C5 Boiler Fault Code: What It Means on Every Brand

The C5 fault code appears on 2 brands — Grant and Viessmann — and it does not mean the same thing on each. The same two or three characters can signal a pressure problem on one boiler and an ignition fault on another, so identify your brand first and use the table below to jump to the right page.

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 C5 means	Severity	DIY?
GGrant	C5 on a Grant Aerona³ heat pump reports a main PCB sensor error: a sensor input on the main (inverter) circuit board itself — not one of the external clamp-on thermistors — has failed its… Full guide →	Stop & check now	Some checks
VViessmann	The C5 fault code on Viessmann boilers indicates that the boiler has entered a degraded 'control mode' because it has lost communication with the variable-speed internal circulation pump. Full guide →	Engineer may be needed	No

C5 brand by brand

Grant C5

C5 on a Grant Aerona³ heat pump reports a main PCB sensor error: a sensor input on the main (inverter) circuit board itself — not one of the external clamp-on thermistors — has failed its checks. The main board carries its own internal measurement points, including the temperature sensing that protects its power electronics, and when one of these on-board channels fails there is nothing to unplug and swap: the fault is in the board. Grant's documented remedy reflects that directly — the main PCB should be replaced. A single power-cycle to rule out a transient is worthwhile; beyond that, this is a parts-and-labour visit.

Causes, fixes & costs for Grant C5 → · All Grant codes

Viessmann C5

The C5 fault code on Viessmann boilers indicates that the boiler has entered a degraded 'control mode' because it has lost communication with the variable-speed internal circulation pump. Rather than locking out entirely, the boiler continues to run but forces the pump to operate at maximum speed as a fallback. This is most commonly associated with Vitodens 200-W models, where the control unit expects speed feedback from the pump electronics — if that signal is absent or corrupted, C5 is triggered. While the boiler may still produce heat in this state, it is working inefficiently and placing extra strain on components, so the fault should be investigated promptly. Note that C5 behaviour can vary slightly across different Viessmann model ranges, so always cross-reference with your specific boiler's documentation or speak to a Viessmann-trained installer if you are unsure.

Causes, fixes & costs for Viessmann C5 → · All Viessmann codes

Frequently asked questions

Is the C5 fault code the same on every boiler?

No. C5 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 C5 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 C5 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.