What the C6 215 code means
Every Greenstar boiler has a fan that pulls in air for combustion and pushes the flue gases out through the flue. The control board sets the fan speed to match how hard the boiler is firing, and it checks the actual speed through a feedback signal. On most Greenstar models, C6 is a fan or airflow fault, and the cause code 215 means the fan is running faster than the board expects.
On the Greenstar i, Worcester lists 215 as "fan speed too high" and its close relation, C6 216, as "fan speed too low". Some older Greenstar manuals simply group both as a fan problem. Because the boiler cannot be sure it is getting the right mix of air and gas when the fan is misbehaving, it stops and shows the code.
A fan that spins too fast is less common than one that is slow or stuck, and it very often turns out to be an electrical or control issue rather than a mechanical one.
Common causes of C6 215
Fan wiring or connector problems
Worcester's fault guidance starts with the fan harness and connector. A loose or corroded connection can corrupt the speed signal, so the board sees the wrong figure.
Faulty fan
Worn bearings or a failing motor inside the fan can make the speed unstable. Fans also wear out with age, especially in boilers that run for long hours.
Control board fault
If the board sends the wrong signal to the fan, or misreads the feedback, the result can be a fan that runs too fast. Worcester lists the control box as a possible replacement if the fan and wiring check out.
Power supply problems
Brief power interruptions or unstable supply can occasionally trigger fan faults, which is why a simple reset sometimes clears C6 215 for good.
How often it happens matters
A single C6 215 after a power cut, which clears with one reset and never returns, is rarely a sign of a failing part. A fault that comes back every few days, or appears alongside new noises from the boiler, is much more likely to be the fan or its wiring. Keeping a simple note of the date and time each time the code appears gives the engineer a clear picture and can save a second visit.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Power blip | One-off fault after a power cut | You, by resetting |
| Loose fan connector or harness | Fault comes and goes | Gas Safe engineer |
| Worn or failing fan | Whining or rattling from the boiler | Gas Safe engineer |
| Control board fault | Fan and wiring test fine | Gas Safe engineer |
Safe checks you can do yourself
- Check the fused spur and the power supply to the boiler. If there has been a power cut, note it down for the engineer.
- Check your other gas appliances work and the gas isolation valve under the boiler is open, with the handle in line with the pipe. This rules out a supply problem hiding behind the fan fault.
- Check your thermostat and timer settings so the boiler is actually being asked for heat.
- Press and hold the reset button as your manual describes. Stop after two or three attempts.
- If the boiler runs, listen for any unusual whining, rattling or grinding from inside the casing and note when it happens.
Do not remove the casing or touch the fan, its wiring or the control board. The fan is part of the combustion and flue system, and faults here can affect how safely the boiler burns gas. Make sure you have a working carbon monoxide alarm near the boiler. If you smell gas, call the National Gas Emergency Service on 0800 111 999.
When to call a Gas Safe engineer
If C6 215 comes back after two or three resets, call a Gas Safe registered engineer. This is not a fault a homeowner can fix, because every likely cause is inside the boiler.
Worcester's manuals direct engineers to check the fan harness, the connector and its connections to the control box, then replace the fan or control box if needed. After any fan work, the engineer should check combustion with a flue gas analyser to make sure the boiler is burning correctly.
When you book, give the full code, the model name and whether the fault started after a power cut, a storm or any electrical work in the house. Mention any new noises too, as a noisy fan points towards the fan itself rather than the board.
Typical repair costs in the UK
If a reset clears the fault and it does not return, there is no cost. If a part is needed, these ranges are typical. Prices vary by region, engineer and model.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Repairing a fan connector or harness | Typically £80 to £150 |
| Fan replacement | Typically £200 to £400 fitted |
| Control board (control box) replacement | Typically £250 to £450 fitted |
How to stop C6 215 coming back
Fan faults are hard to prevent completely, but a yearly service helps. An engineer can check the fan connections, listen for early bearing wear and test combustion before a small problem becomes a breakdown. Our boiler service cost guide explains what is normally included. Keep the flue terminal outside clear of plants and debris, so the fan is not working against a restriction. If your boiler instead shows a flame fault, see our pages on EA 227 and EA 229.