A C6 code on your Worcester Bosch boiler points to a problem with the fan. The boiler flashes the code on its display panel the moment it spots the fault. Here’s what the code means, what causes it, and what an engineer does to put it right.
What the C6 error code means
A C6 fault tells you something is wrong with the boiler fan. The fan clears toxic flue gases out of your home, so it’s a key safety part. The boiler watches the fan through an air pressure switch. If the fan isn’t doing its job, the boiler shuts down rather than risk it.
You may also see a related code:
- 215 — the fan speed is too low.
- 216 — the fan speed is too high.
If the fan stops and won’t restart, you might see a C7 or 214 instead. Both point to a fan speed problem. Junkers boilers use the same code system, so a C6 there means the same thing.
The C6 is one of many — you can check the other Worcester Bosch fault codes if your display is showing something different.
C6 error code causes and fixes
If the fan has been set up correctly and the fault still shows after a reset, there are three common causes. All three are jobs for a Gas Safe engineer, not a DIY fix.
1. Broken fan
An engineer first checks whether the fan can spin freely and reach the correct speed during start-up. They will look for worn bearings, a stiff shaft, debris around the fan, water damage, or a fan that does not respond correctly when the boiler calls for heat.
If those checks confirm the fan has failed, the engineer will decide whether a replacement fan is needed and then test the boiler again after the repair. This is not a homeowner repair, because the fan is part of the boiler’s combustion and flue-safety system.
2. Faulty or loose fan wiring
Before assuming the fan itself has failed, an engineer checks the wiring and connections between the fan and the control board. They will look for loose plugs, damaged insulation, poor continuity, moisture damage, or a broken cable that could stop the fan signal being read properly.
If the wiring is the issue, the engineer will repair or replace the affected connection and confirm the boiler is receiving the correct fan-speed signal. This avoids replacing a working fan unnecessarily.
3. PCB fault
If the fan and wiring test correctly, the engineer checks whether the PCB is sending and receiving the correct signals. They may look for water damage, corrosion, dry joints, burnt connections, or a board fault that stops the boiler communicating with the fan.
A PCB is not usually changed as a first step. The engineer confirms the fault by testing the fan circuit, checking the boiler’s service settings, and making sure the problem is not being caused by wiring or another safety component before deciding whether PCB replacement is justified.
The PCB is the boiler’s communication centre. It sends and receives signals to the fan, pump, burner, thermostat, diverter valve and gas valve. Most PCB faults come from age, water damage or a dry joint. If the board is cracked, burnt or water-damaged, repair is usually not practical and replacement may be the safest option. Only a Gas Safe engineer can diagnose this accurately. A new PCB usually costs £200 to £500 plus fitting, so on an older boiler it’s worth weighing up a replacement instead.
How an engineer diagnoses a C6 fan fault
Serviceteam are Worcester Bosch accredited installers, so your boiler repair is in safe hands. A failed fan is uncommon on boilers under ten years old, but it happens. The engineer checks the wiring and electronics running to and from the fan.
On an older fan, a worn shaft or worn bearings can make it stiff. It can’t spin properly or build the speed needed to clear flue gases. The bearings can be replaced on their own, but the labour usually makes a new fan the better call.
Why the boiler locks out at low fan speed (C6 215)
The fan is the first thing to start up. If it isn’t running as it should, the boiler locks out, because a slow fan is a safety risk. With a C6 215, the fan is turning but too slowly. It shifts some flue gases but not all of them, so the boiler shuts down to protect everyone in the property.
Is there a DIY fix for a C6 fault?
No. We wouldn’t recommend it, even for a confident DIYer. You’re dealing with gas and toxic fumes, so a C6 fault is a job for a qualified engineer. Trying to fix it without gas safety qualifications can have serious consequences.
Get your C6 fault fixed
Got a C6 fault you can’t clear? Serviceteam are WB-approved installers and we know these boilers inside out. Book a visit and we’ll diagnose the fan or PCB fault and get your heat and hot water back on.
