Worcester Bosch F7 fault — what it means and what to do

F7 is a "false flame" fault: your Greenstar has spotted a flame signal when there shouldn't be one. Here's why you should stop resetting it, and how we'll send a Gas Safe registered engineer to fix it.

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What does the Worcester Bosch F7 fault mean?

The Worcester Bosch F7 fault means your boiler's flame-detection circuit is still reading a flame signal when the burner should be off. In plain terms, the boiler "sees" a flame that shouldn't be there, so it shuts down and locks out to keep you safe.

Worcester Bosch's own engineer literature puts it simply: "Although appliance switches off, flame still detected." Because the boiler can't confirm that combustion has truly stopped, it treats this as a safety risk and refuses to fire until an engineer has checked it.

F7 is the exact opposite of the EA fault, where the boiler tries to light and finds no flame at all. With F7 there's a flame signal when there shouldn't be one; with EA there's no flame when there should be. Both are flame-circuit lockouts at opposite ends of the same safety logic, and they share some of the same culprits.

F7 isn't a gas leak and it isn't a 999 emergency on its own. But it is a genuine safety lockout, so the right move is to stop resetting and book a Gas Safe registered engineer. Codes vary a little between Greenstar models, so check your manual for the exact wording.

F7 fault — key facts at a glance

Question	Answer
What it means	A flame signal is detected when the burner should be off (a "false flame").
Severity	Engineer-only lockout. Not a gas or CO emergency on its own — but don't keep resetting.
Can I fix it myself?	No. One reset only, plus simple external checks. Everything else needs a Gas Safe registered engineer.
Is it dangerous?	F7 itself isn't a 999 emergency. The danger is in ignoring it and resetting repeatedly. Smell gas? Call 0800 111 999.
Typical repair cost	Often around £70–£160 if it's the electrode, rising to roughly £300–£600 if it's the PCB (industry ranges, not Smart Plan prices).
What to do	Reset once. If F7 returns, stop and book a Gas Safe registered engineer.

What causes the F7 fault?

To understand F7 it helps to know how your boiler checks for a flame. A small electrode sits in the burner and passes a tiny electrical current through the flame — this is called flame rectification. When the boiler reads that current, it knows a flame is present. F7 happens when the boiler reads that signal at the wrong moment: when the gas should be off and there should be nothing burning.

Worcester Bosch's engineer service booklet gives three ranked causes, checked in order. We've put them in plain English below, alongside the real-world culprits engineers see most often.

1. A dirty, damp or worn electrode

This is the most common cause by a mile. Carbon build-up, dirt or moisture on the flame-sensing electrode can leave a phantom signal that makes the boiler think a flame is still there. Damp electrodes are especially common after cold, wet weather. A good engineer checks the electrode first — it's also the cheapest fix.

2. A flue or combustion problem

If combustion gases are lingering in the boiler's air path — Worcester's test is a CO₂ level above 0.2% in the combustion air — that residual signal can trigger F7. This points to a flue blockage, restriction or routing fault. Confirming it needs a combustion analyser, which is strictly Gas Safe engineer territory.

3. A faulty PCB (control board)

If the electrode and flue both check out, the fault is usually the PCB — the boiler's brain. A failed board, or a cracked solder joint on it, can misread the flame signal and report F7 even though nothing's wrong with the burner. This is the priciest of the three to put right.

A note on the gas valve

A gas valve that isn't sealing fully can also let a flame linger when it shouldn't, which is the thread linking F7 to its FA and FB siblings below. Like everything else here, the gas valve is engineer-only — never something to touch yourself.

F7 symptoms and likely causes

What you'll notice	Most likely cause
F7 showing with the reset button flashing; no heating or hot water.	A genuine lockout. Reset once; if it returns, it needs an engineer.
Pressing reset makes a noise for a while, then F7 comes straight back.	The underlying fault is still there — usually the electrode or PCB. Stop resetting.
F7 appeared after cold or damp weather.	Moisture on the electrode or ignition leads, or a frozen condensate pipe.
F7 keeps returning no matter what you do.	Most often a dirty/worn electrode, sometimes the PCB or gas valve. Book an engineer.
F7 alongside an FA or FB code at other times.	Points to the gas valve or flame circuit — all engineer-only (see FA and FB below).

How do I fix the F7 fault?

There's very little you can safely do yourself with F7, and that's by design — it touches the gas and flame side of the boiler, which is for a Gas Safe registered engineer only. But there are a couple of safe checks worth doing before you call out.

