

• A failing or worn fan — bearings wear out, typically after several years of use, and the motor can seize or struggle to reach speed. This is one of the most common causes on older Greenstar units.
• A blocked flue terminal — leaves, debris, ice, snow or even a wasp nest in the outside flue can restrict airflow. This is more common in autumn and winter.
• A blocked venturi — the small air throat inside the fan assembly can clog with dust or cobwebs, so the fan spins but can't move enough air.
• Wiring or connection faults — loose, corroded or damaged wiring between the fan and the control board.
• A faulty fan speed sensor or PCB issue — a failed hall sensor or a fault on the main circuit board reading the signal incorrectly.
• Condensate or moisture — water tracking back into the fan or affecting sensitive components.

What you can safely check

C6 is an engineer-only fault — every likely cause sits inside the sealed combustion circuit. There is no homeowner DIY repair here, but there is one safe thing worth trying before you call out:

1. Try a single reset. A one-off glitch can occasionally clear with one reset. Follow the correct procedure in our guide on how to reset a Worcester Bosch boiler, and only reset once.

In most cases a genuine C6 comes straight back, because the cause is mechanical. If it returns — at all — do not keep resetting. Repeated resets won't fix a fan or flue fault and can mask a real problem. Stop and book an engineer.

When to call a Gas Safe registered engineer

If C6 doesn't clear after one reset, that's your trigger to call a professional. Unlike a frozen pipe or low pressure, there is nothing else a homeowner can safely do — the fix lives behind the boiler's sealed casing.

It is illegal and dangerous for an unregistered person to work on the gas supply, burner, fan, flue, sealed combustion circuit or PCB. A qualified engineer — you can confirm anyone's credentials on the Gas Safe Register — will typically:

What the engineer will check

• Inspect the flue terminal for blockages and clear them.
• Test the fan for correct speed and check its wiring and connections.
• Check the venturi and clean or replace it if blocked.
• Test the fan speed sensor and the PCB's reading of it.
• Replace the fan assembly or PCB if either has failed.

C6 is one of several Worcester lockout codes — our Worcester error codes guide explains how these protective shutdowns work, and you can read more about boiler lockout generally.

Typical Worcester Bosch C6 repair cost

Costs depend on whether it's a simple blockage or a failed part. These are indicative 2026 UK ranges, fitted, and your actual quote will vary by region and engineer:

Job	Indicative 2026 cost
Diagnostic / call-out visit	£80–£150
Flue terminal clearance	£90–£140
Venturi clean + service	£180–£250
Fan assembly replacement	£280–£450
Wiring repair	£100–£250
Main PCB replacement	£400–£620

If your boiler is more than about 10–12 years old and needs a fan and PCB, it's worth weighing repair against replacement. Our boiler repair costs guide puts these figures in context.

Related Worcester Bosch codes

If you're working through Worcester faults, these related guides may help:

• Worcester Bosch boiler faults hub — all the common codes in one place.
• Worcester boiler no hot water — when a fault leaves you with no heat.
• Worcester boiler repair — what to expect from a callout.

Will boiler cover pay for a C6 repair?

Often, yes — but it depends on your policy. Many boiler cover plans include repairs to internal parts like the fan and PCB, which is exactly what a C6 fault usually needs. The catch is that policies typically won't pay for faults that existed before you took the cover out, and many exclude boilers over a certain age or that haven't been serviced.

If you already hold cover, a C6 lockout is a textbook claim — report it and let your provider send an engineer. If you don't, see what boiler cover includes and our honest take on whether boiler cover is worth it before deciding. Always check your own policy wording. This is general information, not regulated financial or gas-safety advice.

This article is general information, not gas-safety or financial advice. Always have gas appliances checked and repaired by a Gas Safe registered engineer. In a gas emergency, call the National Gas Emergency line on 0800 111 999. Costs are indicative UK guides for 2026.