Worcester Bosch C6 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch C6 fault code mean?
The C6 fault code appears on Worcester Bosch boilers when the control system detects that the flue fan is either not running, running too slowly, or operating outside its expected speed range. The fan plays a critical safety role — it draws combustion air in and pushes harmful flue gases safely out of the home. When the boiler's monitoring system can't confirm the fan is spinning within the correct parameters (typically within around 400–500 rpm of the target speed), it triggers a safety lockout and displays C6. Both heating and hot water will be lost until the fault is cleared. Two sub-codes help narrow down the specific problem: C6 215 means the boiler has detected the fan running faster than expected, while C6 216 — by far the more common variant — indicates the fan speed is too low. Knowing which sub-code your display shows gives an engineer a useful head start when diagnosing the fault.
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 or worn fan motor Common
The fan is the most frequent culprit behind C6. Over time, the motor bearings wear out, the shaft stiffens, or the motor windings develop an electrical fault. The fan may audibly struggle, make a grinding or humming noise, or simply fail to reach its target speed. A worn fan often triggers C6 216 (speed too low). Because of labour costs, engineers almost always replace the full fan assembly rather than the bearings alone.
- Blocked or restricted flue terminal Common
Debris at the outside flue terminal — fallen leaves, compacted snow or ice, a wasp nest, or even a stray object — can restrict airflow enough to prevent the fan reaching the speed the boiler expects. This is especially common in autumn and winter. The fan itself may be perfectly healthy; it simply cannot move enough air against the blockage.
- Blocked venturi tube Sometimes
Inside the fan assembly sits a small plastic venturi (a shaped throat) that channels air and links to the air pressure switch. Dust, fluff, and combustion residue can gradually clog it. When blocked, the fan spins freely but cannot draw sufficient air, causing the boiler to lock out on C6 even though the fan motor itself is fine.
- Loose or damaged fan wiring Sometimes
The fan communicates its speed to the PCB via a tacho signal wire. A loose plug, corroded connector, or chafed cable can break this signal, making the PCB think the fan has stalled when it hasn't. Wiring faults can cause intermittent C6 trips that appear random and disappear briefly after a reset.
- Faulty air pressure switch (APS) Sometimes
The air pressure switch monitors airflow by detecting the pressure difference created by the fan. If the switch's diaphragm splits, the hose connecting it becomes kinked or blocked, or the switch contacts stick, it will fail to confirm adequate airflow to the PCB — leading to a C6 lockout even if the fan and flue are perfectly clear.
- PCB failure Rare
The printed circuit board controls and monitors the fan, including interpreting the tacho signal. Water ingress, age-related component failure, or a dry solder joint on the PCB can cause it to misread or lose the fan signal entirely. PCB faults tend to produce other accompanying error codes and are less likely to cause C6 in isolation, but they cannot be ruled out — especially on older boilers.
How to fix it
- Reset the boiler once or twice DIY safe
Press and hold the reset button (often marked with a spanner icon) for around 3–5 seconds until the boiler attempts to restart. If it fires up and runs normally, the fault may have been a momentary glitch. However, if C6 returns within a short time, do not keep resetting — repeated resets without fixing the underlying cause can mask a genuine safety issue and may cause further damage.
- Check the external flue terminal for visible blockages DIY safe
Go outside and look at the flue outlet on the external wall. If you can clearly see leaves, ice, compacted snow, or other debris blocking the grille from ground level, carefully remove it by hand or with a soft brush. Only do this if you can reach it safely without a ladder. Do not poke anything deep into the flue pipe. If the terminal is at height or the blockage isn't obvious, leave this to an engineer.
- Note the full fault code shown on the display DIY safe
Check whether your boiler display shows C6 215 (fan speed too high) or C6 216 (fan speed too low). Take a photo or make a note of the exact code. This information is genuinely useful to the engineer and can save diagnostic time, as C6 216 points strongly to a worn fan or blocked venturi, while C6 215 may indicate a wiring or PCB signal issue.
- Have a Gas Safe engineer inspect and test the fan assembly Gas Safe engineer
An engineer will remove the boiler casing, access the fan, and test its motor speed, electrical connections, and tacho output against Worcester Bosch's specifications. In most cases the fan is replaced as a complete unit. This is the single most common resolution for C6, particularly where C6 216 is displayed.
- Have the engineer check the venturi, APS hose, and air pressure switch Gas Safe engineer
While the casing is off, the engineer should inspect the venturi for debris and blow it clear if needed, check the pressure switch hose for kinks or cracks, and test the air pressure switch itself. These checks are quick when the boiler is already open and can prevent a second callout if the fan replacement alone doesn't clear the fault.
- Have the engineer inspect fan wiring and connectors Gas Safe engineer
The engineer will check the tacho wire, fan plug, and associated loom for corrosion, looseness, or damage. A wiring repair or harness replacement is significantly cheaper than a fan or PCB replacement, so it's worth confirming before ordering parts.
- If all else fails, have the PCB tested or replaced Gas Safe engineer
If the fan, wiring, venturi, and APS all test correctly but C6 persists, the PCB is the remaining likely cause. A Gas Safe engineer or a specialist PCB repair service can test the board and advise whether replacement is cost-effective given the age of the boiler.
Parts you may need
- Flue fan assembly (Worcester Bosch compatible) · from £145
- Air pressure switch (APS) · from £35
- APS hose / pressure switch tube · from £12
- Venturi insert · from £25
- Fan wiring harness / tacho lead · from £20
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–£400, depending on the underlying cause.
Frequently asked questions
Can I fix a Worcester Bosch C6 fault myself?
The only safe DIY actions are a single reset and a ground-level visual check of the external flue terminal for obvious debris like leaves or ice. Everything beyond that — accessing the fan, testing the air pressure switch, inspecting wiring, or touching the PCB — involves opening the sealed boiler casing and working near gas-carrying components. This work must be carried out by a Gas Safe registered engineer. Attempting it yourself is unsafe, potentially illegal, and could void your warranty or home insurance.
What is the difference between C6 215 and C6 216 on a Worcester Bosch boiler?
C6 215 means the boiler's control system has detected the fan running faster than its target speed, which can sometimes point to a wiring or PCB signal fault. C6 216 is much more common and indicates the fan speed is too low — typically caused by a worn or failing fan motor, a blocked venturi, or a restricted flue. Telling your engineer which sub-code is displayed will help them diagnose the problem more quickly.
How much does it cost to fix a Worcester Bosch C6 fault in the UK?
Most C6 repairs fall between £150 and £400 including parts and labour. Fan replacement — the most common fix — typically costs £100–£200 for the part, plus an engineer's labour charge. A wiring or APS repair will usually cost less. PCB replacement is the expensive end of the spectrum: the board alone can cost £200–£500 or more, and for older boilers it may not be economical. If your boiler is still within its 7–12 year Worcester Bosch warranty and has a full annual service history with an accredited installer, the repair may be covered — worth checking before you pay.
Why does my Worcester Bosch keep coming back with a C6 code after I reset it?
A recurring C6 fault almost always means the underlying cause has not been resolved. Repeated resets can temporarily clear the lockout, but if the fan is worn, the venturi is blocked, or the air pressure switch is faulty, the boiler will keep tripping out — sometimes within minutes. Resetting more than two or three times without fixing the root cause is not recommended, as it can put additional strain on a struggling fan motor and delays getting the proper repair done.