What you can safely do yourself

Reset the boiler once. Then, in cold weather, check whether your external condensate pipe has frozen — a blocked condensate can sometimes show up here. If it has, you can gently thaw the visible outside pipe with warm (not boiling) water, as long as it's safe to reach. That's the limit of safe DIY.

When to call a Gas Safe registered engineer

Everything else. Cleaning or replacing the electrode, testing the flame circuit, checking the gas valve, analysing combustion and any PCB work all need a Gas Safe registered engineer. It's illegal and unsafe for anyone who isn't Gas Safe registered to work on the gas side of a boiler — so if your one reset doesn't clear it, that's your cue to book one in. You can also call Worcester Bosch's customer line on 0330 123 9559.

When the simple checks don't sort it, that's where we come in. We'll send a Gas Safe registered engineer to diagnose the fault properly and put it right.

The safe F7 reset — step by step

If you want to try the one permitted reset, here's how to do it safely. One attempt only — don't keep going if F7 returns.

Make a note of the F7 code and whether the reset button is flashing.
Switch the boiler off at its fused spur or wall switch and wait about a minute.
Switch it back on and let it settle.
Press and hold the reset button for about three seconds, then release, so the boiler tries to restart.
If the boiler fires and stays running, you're back on — keep an eye on it.
If F7 comes straight back, stop. Don't reset again — book a Gas Safe registered engineer.
In cold weather, also check the external condensate pipe for ice and thaw it gently with warm (not boiling) water if it's safe to reach.

Why you should stop resetting an F7 fault

It's tempting to keep hitting reset, but with F7 that's the wrong move — and it's worth knowing why.

F7 is a safety lockout, not a glitch. The boiler has detected a flame signal it can't explain, which could mean gas behaving in a way the boiler didn't expect. Repeatedly resetting forces the boiler to keep trying to light while its flame-detection circuit is unreliable — masking a genuine hazard instead of fixing it.

It's also pointless. Resetting changes nothing about a dirty electrode, a flue fault, a leaking gas valve or a cracked board. The underlying cause is still there, so the lockout just comes back. The honest rule is simple: one reset maximum. If F7 returns, stop and book a Gas Safe registered engineer.

Is the F7 fault dangerous?

On its own, F7 is not a 999 emergency. It isn't a code that means "gas leak" or "carbon monoxide", and the National Gas Emergency line, 0800 111 999, is only for when you can smell gas or your CO alarm is going off — not for an F7 lockout.

What makes F7 worth taking seriously is that it's a safety system doing its job. The boiler has locked itself out because it can't trust its own flame reading. The safe response is to leave it locked out and get a Gas Safe registered engineer to find the cause, rather than overriding it with repeated resets.

If you ever smell gas or your CO alarm sounds — whatever code is showing — treat it as an emergency: call 0800 111 999, open windows, turn off the gas at the meter, don't touch electrical switches, and leave the property.

F7's siblings: the FA and FB gas-valve faults

F7 belongs to a small family of flame and gas-valve safety lockouts on Worcester Bosch boilers. If you've seen F7, you may come across its close cousins FA and FB, which sit right next to it. They all share the same rule: stop resetting and call a Gas Safe registered engineer.

FA — flame still detected, or a gas valve leak test failure

FA covers two closely related faults, so check the exact number against your manual. FA 306 means the boiler still detects a flame (ionisation) signal after the burner should have gone out — that's the one most like F7, often a gas valve not shutting off cleanly, moisture on the electrode, a false reading from the flame circuit or a PCB fault. FA 364 is different: it means the gas valve's EV2 internal leak test has failed (a sibling of FB 365's EV1 leak-test failure). Like F7, every version of FA is engineer-only and shouldn't be repeatedly reset.

FB — gas valve internal leak-test failure

FB (often FB 365) means the gas valve has failed its own internal sealing check — the boiler tests whether the valve seals properly and the valve hasn't passed. This is firmly a Gas Safe registered engineer job; gas valve faults are serious and should never be ignored or repeatedly reset.

The common thread across F7, FA and FB is the flame circuit, the gas valve and the PCB — and the same blunt advice: don't keep resetting, and book a Gas Safe registered engineer without delay.

How much does it cost to repair an F7 fault?

How much an F7 repair costs depends on which of the three causes is behind it, and the gap between the cheapest and dearest fix is wide. The figures below are typical industry ranges to give you a feel for it — they're not Smart Plan's prices.

