Boiler
F10
The F10 error code on a Vaillant boiler indicates a problem with the flow temperature sensor — specifically, that the sensor is either reading an abnormally low temperature, has developed a fault, or has become disconnected. It’s one of the more common Vaillant fault codes and, while it will lock out your boiler and leave you without heating or hot water, it doesn’t necessarily mean an expensive repair. In this guide, you’ll learn exactly what the F10 code means, what causes it, the one safe check you can do yourself, and when you need to call a Gas Safe registered engineer.
What does this error mean?
The F10 fault code on Vaillant boilers — including popular ranges like the ecoTEC Plus, ecoTEC Pro, and ecoFIT Pure — means the boiler’s flow temperature sensor (also called the NTC flow sensor or flow thermistor) has detected a temperature that is too low, or is sending a signal that falls outside the expected range. The flow sensor sits on the primary heat exchanger and monitors the temperature of the water leaving the boiler to be circulated around your heating system. When the boiler fires up, this sensor should quickly detect a rise in water temperature. If it doesn’t — because the sensor is faulty, has a broken or loose electrical connection, or because there genuinely isn’t enough water flowing through the system — the boiler’s safety controls step in and trigger the F10 lockout to prevent damage. In short: your boiler thinks something is wrong with the water temperature on the output side, so it shuts itself down rather than risk overheating or running dry.
Est. repair cost
£80–£180 for an NTC sensor replacement (parts + labour); £200–£350 for a pump replacement; £300–£500+ for a PCB fault — all carried out by a Gas Safe engineer
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Check your boiler pressure gauge: Look at the pressure gauge on the front of your Vaillant boiler. It should read between 1.0 and 1.5 bar when cold. If it reads below 1.0 bar, your system is low on pressure. Locate the filling loop (usually a braided silver hose beneath the boiler) and slowly open both valves until the gauge reads 1.2–1.3 bar, then close both valves. Low pressure alone can sometimes trigger an F10.
Step 2 — Reset the boiler: Once you’ve checked the pressure, press and hold the reset button on your Vaillant boiler (it’s usually marked with a flame symbol or a rotating arrow, depending on your model). Hold it for 3–5 seconds. The boiler will attempt to restart. Watch to see if it fires up normally or if the F10 code returns within a few minutes.
Step 3 — Check the boiler pressure again after reset: If the boiler fires but the pressure drops again quickly, this suggests a leak somewhere in the system — do not keep refilling without investigating, and call a Gas Safe engineer.
Step 4 — Check for obvious visible leaks: While the boiler is off, visually inspect the pipework around the boiler and any visible radiator valves for signs of drips or damp patches. Do not remove any panels or touch internal components.
Step 5 — Bleed your radiators: If you suspect air in the system, use a radiator bleed key (available for under £2 at any hardware shop) to bleed each radiator in your home, starting from the ground floor and working upwards. This releases trapped air that can disrupt water flow and confuse the temperature sensor. After bleeding, re-check and top up boiler pressure to 1.2–1.3 bar if needed, then reset the boiler again.
Step 6 — If the fault returns immediately or persists after reset, stop here: At this point, the issue is almost certainly an internal component — the flow NTC sensor, the pump, the wiring, or the PCB. These are not tasks a homeowner should attempt on a gas appliance. Do not attempt to remove the boiler casing, touch internal wiring, or replace any components yourself. Gas appliances must be serviced by a Gas Safe registered engineer.
Step 7 — Call a Gas Safe registered engineer and describe the fault clearly: Tell them you have a Vaillant boiler showing fault code F10, that you’ve checked and topped up the pressure, bled the radiators, and reset the boiler, but the fault has returned. This gives them the information they need to diagnose efficiently and may save you time and call-out cost.
📞 When to call a professional
You should call a Gas Safe registered engineer if the F10 error returns after you’ve topped up the boiler pressure and performed a reset — which, for most homeowners, will be immediately. Because the F10 relates to an internal temperature sensor, pump, or wiring fault on a gas boiler, virtually all fixes beyond a basic pressure top-up and reset fall outside what any homeowner should attempt. When you call, tell the engineer: ‘My Vaillant boiler is showing fault code F10. I’ve checked the pressure (and topped it up if needed), bled the radiators, and reset the boiler, but the fault keeps coming back.’ This helps them arrive prepared with the right parts — most likely a replacement flow NTC sensor, which is a standard stock item for Vaillant engineers. Do not attempt to open the boiler casing, disconnect any wiring, or replace any parts yourself. This is not only dangerous on a gas appliance but will also invalidate any remaining warranty and may be illegal without Gas Safe registration.
⚖️ Repair or replace?
For most Vaillant boilers showing F10, the most common fix is a replacement flow NTC temperature sensor — a relatively inexpensive repair. The sensor itself typically costs £15–£40 for a genuine Vaillant part, and a Gas Safe engineer should be able to diagnose and replace it within a single visit. If the fault is a wiring issue, it may be resolved even more cheaply. A pump replacement is more involved, typically costing £200–£350 including labour. A PCB replacement is the most expensive scenario, ranging from £300–£500+ depending on the model.
Repair vs Replace: If your Vaillant boiler is under 8 years old, repair is almost always the right call — Vaillant parts are widely available and the brand has strong engineer support across the UK. If your boiler is 10–15 years old and this isn’t its first fault, it’s worth having an honest conversation with your engineer about whether repair makes economic sense. A new Vaillant ecoTEC or ecoFIT boiler costs £1,500–£3,000 supplied and installed, and comes with a manufacturer warranty of up to 10 years if installed and registered correctly. If your boiler is over 12 years old and repairs are stacking up, replacing it — especially with a modern A-rated condensing boiler — will likely save you money on energy bills and call-out costs over the next few years.