The C1 fault code appears on 3 brands — Grant, Viessmann and Worcester Bosch — 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 C1 means | Severity | DIY? |
|---|---|---|---|
G Grant | The C1 code on the Grant Aerona³ indicates a fault detected in the compressor discharge temperature sensor — the thermistor that monitors the temperature of refrigerant as it leaves the… Full guide → | Lockout — engineer likely | No |
V Viessmann | The C1 fault code on Viessmann boilers signals that the burner control unit has gone into lockout because the fan failed to reach a complete standstill at the point in the firing sequence… Full guide → | Lockout — engineer likely | No |
WB Worcester Bosch | The C1 fault code on Worcester Bosch boilers — most commonly displayed as C1 264 on Greenstar i, Si, CDi, and Greenstar 1000/2000 models — indicates that the boiler's fan has stopped… Full guide → | Lockout — engineer likely | No |
The C1 code on the Grant Aerona³ indicates a fault detected in the compressor discharge temperature sensor — the thermistor that monitors the temperature of refrigerant as it leaves the compressor. The heat pump's PCB relies on this sensor as a safety check; if the reading falls outside expected parameters (whether due to a sensor failure or a genuine high-discharge condition), the unit will trigger a protective shutdown to prevent compressor damage.
The C1 fault code on Viessmann boilers signals that the burner control unit has gone into lockout because the fan failed to reach a complete standstill at the point in the firing sequence where it should have stopped — or because the electrical connection to the fan was detected as unstable. In plain terms, the boiler's control board expects the fan to be fully idle before it proceeds with ignition. If that condition isn't met, the boiler shuts itself down as a precaution against unsafe combustion. This is a deliberate safety lockout, not a minor glitch, and the boiler will not attempt to fire again until the underlying problem has been resolved. Note that because Viessmann produces a wide range of boilers (including the Vitodens, Vitopend, and Vitowall ranges), the precise trigger for C1 can vary slightly by model — always cross-reference with your specific boiler's documentation.
Causes, fixes & costs for Viessmann C1 → · All Viessmann codes
The C1 fault code on Worcester Bosch boilers — most commonly displayed as C1 264 on Greenstar i, Si, CDi, and Greenstar 1000/2000 models — indicates that the boiler's fan has stopped operating or has lost speed during a heating or hot water cycle. The fan is responsible for drawing fresh air into the combustion chamber and expelling flue gases safely outside. When the air pressure switch detects that airflow has dropped or disappeared, it signals the PCB, which shuts the boiler down as a safety measure. Without a working fan, dangerous combustion gases could build up inside the appliance. Note: on some older Worcester models such as the R25 HE, C1 can indicate an ignition fault rather than a fan issue — always cross-reference with your boiler's model documentation if you are unsure.
Causes, fixes & costs for Worcester Bosch C1 → · All Worcester Bosch codes
No. C1 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, C1 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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