F.04 Boiler Fault Code: What It Means on Every Brand

The F.04 fault code appears on 3 brands — Ferroli, Protherm and Vaillant — 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 F.04 means	Severity	DIY?
FFerrolishown as F04	The F04 code on a Ferroli boiler is a safety lockout that points to one of two related problems, depending on your specific model. Full guide →	Lockout — engineer likely	No
PProtherm	The F.04 code appears on Protherm boilers when the control board detects a short circuit in the domestic hot water (DHW) storage tank temperature sensor — the NTC thermistor responsible… Full guide →	Engineer may be needed	No
VVaillant	The F.04 fault code on Vaillant ecoTEC boilers signals that the control board has lost a valid reading from the heating flow NTC thermistor — the small temperature sensor that monitors the… Full guide →	Lockout — engineer likely	No

F.04 brand by brand

Ferroli F04

The F04 code on a Ferroli boiler is a safety lockout that points to one of two related problems, depending on your specific model. On older or open-flued Ferroli models, F04 signals that the flue gas thermostat has tripped — meaning the boiler detected excessive flue temperatures and shut itself down for a minimum of 20 minutes to protect the appliance. On newer, sealed-system models, F04 relates to a fault in the fan or air pressure switch circuit: the air pressure switch monitors whether the fan is generating sufficient draught to safely vent combustion gases, and if it cannot confirm this, the boiler refuses to fire. In both cases the boiler is locked out as a safety measure, and a simple reset may not resolve the underlying problem. Check your model documentation if you are unsure which definition applies, but the diagnostic steps and likely causes are broadly the same.

Causes, fixes & costs for Ferroli F04 → · All Ferroli codes

Protherm F.04

The F.04 code appears on Protherm boilers when the control board detects a short circuit in the domestic hot water (DHW) storage tank temperature sensor — the NTC thermistor responsible for monitoring hot water temperature. Rather than an open circuit (a broken wire), F.04 specifically indicates the sensor signal has collapsed in a way the board reads as a short. When this happens, the boiler typically continues to provide central heating, but hot water production is suspended until the fault is resolved. On compatible Protherm models, the letter F will flash on the display alongside the error number. Note that a broken sensor circuit alone does not trigger F.04 — the board interprets a fully open circuit as simply 'no storage tank connected', so F.04 is reserved for the short-circuit condition specifically.

Causes, fixes & costs for Protherm F.04 → · All Protherm codes

Vaillant F.04

The F.04 fault code on Vaillant ecoTEC boilers signals that the control board has lost a valid reading from the heating flow NTC thermistor — the small temperature sensor that monitors the water leaving the heat exchanger. Without a reliable signal from this sensor, the boiler cannot regulate heat output safely, so it shuts down to protect itself and the system. In practical terms the boiler is telling you it cannot 'see' how hot the flow water is, which could be because the sensor plug has come loose, the wiring between the sensor and the PCB is damaged, or the sensor itself has failed. Note: on certain older Vaillant Ecomax Pro models, NTC-related faults are grouped under codes including F.00, F.04, F.05, F.10, F.14 and F.15 — if your boiler is an Ecomax Pro, cross-check your model's manual to confirm which sensor the code refers to. On some older Vaillant ranges the similar-looking code F.4 (no leading zero, no dot) indicates an ignition fault rather than a sensor fault — if your display shows F.4 without the dot, this is a different fault requiring different diagnosis.

Causes, fixes & costs for Vaillant F.04 → · All Vaillant codes

Frequently asked questions

Is the F.04 fault code the same on every boiler?

No. F.04 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 F.04 fault myself?

Not usually. On every brand listed, F.04 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 F.04 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.