Quick answer
Vaillant ecoTEC boilers show a fault on the display as an "F" code that points to a specific problem. F22 (low water pressure) is the one most homeowners can fix themselves by topping up via the filling loop to about 1–1.5 bar.
F28 and F29 are ignition and flame faults where you can check the gas supply and reset once but no more. F75 is a pump or pressure-sensor fault with no safe DIY fix.
The rule of thumb is to reset only once — if a code returns, or you see F75, smell gas or have a leak that won't hold pressure, call a Gas Safe registered engineer.
Vaillant is one of the most widely fitted boiler brands in UK homes, and the ecoTEC range (ecoTEC plus, ecoTEC pro and ecoTEC exclusive) is known for being reliable. But like any condensing boiler, it can throw up a fault from time to time.
The good news is that Vaillant boilers tell you exactly what's wrong: a code beginning with F appears on the digital display, each one pointing to a specific problem.
This guide is a hub for the most common Vaillant faults. Some have a homeowner-safe fix you can try in a couple of minutes; others are a clear sign to call a Gas Safe registered engineer. We'll always tell you which is which.
Safety first. Your boiler burns gas, so there are strict limits on what you should touch. Bleeding radiators, topping up pressure via the filling loop, resetting once from the front panel and checking the power supply are all fine.
Anything involving the gas valve, internal pipework, the flue, the sealed heating circuit or removing the casing is a job for a Gas Safe registered engineer only — never DIY. If you smell gas, call the National Gas Emergency line on 0800 111 999 immediately.
The most common Vaillant F codes at a glance
Use the table below to find your code, see what it usually means, and jump to a detailed guide where we have one. Some Vaillant displays show the code with a dot — for example F.22 or F.75 — which means exactly the same thing as F22 or F75.
The "Safe to try?" column tells you whether there's a homeowner check worth attempting first, or whether it's an engineer-only fault from the start.
| Code | What it usually means | Safe to try? / engineer required |
|---|---|---|
| F.0 (F0) | Flow temperature sensor (NTC) interrupted or open-circuit | No — Gas Safe engineer (internal sensor) · F0 guide |
| F.1 (F1) | Return temperature sensor (NTC) interrupted or open-circuit | No — Gas Safe engineer (internal sensor) · F1 guide |
| F.13 (F13) | Cylinder / hot-water temperature sensor short-circuit | No — Gas Safe engineer (internal sensor) · F13 guide |
| F.22 (F22) | Low water pressure / dry-fire protection — too little water in the system | Yes — top up pressure (see checklist below) · F22 guide |
| F.25 (F25) | Flue-gas temperature too high (overheat safety shut-off) | No — Gas Safe engineer (flue / combustion) · F25 guide |
| F.27 (F27) | Flame detected while the burner should be off (flame-simulation fault) | No — Gas Safe engineer (gas / ionisation) · F27 guide |
| F.28 (F28) | Ignition failure on start-up — the boiler won't light | Limited — check gas & reset once · F28 guide |
| F.29 (F29) | Flame lost during operation — the boiler lights then cuts out | Limited — check gas & reset once · F29 guide |
| F.35 (F35) | Air / flue duct fault — typically a blocked or obstructed flue | No — Gas Safe engineer (flue) · F35 guide |
| F.61 (F61) | Gas valve actuation fault — the control can't drive the gas valve correctly | No — Gas Safe engineer (gas valve) · F61 guide |
| F.62 (F62) | Gas valve switch-off delay — the flame lingers after the valve should have closed | No — Gas Safe engineer (gas valve) · F62 guide |
| F.75 (F75) | Pump or pressure-sensor fault — no pressure change detected when the pump runs | No — Gas Safe engineer · F75 guide |
Code meanings above are based on Vaillant's published fault-code list for the ecoTEC range; the exact wording and the parts implicated can vary slightly by model. Always check your boiler's manual, and have a Gas Safe registered engineer confirm the diagnosis before any parts are replaced.
Two other common Vaillant problems don't always show a single code, so we've covered them separately: a boiler giving heating but no hot water, and our general guide to Vaillant boiler repair covering costs, parts and what to expect.
F22 — low water pressure
F22 is the fault most homeowners can fix themselves. Vaillant ecoTEC boilers need the heating system to hold a minimum amount of water; if the pressure drops too low, the boiler shuts down to protect itself. Check the pressure gauge on the front of the boiler — when cold it should read roughly 1 to 1.5 bar, rising towards 2 bar when the system is hot.