If it's the electrode — the most common and the cheapest fix — you're usually looking at roughly £70–£160 all in, often under an hour's work. A flue problem depends on what's found. If it turns out to be the PCB, expect something more like £300–£600, because the control board is one of the pricier parts on a Worcester boiler.

If your boiler's getting on in years and the fault is a costly one like the PCB, it can be worth weighing up a repair against a replacement. A common rule of thumb is that once a repair creeps past about a third of the cost of a new boiler — or the boiler's past the ten-year mark — replacing starts to make more sense. A cheap electrode fix, though, is usually a no-brainer at any age.

Typical F7 repair costs (industry ranges)

Part or job	Typical industry range
Call-out and diagnostics	Around £80–£150
Flame-sensing / ignition electrode (the most likely F7 fix)	Often £70–£160 fitted
Gas valve repair or replacement	Roughly £250–£450+
PCB (control board) replacement	Around £300–£600

When F7 won't clear — how Smart Plan helps

If you've done your one reset, checked the obvious externals, and F7 is still there, the next step is a Gas Safe registered engineer. That's where we come in.

Smart Plan is a service plan, not insurance. You've got two easy options when your boiler plays up.

Need it fixed now?

Book a one-off repair and we'll send a Gas Safe registered engineer to fix it. They diagnose the fault and get your heating and hot water back on.

Want cover for next time?

Take out an ongoing Smart Plan boiler module and you're sorted when the next code appears. Parts and labour are included up to your cover limit.

Cover is modular, so you only pick what you want — don't pay for what you don't use. Boiler cover runs up to £500 per year if your boiler's under 7 years old, or up to £200 if it's older. A £95 call-out fee applies.

We've looked after over 15,000 customers, we've been trading since 2014, and we're rated on Trustpilot. When a fault code won't budge, we'll send a Gas Safe registered engineer to put it right.

Worcester Bosch F7 fault FAQs

Is the F7 fault an emergency?

No — F7 on its own isn't a 999 emergency. It's a safety lockout, not a gas leak. The National Gas Emergency line, 0800 111 999, is only for when you can smell gas or your CO alarm sounds. For F7, reset once and, if it returns, book a Gas Safe registered engineer.

Can I keep resetting an F7 fault?

No. Reset once at most. F7 is a safety lockout, so repeatedly resetting masks a genuine hazard and changes nothing about the underlying fault — the code just comes back. If F7 returns after one reset, stop and book a Gas Safe registered engineer.

What's the difference between F7 and EA?

They're opposites. F7 means the boiler detects a flame when there shouldn't be one (a false flame). EA means the boiler tries to light and finds no flame at all. Both are flame-circuit lockouts and can share causes like the electrode or PCB. You can read more on our EA / 227 fault guide.

Can I fix the F7 fault myself?

Only the basics: one reset, and checking for a frozen external condensate pipe in cold weather. Everything else — the electrode, flue, gas valve and PCB — needs a Gas Safe registered engineer. It's illegal and unsafe for a non-registered person to work on the gas side of a boiler.

What do FA and FB faults mean on a Worcester Bosch?

They're F7's siblings. FA 306 means a flame signal is still detected after the burner should have gone out, often a gas valve not sealing properly; FA 364 is the gas valve's EV2 internal leak test failing. FB (often FB 365) means the gas valve has failed its EV1 leak test. Check the exact number against your manual. All are engineer-only, and like F7 you shouldn't keep resetting them.

How much does an F7 repair cost?

It depends on the cause. If it's the electrode — the most common fix — it's often around £70–£160. If it's the PCB, expect something more like £300–£600. These are typical industry ranges, not Smart Plan prices; your engineer will quote for your boiler.

Which Worcester Bosch boilers get the F7 fault?

F7 = false flame is documented on the Greenstar CDi and CDi Classic ranges in Worcester's engineer booklets, and the same flame-signal logic runs across the wider modern Greenstar line. The exact wording can vary by model, so always check your boiler's own manual.

Is boiler cover the same as insurance?

No — it's a service plan, not insurance. You pick the cover modules you want, and when something breaks we send a Gas Safe registered engineer to fix it, with parts and labour included up to your cover limit.

F7 still showing? We'll send a Gas Safe registered engineer.

Book a one-off repair or set up an ongoing Smart Plan boiler module. A service plan, not insurance — parts and labour included up to your cover limit.