The C2 fault code appears on 4 brands — Grant, Ideal, Keston 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.
The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.
| Brand | What C2 means | Severity | DIY? |
|---|---|---|---|
G Grant | The C2 code on a Grant Aerona³ heat pump indicates a fault within the compressor protection system, most likely triggered by an out-of-range reading from the outlet (flow) water… Full guide → | Lockout — engineer likely | No |
I Ideal | The C2 fault code on an Ideal boiler points to an internal error with the Boiler Chip Card (BCC) — a small device that sits on the PCB and tells it exactly which boiler model it is fitted… Full guide → | Lockout — engineer likely | No |
K Keston | The C2 code on a Keston boiler indicates a BCC fault — a problem with the Boiler Chip Card (BCC). Full guide → | Lockout — engineer likely | No |
V Viessmann | The C2 code on a Viessmann boiler signals one of two problems depending on your model. Full guide → | Engineer may be needed | No |
The C2 code on a Grant Aerona³ heat pump indicates a fault within the compressor protection system, most likely triggered by an out-of-range reading from the outlet (flow) water temperature sensor, or by an overtemperature condition at the heat pump's water outlet. When the unit detects that the leaving water temperature has exceeded safe limits — or that the sensor monitoring it is giving an implausible signal — it shuts down automatically to protect the compressor from damage.
The C2 fault code on an Ideal boiler points to an internal error with the Boiler Chip Card (BCC) — a small device that sits on the PCB and tells it exactly which boiler model it is fitted to. Without a correctly functioning chip card, the PCB cannot configure itself properly, and the boiler will lock out as a safety measure. This is subtly different from the C0 code, which signals that the chip card activation process has not completed successfully; C2 means the card itself has developed an internal fault or is not being read correctly. A single reset attempt is always worth trying, but if the fault returns the boiler will need an engineer's attention.
The C2 code on a Keston boiler indicates a BCC fault — a problem with the Boiler Chip Card (BCC). The BCC is a small programmable card that holds your boiler's specific configuration and output settings. When the control board detects that the BCC is missing, mismatched, corrupted, or has lost its stored parameters, it throws the C2 code and shuts the boiler down into block or lockout mode. This is closely related to the C0 code (BCC Start Fault), which appears at start-up; C2 tends to occur once the boiler is already running. Because the BCC is tied directly to the PCB, any recent board work or electronic disturbance is a primary suspect. The boiler will not restart automatically — you must press the reset button once the underlying issue is resolved.
The C2 code on a Viessmann boiler signals one of two problems depending on your model. On most Viessmann boilers it means the burner has entered a fault or emergency mode because the fan failed to reach a standstill after shutdown, or because a plug-in connection to the fan or control board is unstable. On models fitted with solar control modules (such as a Vitosolic unit) or certain Vitotronic controllers, C2 instead points to a communication fault with that external module — or to a short-circuit or broken cable in the outdoor temperature sensor, causing the boiler to fall back to a fixed constant-temperature operating mode. Because the code has these two distinct meanings across the Viessmann range, the first step is identifying which scenario applies to your boiler model.
Causes, fixes & costs for Viessmann C2 → · All Viessmann codes
No. C2 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.
Not usually. On every brand listed, C2 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.
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.
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