C7 Boiler Fault Code: What It Means on Every Brand

C7 is a fan-speed fault on Worcester Bosch and a controller-board communication fault on the Grant Aerona³ heat pump — unrelated problems that happen to share a code. The Worcester version is far more common and is engineer work; the Grant version often resolves with a reseated connector.

Up to: Stop & check now 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 C7 means	Severity	DIY?
GGrant	C7 on a Grant Aerona³ heat pump is a controller PCB serial error: the digital communication link between the unit's circuit boards — the controller board and the main inverter board… Full guide →	Stop & check now	Some checks
WBWorcester Bosch	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. Full guide →	Lockout — engineer likely	No

C7 brand by brand

Grant C7

C7 on a Grant Aerona³ heat pump is a controller PCB serial error: the digital communication link between the unit's circuit boards — the controller board and the main inverter board talking over a serial connection — has failed. Grant's documentation attributes it to miswiring or poor contact between the PCBs, with controller board replacement the fallback when the link itself is proven and the fault persists. In practice that makes C7 one of the more hopeful-looking board codes: a reseated connector or corrected wiring often restores communication without any parts at all, and the engineer works through exactly that order — connections first, board second.

Causes, fixes & costs for Grant C7 → · All Grant codes

Worcester Bosch C7

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.

Causes, fixes & costs for Worcester Bosch C7 → · All Worcester Bosch codes

Frequently asked questions

Is the C7 fault code the same on every boiler?

No. C7 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 C7 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. Where the code is a simple pressure or reset condition, that may be all it needs. Never open the boiler casing yourself.

Who should I call about a C7 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.