4C Boiler Fault Code: What It Means on Every Brand

The 4C fault code appears on 2 brands — Buderus 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: Lockout — engineer likely 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 4C means	Severity	DIY?
BBuderus	Fault code 4C on a Buderus boiler signals that one or both of the built-in safety temperature limiters (STBs) — covering the primary heat exchanger and the flue gas circuit — have tripped… Full guide →	Lockout — engineer likely	No
VViessmann	The 4C fault code (grouped by Viessmann alongside code 48) indicates that the boiler's control system has detected a broken cable or open circuit on the supply temperature sensor for… Full guide →	Engineer may be needed	No

4C brand by brand

Buderus 4C

Fault code 4C on a Buderus boiler signals that one or both of the built-in safety temperature limiters (STBs) — covering the primary heat exchanger and the flue gas circuit — have tripped due to temperatures exceeding safe operating limits. This is a protective overheat shutdown: the boiler blocks heat demand immediately and, if the condition persists, stops the circulation pump around three minutes later. If the fault clears quickly it may display as a steady blocking fault; if it lingers or recurs, it escalates to a flashing locking fault that requires a manual reset after the root cause has been resolved. Do not repeatedly reset without addressing the underlying problem, as doing so can mask a serious issue.

Causes, fixes & costs for Buderus 4C → · All Buderus codes

Viessmann 4C

The 4C fault code (grouped by Viessmann alongside code 48) indicates that the boiler's control system has detected a broken cable or open circuit on the supply temperature sensor for Heating Circuit 2 — the secondary heating circuit managed by a mixing or blending valve. In response, the boiler closes the mixing valve as a protective measure. An open circuit means the electrical signal from the sensor to the control board has been interrupted, either by a physical break in the wiring, a corroded or loose connector, or a failed sensor. This is the opposite of fault codes 40 and 44, which indicate a short circuit on the same sensor — if you're seeing 4C, the circuit is broken (open); if you see 40/44, the sensor wiring is shorted. Because this fault sits within the secondary heating circuit's temperature monitoring, it is an internal wiring or sensor issue that requires a qualified engineer to investigate beyond a basic reset.

Causes, fixes & costs for Viessmann 4C → · All Viessmann codes

Frequently asked questions

Is the 4C fault code the same on every boiler?

No. 4C 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 4C fault myself?

Not usually. On every brand listed, 4C 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 4C fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. 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.