Worcester Bosch C6 215 Fault: Fan Speed Out of Range — Causes & What To Do

C6 215 indicates the boiler's fan is running at the wrong speed (too high or too low), so the boiler can't confirm flue gases will be safely expelled. It locks out before ignition as a safety measure — a boiler can't fire unless it's certain the flue is being cleared.

The most common causes, in order of how often we see them:

• • Faulty fan — the fan motor can fail or slow with age, so it can't reach or maintain the correct speed.
• • Loose or damaged fan lead or connector — a poor connection between the fan and the PCB interrupts the speed signal. A common and relatively simple fix.
• • PCB fan signal fault — the control board may fail to drive the fan correctly or read its speed.
• • Flue blockage or restriction — a blocked or restricted flue can affect fan performance and trigger the speed fault.

You can try one reset — occasionally a transient fan signal will clear. But fan faults don't usually self-correct, and the boiler won't fire unless it's certain the flue is being cleared. If C6 215 persists after one reset, book a Gas Safe engineer.

• • Check the pressure gauge — confirm it's in the healthy 1.0–1.5 bar range, then try one reset.
• • Listen for the fan — when the boiler tries to fire, you should hear the fan spin up before ignition. If you don't hear it, or it's noisy/scratching, the fan may have failed.
• • Check the flue terminal outside is clear of debris, nests or snow — a blocked flue can affect fan operation.
• • If C6 215 persists after one reset, the fault is the fan, its lead, or the PCB — book a Gas Safe engineer.

The fan and its wiring sit inside the boiler casing, and the flue involves gas safety. Replacing the fan, repairing its lead, or any work inside the casing is gas work that must be done by a Gas Safe Registered engineer.

• • Test the fan — confirm it runs and reaches the correct speed.
• • Inspect the fan lead and connector for damage or poor contact.
• • Test the PCB fan drive and speed-sensing signal.
• • Check the flue for blockage or restriction.
• • Replace the fan or repair the lead as needed, then re-commission with a flue check.

Our Gas Safe Registered (594547) engineers repair every Worcester Bosch model — Greenstar CDi, Si, Ri and 8000 — across Surrey, Berkshire, Buckinghamshire and West London, and carry common sensors, fans and ignition parts on the van. Most Worcester Bosch fault-code repairs are completed in a single visit, with any part quoted and agreed before fitting.