Boiler
C7
The C7 error code on a Worcester Bosch boiler indicates a fault with the fan — specifically that the fan is not reaching the correct speed or is failing to respond as expected during start-up. This is a moderately serious fault: your boiler will lock out and stop working until the issue is resolved.
What does this error mean?
Worcester Bosch boilers use the C7 code to signal that the fan, which expels combustion gases safely out of the flue, is not operating within its required parameters. The boiler’s control board monitors fan speed via a signal from the fan’s Hall effect sensor; if that signal is absent, too slow, or erratic, the boiler locks out as a safety measure. Without a correctly functioning fan, combustion gases could back up into your home, so the boiler is right to stop.
Est. repair cost
£200–£400 for professional fan replacement (parts + labour); fan units alone cost £80–£150 as a supply-only part
🔍 Likely causes
🛠️ Step-by-step fix
Press the reset button on your Worcester Bosch boiler (usually a flame symbol with a line through it) and wait 2–3 minutes to see if the boiler fires up normally. A one-off lockout can occasionally be cleared this way.
Check your flue terminal on the outside wall — ensure it is not blocked by debris, a bird nest, snow, or overgrown vegetation. Clear any visible obstruction carefully from outside.
Listen when you attempt to restart the boiler: if you can hear the fan failing to spin up or making a grinding or rattling noise, this confirms a mechanical fan fault. Do not attempt to open the boiler casing — stop here.
If the boiler locks out again with C7 after a reset, do not keep resetting it repeatedly. Persistent lockouts indicate a genuine component fault that requires a qualified engineer.
Call a Gas Safe registered engineer and tell them the exact error code (C7) and the boiler model (e.g. Worcester Bosch Greenstar 8000, 2000, CDi). This allows them to bring the correct parts — typically a replacement fan or Hall effect sensor.
📞 When to call a professional
You should call a Gas Safe registered engineer as soon as the C7 code returns after a single reset attempt. Any work beyond a visual flue check or boiler reset on a gas appliance must legally be carried out by a Gas Safe registered engineer — this is not optional. Tell the engineer it is a Worcester Bosch C7 fan fault, and give them the full boiler model number (found on the front casing label) so they can source the correct fan unit before attending.
⚖️ Repair or replace?
Fan replacement on a Worcester Bosch boiler typically costs £200–£400 all-in, including parts and labour, depending on the model and your location in the UK. If your boiler is under 10 years old and otherwise in good condition, repair is almost always the right call. If the boiler is over 12–15 years old and has had multiple faults, a new A-rated combi boiler (typically £1,800–£3,000 installed) is likely more cost-effective in the long run given improved efficiency savings.