Worcester Bosch C6 215 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch C6 215 fault code mean?
The C6 fault family on Worcester Bosch boilers relates to the flue fan, and sub-code 215 specifically indicates that the boiler's control system has detected an abnormal fan speed. In most reported cases across current models, C6 215 points to the fan running faster than it should — however, some older model generations and a minority of trade sources describe 215 as the fan running too slowly or not at all. Regardless of the direction of the fault, the core diagnosis is the same: the fan is not operating within the speed range the boiler expects. Because the fan is responsible both for supplying air to the burner and for safely expelling combustion gases through the flue, the boiler locks out immediately as a safety precaution. It will not restart until the underlying cause has been found and resolved. Other C6 sub-codes you may see on Worcester Bosch boilers include C6 211 (fan failing to reach the required speed at startup), C6 212 (fan speed lost mid-run), and C6 216 (fan speed signal absent). All of these point to the same general area — fan, wiring, air pressure switch, or PCB — and the diagnostic approach is broadly the same.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty fan motor or worn impeller Common
The fan motor is the most frequent cause of a C6 215 lockout. After 8–12 years of use, motor bearings wear and the impeller can seize or become loose on the shaft. A worn motor may spin erratically — sometimes faster than expected, sometimes too slowly — and the boiler's speed sensor picks up the irregular signal. This is particularly common on Greenstar i and CDi models manufactured between 2010 and 2016.
- Loose or damaged fan wiring Common
The fan communicates its speed back to the PCB via a feedback wire. If a connector has worked loose, a cable has chafed against the casing, or a terminal has corroded, the PCB either receives an incorrect speed reading or no signal at all. This can produce a C6 215 code even when the fan motor itself is perfectly healthy, making wiring checks an early priority during diagnosis.
- Blocked or restricted flue Common
Leaves, bird nests, ice, or accumulated debris at the flue terminal can restrict the airflow path. With the outlet partially blocked, the fan has to work harder, its speed behaviour changes, and the boiler detects a fault. A blockage at the external terminal is worth checking visually from ground level before calling an engineer.
- Blocked venturi or air intake Sometimes
Inside the fan assembly sits a small plastic venturi — a shaped throat that controls airflow into the combustion chamber. Dust, spider webs, or fine debris can gradually clog this component. The fan motor spins normally but cannot pull sufficient air through, altering the effective speed profile that the boiler monitors.
- Air pressure switch fault Sometimes
The air pressure switch monitors the pressure differential created by the fan. If the switch itself is faulty, its sensing tube is kinked or blocked, or the diaphragm inside has split, it can send incorrect signals to the PCB that mimic a fan speed problem. A Gas Safe engineer can test the switch independently of the fan to rule this out.
- PCB fault Rare
The printed circuit board both sends the drive signal to the fan and reads the speed feedback. If the PCB develops a fault — due to age, moisture ingress, or electrical surge — it may misinterpret normal fan operation as a speed error, or it may fail to drive the fan to the correct speed in the first place. PCB faults are less common but more expensive to resolve, and are usually diagnosed after all other causes have been ruled out.
How to fix it
- Check your gas supply is working DIY safe
Before anything else, confirm that other gas appliances in your home — a gas hob, for example — are working normally. If you have no gas supply, contact your gas supplier rather than investigating the boiler further.
- Inspect the external flue terminal for obvious blockages DIY safe
Go outside and look at the point where the flue exits the building. If you can see leaves, a bird nest, ice, or debris covering the grille and you can reach it safely from ground level without using a ladder, carefully clear the obstruction. Do not attempt to access the flue pipe itself or remove any covers.
- Check whether the condensate pipe is frozen (winter months) DIY safe
If the weather is freezing, locate the white plastic condensate pipe that exits the boiler and runs outside. Pour warm (not boiling) water over any frozen section to thaw it, or wrap it with a warm cloth. A frozen condensate can create back-pressure conditions that contribute to fan-related faults.
- Attempt a single boiler reset DIY safe
Press and hold the reset button (often marked with a spanner icon or flame symbol) for approximately 5 seconds. Allow the boiler a full minute to attempt ignition. If it starts and runs normally, monitor it over the next few hours. If C6 215 returns, do not reset more than two or three times in total — repeated resets will not cure an internal component fault and can mask the problem from a diagnosing engineer.
- Call a Gas Safe registered engineer for full diagnosis Gas Safe engineer
If the fault code returns after resetting, a qualified engineer is needed. They will inspect the fan harness and all connectors for looseness or corrosion, test voltage at the fan motor terminals with a multimeter, check the impeller spins freely by hand, verify the air pressure switch and its sensing tube, and assess PCB output signals. Based on their findings they will repair or replace the faulty component — most commonly the fan assembly or wiring. The majority of C6 215 faults can be diagnosed and repaired in a single visit.
Parts you may need
- Boiler fan assembly (Worcester Bosch compatible) · from £145
- Air pressure switch · from £35
- Fan wiring harness / connector set · from £25
- Venturi / air intake assembly · from £30
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£380, depending on the underlying cause.
Frequently asked questions
Is C6 215 always a fan replacement, or could it be something simpler?
Not always a fan replacement. A loose wiring connector or a partially blocked flue terminal can produce exactly the same C6 215 code as a failed motor. An experienced engineer will work through the simpler checks first — testing connections and inspecting the flue — before recommending a component swap. In some cases the fix is a £20 connector repair rather than a £150 fan. That said, if the motor bearings have worn or the impeller is damaged, a full fan assembly replacement is the most reliable long-term solution.
Can I reset a C6 215 fault myself and keep using the boiler?
You can try one or two resets, but if the fault returns each time you should stop. The boiler has locked out because it detected the fan running outside safe speed limits, and continuing to force resets does not address the root cause. Running the boiler in a compromised state could affect flue gas extraction, which is a safety concern. Book an engineer promptly rather than relying on repeated resets.
How much does it cost to replace a Worcester Bosch fan?
A genuine Worcester Bosch compatible fan assembly typically costs £100–£200 for the part. Add an engineer's labour — usually £50–£100 per hour depending on your region — and a typical fan replacement comes to around £200–£380 all in. If the fault turns out to be a wiring issue or a blocked flue rather than a failed motor, the total cost is usually lower. In rare cases where the PCB has failed and needs replacing, costs can rise to £400–£600 or more; however, PCB faults are much less common and would normally only be diagnosed after other causes have been ruled out.
What is the difference between C6 211, C6 212, C6 215, and C6 216 on a Worcester Bosch boiler?
All of these are sub-codes within the C6 fan fault family, and they all point to the same general area of the boiler. C6 211 typically means the fan failed to reach its target speed during startup; C6 212 means the fan speed was lost while the boiler was running; C6 215 indicates an abnormal fan speed — most commonly reported as running too fast, though some model generations flag it as too slow; and C6 216 means the speed feedback signal from the fan was absent entirely. The diagnostic and repair process is similar for all of them, and a Gas Safe engineer will use the specific sub-code alongside live testing to pinpoint the exact component at fault.
Will my boiler warranty cover a C6 215 repair?
It depends on your boiler's age and whether you have kept up with annual services. Worcester Bosch's standard manufacturer warranty (typically 2–10 years depending on the model and whether it was registered) may cover fan faults within the warranty period, provided the boiler has been serviced annually by a Gas Safe registered engineer and the fault is not due to external damage or installer error. If your boiler is out of warranty, a Worcester Bosch service plan or a separate boiler cover policy may help spread the cost. Always check your warranty documentation before booking a repair.