If it's reading below 1 bar, you can usually top it up using the filling loop. Here's the safe sequence to repressurise:
- Turn the boiler off and let it cool — you want the system cold so the pressure reading is accurate.
- Locate the filling loop: the small braided silver hose connecting the cold and central-heating pipes underneath or near the boiler, with a valve at each end.
- Open both valves slowly. You'll hear water flowing in and the gauge will start to rise.
- Watch the gauge and close the valves firmly once it reaches roughly 1.2 to 1.5 bar. Don't overfill — too much pressure can trip the relief valve.
- Turn the boiler back on and press reset once. The F22 should clear.
Our full F22 guide walks through it with more detail. Important: if the pressure keeps dropping after you top up, or you have to repressurise repeatedly, there's a leak somewhere in the system — stop topping up and call a Gas Safe registered engineer to find and fix it.
Don't keep adding fresh water, as it introduces oxygen and accelerates internal corrosion.
F28 and F29 — ignition and flame faults
F28 means the boiler failed to ignite when it tried to fire up; F29 means it lit but then lost the flame.
Both can be triggered by something simple, such as a temporary gas-supply interruption, so it's worth a couple of basic checks: make sure other gas appliances (your hob or oven) are working, that your gas supply hasn't been cut off, and that the boiler's condensate pipe isn't frozen in cold weather.
After that, you can press the reset button on the front panel once.
If the code returns straight away, stop there. Persistent F28 or F29 faults usually point to the gas valve, ignition electrodes, flue or condensate system — none of which a homeowner should touch. Book a Gas Safe registered engineer. See the F28 guide and F29 guide for the full picture.
The one-reset rule. It's fine to reset a Vaillant boiler once from the front panel. If the same fault comes back, don't keep resetting — repeatedly cycling a boiler that's locked out can mask a genuine fault and, in the case of ignition faults, isn't safe. One reset, then call an engineer.
F75 — pump and pressure-sensor faults
F75 appears when the boiler can't detect a change in water pressure when the pump starts. It usually points to a faulty pump, a stuck or sludged-up pressure sensor, or trapped air in the system.
There's no safe homeowner fix for this one — it involves the sealed heating circuit and internal components — so it's a Gas Safe engineer job. An engineer will establish whether it's the pump or the sensor at fault, as either can trigger the same code.
Our F75 guide explains the likely causes and typical repair costs.
One useful piece of background: F75 is especially associated with ecoTEC models built roughly between 2005 and 2010, which used Wilo pumps and pressure sensors that proved prone to failure.
Later boilers generally moved to Grundfos pumps, which are widely regarded as more durable — but F75 can still appear on any model if the sensor clogs with debris or magnetite (sludge). Whichever pump is fitted, diagnosis and replacement are engineer-only.
Overheat and flue codes — F25 and F35
F25 is a safety shut-off triggered when the flue-gas temperature reads too high, and F35 indicates an air/flue duct fault, typically a blocked or obstructed flue. Both involve combustion and the flue system, so there is no homeowner fix — these are Gas Safe engineer faults from the outset. Do not attempt to inspect or clear the flue yourself.
Sensor codes — F0, F1 and F13
F0 and F1 point to the flow and return temperature sensors (NTC thermistors) being interrupted or open-circuit, while F13 relates to the cylinder/hot-water sensor short-circuiting. These are internal electronic faults inside the boiler.
They aren't usually dangerous in themselves, but the fix means working inside the appliance, so they're a job for a Gas Safe registered engineer.
What you can safely check yourself
Before calling anyone out, these homeowner-safe checks resolve a surprising number of "faults":
- Power and fuse: confirm the boiler has power and the fused spur switch is on. A tripped fuse can look like a dead boiler.
- Thermostat and programmer: check the room thermostat is calling for heat and the timer/programmer is set correctly — flat thermostat batteries are a common culprit.
- Pressure: top up via the filling loop if the gauge reads below 1 bar.
- Bleed radiators: cold spots at the top of radiators mean trapped air; bleeding them can restore even heating.
- Frozen condensate pipe: in freezing weather the external white plastic condensate pipe can ice up, causing lock-out. Thawing it gently with warm (not boiling) water often clears the fault.
When to call a Gas Safe registered engineer
Call a professional if: a fault code returns after one reset; the pressure won't hold after topping up (a leak); you have ignition faults (F28/F29) that don't clear; you see F75 or any pump/sensor code; or you ever smell gas or see sooting around the boiler.
