C6 Boiler Fault Code: What It Means on Every Brand

C6 is a fan fault on Worcester Bosch boilers (the flue fan not running or not reaching speed) and a fan / air-pressure fault on the Grant Aerona³ heat pump. On both, the fan is an engineer's component; the homeowner check is a clear flue terminal or, on the heat pump, an unobstructed outdoor unit.

Up to: Lockout — engineer likely 2 brands use this code Meaning differs by brand

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What C6 means	Severity	DIY?
GGrant	The C6 code on a Grant Aerona³ heat pump indicates that the unit has detected a fan failure or an air pressure switch fault. Full guide →	Lockout — engineer likely	No
WBWorcester Bosch	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… Full guide →	Lockout — engineer likely	No

C6 brand by brand

Grant C6

The C6 code on a Grant Aerona³ heat pump indicates that the unit has detected a fan failure or an air pressure switch fault. The air pressure switch monitors whether the outdoor fan is generating sufficient airflow; if the fan stops or slows significantly, the pressure switch trips and C6 is logged. On the Aerona³ system the code appears simultaneously on the LCD of the Aerona Remote Controller (accompanied by a warning icon and a flashing red LED) and on the LED display of the Terminal PCB inside the heat pump itself. This is considered a serious ventilation fault — without adequate airflow across the outdoor coil the heat pump cannot operate safely and will typically lock out to protect the compressor.

Causes, fixes & costs for Grant C6 → · All Grant codes

Worcester Bosch C6

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.

Causes, fixes & costs for Worcester Bosch C6 → · All Worcester Bosch codes

Frequently asked questions

Is the C6 fault code the same on every boiler?

No. C6 means different things on different brands — see the table above. Always identify your boiler's brand (it is printed on the front casing) before acting on any advice about the code.

Can I fix a C6 fault myself?

Not usually. On every brand listed, C6 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.

Who should I call about a C6 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. For heat pumps, an MCS or F-Gas qualified heat pump engineer. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.