The C3 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.
The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.
| Brand | What C3 means | Severity | DIY? |
|---|---|---|---|
G Grant | The C3 fault code on the Grant Aerona³ air source heat pump indicates that the unit has detected an abnormally high leaving-water temperature — the temperature of the water exiting the… Full guide → | Lockout — engineer likely | No |
V Viessmann | The C3 fault code on Viessmann boilers indicates a problem with the outdoor temperature sensor used for weather compensation. Full guide → | Engineer may be needed | No |
The C3 fault code on the Grant Aerona³ air source heat pump indicates that the unit has detected an abnormally high leaving-water temperature — the temperature of the water exiting the heat pump and entering your heating circuit has exceeded a safe operating threshold. As a protective measure, the heat pump shuts itself down to prevent damage to internal components. A flashing red LED on the remote controller and a warning icon on the LCD display will accompany the code.
The C3 fault code on Viessmann boilers indicates a problem with the outdoor temperature sensor used for weather compensation. Depending on your specific model, the boiler has detected either a short circuit (the sensor's resistance has collapsed and is feeding a false reading) or an open circuit (the cable or sensor element is broken and sending no signal at all). The practical effect is the same: your boiler can no longer read the external air temperature, so it falls back to operating at a fixed or assumed outdoor temperature — typically 0°C — rather than modulating its output intelligently. This means your radiators may stay cooler than expected even though the boiler appears to be running normally, and you'll lose the efficiency benefits of weather compensation entirely. On the Vitodens 100-W specifically, C3 places the burner into fault mode and may be linked to DIP switch S2 being set incorrectly. On Vitodens 200-W, Vitotronic 200, 300, 300-K, 050, 200-H, and Vitocontrol-S models, the boiler operates in constant mode due to a sensor short circuit or lead break. Always cross-reference with your model's own manual, as Viessmann use the same code across a wide range of controllers with subtly different behaviour.
Causes, fixes & costs for Viessmann C3 → · All Viessmann codes
No. C3 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, C3 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.
Tell us what you need — we'll help, and we can put you in touch with a Gas Safe registered engineer.