Vaillant F0 fault code: what it means and how to fix it

What does F0 mean on a Vaillant boiler?

On Vaillant ecoTEC, ecoFIT and similar models, the fault code F0 (displayed on some units as F.00) means a flow temperature sensor interruption. Vaillant's own description of F.0 is that “the flow temperature sensor has been interrupted”.

The flow sensor is a small NTC thermistor that measures the temperature of the water leaving the boiler and feeds that reading to the control board.

If the board stops receiving a valid signal from this sensor — because the reading has gone “open-circuit” — it can no longer safely control the burner.

The boiler responds by shutting down and showing F0, which is a protective boiler lockout rather than a sign of immediate danger. In practice you will normally have no heating and no hot water until the fault is cleared.

The exact wording can vary slightly between Vaillant model ranges, but across the ecoTEC and ecoFIT families F0 consistently relates to the flow (NTC) temperature sensor circuit, not to gas pressure or water pressure.

Common causes of F0

F0 almost always traces back to the sensor itself or its electrical path. Typical causes include:

• A failed flow NTC thermistor — the sensor has worn out or gone open-circuit.
• A loose, corroded or disconnected plug at the sensor or on the wiring harness.
• Damaged wiring between the sensor and the control board.
• A fault on the PCB (control board) so it cannot read the sensor correctly.
• An after-effect of a power cut or recent reset, which can occasionally surface a marginal connection.

Because every one of these items sits inside the sealed boiler casing, identifying which is to blame is a job for a qualified engineer with the correct test equipment.

What you can safely check

You should not attempt any internal repair for F0, but there are a few universal, no-risk checks worth doing before you call out an engineer:

1. Reset the boiler once. Press and hold the reset button as described in your manual, or follow our guide on how to reset your boiler. Do this only once — if F0 comes straight back, stop and book an engineer. Repeatedly resetting a boiler that keeps locking out can be dangerous and will not fix the underlying fault.
2. Check the pressure gauge. Confirm the needle sits roughly between 1 and 1.5 bar when the system is cold. See what your boiler pressure should be; if it is low, our guide on low boiler pressure explains the safe top-up via the filling loop.
3. Confirm the gas and electricity are on. Make sure no other appliance has tripped the supply and that the gas is on.

If those checks do not clear F0 — and with a genuine sensor fault they usually will not — the next step is professional diagnosis. Repressurising and thawing a frozen condensate pipe are the only hands-on tasks a homeowner should ever do; everything to do with F0 beyond that belongs to an engineer.

When to call a Gas Safe registered engineer

Call an engineer whenever F0 returns after a single reset. This is the clear boundary: anything that involves opening the casing or touching the sensor, wiring, burner, gas valve, flue or PCB is illegal and dangerous for an unregistered person to attempt. Always use a Gas Safe Register engineer.

A competent engineer will typically:

• Read the fault history and live sensor values on a diagnostic tool.
• Test the flow NTC thermistor's resistance against Vaillant's reference figures.
• Inspect and reseat the sensor plug and wiring harness.
• Replace the flow sensor, repair the wiring, or — less often — replace the PCB if the board itself is at fault.

If you ever smell gas or suspect a leak, leave the property and call the National Gas Emergency line on 0800 111 999.

Typical Vaillant F0 repair cost

F0 is usually an affordable repair because the flow sensor is an inexpensive part and quick to change. Costs rise only if the wiring or PCB is involved. The figures below are indicative 2026 UK ranges — get a firm quote before any work.

Work	Indicative 2026 cost
Diagnostic / call-out visit	£70–£120
Flow (NTC) temperature sensor replacement	£90–£200
Wiring harness / connector repair	£100–£220
PCB replacement (worst case)	£300–£500+

Most F0 jobs land at the lower end — commonly £90–£300 all in. For wider context on pricing, see our boiler repair costs guide.

Related Vaillant codes

If your boiler shows other codes alongside or instead of F0, these guides may help:

• Vaillant F22 — low water pressure.
• Vaillant F28 — ignition failure.
• Vaillant F75 — pump or pressure sensor fault.

You can also browse the full Vaillant fault codes hub.

Will boiler cover pay for an F0 repair?

Often, yes — a flow sensor failure is a standard boiler breakdown, which is the kind of fault boiler cover is designed for.

A policy that includes parts and labour would normally cover the diagnosis and the sensor replacement, subject to any excess and the usual exclusions (such as a boiler beyond a maximum age or one that failed an initial inspection).

Always check your own policy wording, as cover varies between providers.

To understand what a typical policy includes, read what boiler cover includes, and weigh up the economics in is boiler cover worth it. This is general information, not financial advice.

As an inexpensive one-off repair, F0 on its own may cost less than a year's premium — cover tends to pay off against larger or repeated faults rather than a single sensor change.

This article is general information, not gas-safety or financial advice. Always have gas appliances checked and repaired by a Gas Safe registered engineer. In a gas emergency, call the National Gas Emergency line on 0800 111 999. Costs are indicative UK guides for 2026.