Worcester Bosch C7 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch C7 fault code mean?
The C7 fault code appears when the boiler's fan fails to reach the correct operating speed — or fails to start at all — during the ignition sequence. Because the fan is responsible for expelling combustion gases safely through the flue before and during firing, the boiler will refuse to light until it confirms the fan is running properly. You may also see this displayed as 'C7 214' on some models; the sub-code 214 specifically identifies a fan or airflow signal fault. Without a working fan, harmful gases could build up inside the heat exchanger, so the boiler locks out as a safety measure. Symptoms typically include the boiler failing to ignite, shutting down mid-cycle, or making a humming or rattling noise before locking out.
General guidance only — not a substitute for professional advice. 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 seized fan motor Common
The fan motor is the most common culprit. Over time, bearings wear out, the motor can seize, or the impeller becomes clogged with debris. If the fan physically cannot spin up to the required speed within the start-up window, the boiler's PCB detects the failure and locks out immediately.
- Wiring or connector harness fault Common
The cables connecting the fan to the PCB can suffer from corrosion, loose terminals, or heat damage. A broken connection means the PCB either cannot send power to the fan or cannot receive the tachometer signal confirming the fan is spinning — either way, a C7 fault results.
- Low system pressure Common
Worcester Bosch boilers monitor system pressure closely. If pressure has dropped below roughly 0.8 bar, the boiler may abort the start-up sequence before the fan completes its pre-purge cycle. Check the pressure gauge on the boiler front; it should sit between 1.0 and 1.5 bar when the system is cold.
- Blocked or obstructed flue Sometimes
A bird's nest, leaves, or other debris lodged in the flue terminal can create back-pressure that prevents the fan from moving air at the expected rate. The boiler's airflow sensing will detect this and trigger the C7 lockout. You can safely inspect the external flue terminal for obvious blockages without opening the boiler.
- Faulty PCB Sometimes
The PCB controls fan speed and interprets the feedback signal. If the board has suffered water ingress, a dry solder joint, or component failure, it may incorrectly read the fan as not running even when it is, or fail to send the start signal altogether. PCB faults tend to arise on older appliances or after a water leak inside the casing.
- Pressure or airflow sensor fault Rare
A dedicated pressure differential or airflow sensor monitors the fan's performance. If the sensor itself or its connecting tubing becomes blocked, cracked, or detached, the reading sent to the PCB will be incorrect and can trigger a C7 fault even when the fan is spinning normally.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If the needle is below 1.0 bar, top up the pressure using the filling loop — your boiler manual shows exactly where this is located. Once pressure is restored, proceed to a reset.
- Inspect the external flue terminal for obstructions DIY safe
Go outside and visually check the flue outlet (usually a white or grey plastic terminal on an external wall or roof). Look for obvious blockages such as leaves, bird nesting material, or ice in winter. Do not reach inside the flue terminal or attempt to remove an internal blockage — if the obstruction is inside the flue run, a Gas Safe engineer must clear it.
- Reset the boiler DIY safe
Once you have checked pressure and the external flue, press the reset button (refer to your model's manual for its exact location — it is often a flame symbol or a dedicated button). Hold it for around 3 seconds. Allow the boiler a full minute to attempt ignition. Only attempt a reset up to 2–3 times; repeated resets without identifying the cause will not fix the fault and may cause further damage.
- Engineer to inspect and test the fan and wiring Gas Safe engineer
A Gas Safe registered engineer will remove the boiler casing, check that the fan spins freely, measure the supply voltage at the fan terminals, and verify the tachometer feedback signal. A seized or underpowered fan will be cleaned or replaced. Damaged wiring or connectors will be repaired or renewed at the same time.
- Engineer to check the airflow or pressure differential sensor Gas Safe engineer
If the fan tests fine electrically and mechanically, the engineer will inspect the pressure differential sensor, its rubber sensing tubes, and the connections to the PCB. A cracked tube or failed sensor will be replaced to restore accurate airflow monitoring.
- Engineer to assess the PCB if other components check out Gas Safe engineer
Should the fan, wiring, and sensors all prove to be in working order, the PCB itself becomes the likely cause. The engineer will check for signs of water damage, burnt tracks, or failed components. A replacement PCB is a significant cost, so the engineer should discuss whether repair is economical given the boiler's age before proceeding.
- Call a Gas Safe registered engineer if the fault persists or returns Gas Safe engineer
If the C7 code returns after a reset or you have not been able to identify the cause through the DIY checks above, do not continue resetting the boiler. Contact a Gas Safe registered engineer — you can verify any engineer's credentials at GasSafeRegister.co.uk — to carry out a full fan and flue system inspection.
Parts you may need
- Worcester Bosch fan assembly (model-specific) · from £95
- Fan wiring harness · from £25
- Pressure differential (airflow) sensor · from £35
- PCB (printed circuit board) · from £220
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
What does C7 214 mean on a Worcester Bosch boiler?
The C7 code is the main fault category indicating the fan has not started or is not running at the correct speed. The sub-code 214 narrows this down to a fan or airflow signal fault specifically — confirming the boiler's control board has detected a problem with fan operation or the feedback it receives from the fan. Both codes point to the same set of causes and require the same diagnostic approach.
Can I fix the Worcester Bosch C7 fault code myself?
There are a few safe checks you can carry out: topping up system pressure if it has dropped below 1.0 bar, checking the external flue terminal for obvious blockages, and resetting the boiler once or twice. However, the underlying cause — a worn fan, faulty wiring, or a defective sensor — requires a Gas Safe registered engineer to diagnose and repair. Opening the boiler casing yourself is unsafe and will void any remaining warranty.
How much does it cost to fix a C7 fault on a Worcester Bosch boiler?
Most C7 repairs fall in the range of £150 to £400. A fan motor replacement — the most common fix — typically costs £180 to £300 including labour. Wiring repairs tend to be at the lower end. If the PCB turns out to be the cause, costs can climb to £350–£550 or more when parts and labour are combined. For boilers over 10–12 years old, it is worth getting the engineer's honest opinion on whether repair is economical before committing to a PCB replacement.
Why does my Worcester Bosch keep showing C7 after I reset it?
If the C7 fault returns shortly after each reset, the underlying component fault has not been resolved. Repeatedly resetting the boiler does not fix the cause and can occasionally cause additional wear. A pattern of recurring C7 lockouts almost always means the fan, its wiring, or a related sensor needs hands-on inspection by a Gas Safe engineer rather than further resets.