C4 Boiler Fault Code: What It Means on Every Brand

The C4 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.

Up to: Lockout — engineer likely 3 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 C4 means	Severity	DIY?
GGrant	The C4 fault code on the Grant Aerona³ heat pump indicates a problem with the outgoing (flow) water temperature sensor — the NTC thermistor that monitors the temperature of water leaving… Full guide →	Lockout — engineer likely	No
VViessmann	The C4 code on Viessmann boilers points to one of two underlying issues depending on your specific model. Full guide →	Engineer may be needed	No
WBWorcester Bosch	The C4 fault code indicates that Worcester Bosch's boiler control system has detected a problem with the fan or the way combustion gases are being expelled through the flue. Full guide →	Lockout — engineer likely	No

C4 brand by brand

Grant C4

The C4 fault code on the Grant Aerona³ heat pump indicates a problem with the outgoing (flow) water temperature sensor — the NTC thermistor that monitors the temperature of water leaving the heat pump. When this sensor reports a reading outside its expected range, the controller flags C4 and may lock out the unit to protect the system. The fault code appears on the Aerona³ Remote Controller display alongside a warning icon and a flashing red LED, and is simultaneously shown on the Terminal PCB LED display inside the outdoor unit.

Causes, fixes & costs for Grant C4 → · All Grant codes

Viessmann C4

The C4 code on Viessmann boilers points to one of two underlying issues depending on your specific model. On Vitodens 200-W and similar models, it signals a breakdown in communication between the boiler and an OpenTherm-connected device — such as a smart thermostat or mixer extension — causing the boiler to drop into a reduced fallback control mode. On Vitodens 200-W, 222-F, and models running Vitotronic 300, 200, 200-H, 050, or Vitocontrol-S controls, C4 typically means the outdoor temperature sensor has developed a short-circuit or cable break, so the boiler defaults to operating as though the outdoor temperature is a fixed standard value. On Vitodens 100-W models, C4 can place the burner into fault mode, sometimes linked to DIP switch S2 being set to ON. In all cases the boiler generally remains operational in a safe fallback state, but heating efficiency is compromised and the underlying fault needs attention.

Causes, fixes & costs for Viessmann C4 → · All Viessmann codes

Worcester Bosch C4

The C4 fault code indicates that Worcester Bosch's boiler control system has detected a problem with the fan or the way combustion gases are being expelled through the flue. The fan is responsible for pushing exhaust gases safely out of your home — without it working correctly, the boiler's safety controls will shut the unit down and refuse to restart. In practice, C4 covers a range of fan-related and airflow-related failures: from a physically worn-out fan motor, to a blocked flue, to wiring faults between the fan and the PCB. Because the fan is directly tied to combustion safety, this fault must be resolved before the boiler will fire again.

Causes, fixes & costs for Worcester Bosch C4 → · All Worcester Bosch codes

Frequently asked questions

Is the C4 fault code the same on every boiler?

No. C4 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 C4 fault myself?

Not usually. On every brand listed, C4 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.

Who should I call about a C4 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.