By law, only an engineer on the Gas Safe Register may work on the gas side of your boiler. You can check any engineer's registration on that site before they start.
Typical Vaillant repair costs (indicative)
If a fault needs parts, it helps to know roughly what's involved before you get a quote. The ranges below are indicative UK figures, last checked 2026, and usually include the part plus a Gas Safe engineer's labour — real prices vary by model, your location and whether it's an emergency call-out. Treat them as a sense-check on a quote, not a fixed price.
| Part / job | Codes it often relates to | Indicative cost (2026) |
|---|---|---|
| Pressure top-up / repressurise | F22 | £0 DIY; ~£30–£60 if an engineer attends |
| Pressure sensor | F22, F75 | £100–£200 |
| Circulation pump | F75 | £150–£300 |
| Fan | F28, F29 (and related) | £160–£490 |
| Gas valve | F28, F29, F61, F62 | £200–£400 |
| PCB (control board) | various electronic faults | £300–£600 (up to ~£790 on some models) |
| Heat exchanger | overheat / F25-type faults | £460–£840 |
Indicative ranges only, last checked 2026, based on published UK repair-cost guides. By law, only a Gas Safe registered engineer may replace these parts. On an older boiler, weigh a big repair (a PCB or heat exchanger) against the cost of a replacement.
How to avoid Vaillant faults — prevention & servicing
Many of the faults above are far less likely on a boiler that's looked after. A well-maintained Vaillant should last around 10 to 15 years, which is the typical lifespan the Energy Saving Trust gives for a gas boiler. A few habits go a long way:
- Book an annual service with a Gas Safe registered engineer. It keeps the boiler running safely and efficiently, catches wear before it becomes a breakdown, and is usually a condition of keeping your manufacturer's warranty valid.
- Keep the system clean. A central-heating inhibitor protects the inside of the system against rust and the sludge (magnetite) that clogs pumps and pressure sensors — a leading cause of F75. Ask your engineer to check and top up the inhibitor level.
- Fit a magnetic filter. A magnetic system filter captures magnetite before it reaches the boiler's pump and heat exchanger. It's especially worthwhile on older systems and in hard-water areas, and it's cleaned out at each service.
- Watch your pressure. A gauge that creeps down over weeks points to a slow leak worth investigating early, before it leaves you with no heating.
- Lag the condensate pipe. Insulating the external condensate pipe helps prevent the winter freeze-ups that cause ignition lock-outs.
Does boiler cover pay for Vaillant repairs?
Most boiler cover and home-emergency policies will send a Gas Safe engineer to diagnose and repair faults like these, often with parts and labour included up to a claim limit.
Cover can make sense if an out-of-warranty repair would otherwise land you with an unexpected bill — though it isn't right for everyone. We weigh it up in is boiler cover worth it? and explain the basics in what is boiler cover?.
You can also compare boiler cover from our selected panel of providers to see what's included.
It pays to compare boiler & central heating cover — prices and cover levels vary more than most people expect.
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Compare boiler coverVaillant fault FAQs
Where is the reset button on a Vaillant ecoTEC?
On most ecoTEC plus and pro models it's a clearly marked reset button on the front control panel; on some it's combined with the dial. Press it once. If the fault returns, don't keep resetting — book a Gas Safe registered engineer.
What pressure should my Vaillant boiler be at?
Roughly 1 to 1.5 bar when the system is cold, rising towards about 2 bar when it's hot. If it's below 1 bar you can top it up via the filling loop. If it keeps dropping, you likely have a leak that needs an engineer.
Can I fix a Vaillant F28 or F29 myself?
Only the basic checks — confirm your gas supply is on, other gas appliances work, the condensate pipe isn't frozen, then reset once. Ignition and flame faults usually involve the gas valve, electrodes or flue, which only a Gas Safe registered engineer should touch.
Is a Vaillant fault code dangerous?
Most lock-outs are the boiler protecting itself and aren't an immediate danger. But if you smell gas, see soot marks, or feel unwell with symptoms that ease when you leave the house, treat it as an emergency: turn the boiler off and call the National Gas Emergency line on 0800 111 999.
This article is general information, not professional or financial advice. Always follow your boiler's manual and use a Gas Safe registered engineer for any gas work. Boiler Cover UK is a comparison site and may earn commission; we show a selected panel of providers, not the whole market.