Worcester Bosch A1 fault — is it the pump, and what will it cost?

A1 (often shown as A1 281) means your Greenstar can't circulate water properly. Start with a free two-minute pressure check — and only if that doesn't hold is it likely the pump. Here's how to tell, and when to call us out.

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What does the Worcester Bosch A1 fault mean?

The Worcester Bosch A1 fault means your boiler isn't getting water moving through it properly — the circulation pump is either stuck, running dry, or running with air in the system. The boiler senses no proper flow through the heat exchanger and shuts down to protect itself, because firing up with no water moving is what cracks a heat exchanger.

On newer menu-display Greenstars the code usually shows as "A1 281". The A1 is the pump/circulation fault and the 281 is the cause code that pins it down: the pump is running dry or with air in the system, which almost always traces back to low system pressure. On simpler displays you may just see "A1".

Here's the honest split that decides everything: if your pressure is low, topping it back up may clear A1 for free in a couple of minutes. If your pressure is fine and A1 keeps coming back, you're into circulation territory — usually a stuck or worn pump — and that's the part with a real bill attached.

Codes vary a little between Greenstar models — on a heat-only CDi Classic Regular, Worcester's wording leans more towards a leak or a pump needing freeing or replacing. Check your boiler's manual for the exact wording on your model, or call Worcester Bosch on 0330 123 9559.

A1 at a glance — the key facts

Question	Answer
What the code means	Poor water circulation — the pump is stuck, running dry, or running with air in the system. Often shown as A1 281.
Severity	Mixed. Not an emergency, not a gas or carbon monoxide risk. But don't run it on repeat — a dry or seized pump can damage itself further.
Can I fix it myself?	Maybe. Checking and topping up pressure and bleeding radiators are safe homeowner jobs. Freeing or replacing the pump is not — that's a Gas Safe registered engineer.
First thing to check	Your pressure gauge. If it's below about 1 bar, top up to 1.0–1.5 bar cold and reset once. If that holds, low pressure was the whole problem.
When to call an engineer	If pressure is fine but A1 returns, if it comes back within a day or two, if you can hear the pump grinding or it's hot to the touch, or if pressure won't hold (a leak).
Typical repair cost (if it's the pump)	A fitted circulation pump replacement is commonly around £250–£450 (industry range, not our price). A power flush for sludge is often £300–£600. Freeing a jammed pump or bleeding air is cheaper.

What causes the A1 fault?

A1 is a circulation fault, so every cause comes back to one question: why can't the pump move water? Sometimes there simply isn't enough water in the system (low pressure), sometimes there's air the pump is spinning against, and sometimes the pump itself has had enough. The trick is working through them cheapest-first.

It often shows up on the first cold snap of the year — the heating fires up hard after a quiet summer, sludge that settled over the warm months gets stirred up, and a tired pump that was coping in spring finally jams. If your A1 appeared the first really cold morning, you're not alone.

Is low system pressure behind A1?

This is the number-one cause and the first thing to check. If pressure drops below about 1 bar, there isn't enough water for the pump to circulate, so it pulls air and effectively runs dry — and trips A1 (281). Top up to 1.0–1.5 bar when cold, reset once, and if it runs normally and the pressure holds, that was the whole problem. A free fix.

Could air in the system be causing A1?

Yes. Air trapped after a drain-and-refill, poor bleeding, or air drawn in through a small leak all stop the pump getting a clean prime. The tell-tale sign is radiators that are cold at the top and warm at the bottom, plus gurgling or bubbling from the boiler or pipes. Bleeding the radiators (start downstairs) and topping the pressure back up afterwards often clears it.

Is the pump seized or jammed?

If pressure is fine and there's no air, the pump itself is the usual culprit. Bearings and the shaft wear out over the years, or the impeller seizes — a lack of water circulation usually points to a seized impeller in the pump. You might hear a grinding or vibrating noise, or feel the pump body running hot. This is a Gas Safe registered engineer's job — freeing or replacing the pump, never a DIY one.

Could sludge build-up be blocking the pump?

On older systems without a magnetic filter, black iron-oxide sludge clogs the pump impeller and the heat exchanger. It's the reason a brand-new pump can fail again within months if the underlying sludge isn't dealt with. The fix is a power flush and, ideally, fitting a magnetic filter to catch it in future — both engineer jobs.

Is the flow temperature sensor causing a false A1 reading?

Less commonly, A1 is triggered by a faulty flow temperature sensor or a pressure-sensor connection rather than the pump itself — the boiler misreads the flow and assumes the pump has failed. There's no way to tell this apart from a real pump fault without testing, so it's firmly an engineer diagnosis.

Is there an underlying leak?

If your pressure keeps dropping within days of topping up, you have a leak somewhere — a radiator valve, a pipe joint, or the pressure-relief valve. Each top-up reintroduces air, so A1 keeps coming back. Repeated topping-up just masks it: the leak has to be traced and fixed by an engineer.

A1 causes and symptoms

Likely cause	What you'd notice	Who fixes it
Low system pressure	Gauge below about 1 bar; tepid hot water; heating drops out. Topping up and resetting clears it.	You — top up via the filling loop, reset once.
Air in the system	Radiators cold at the top, warm at the bottom; gurgling or bubbling noises; often after a recent drain or refill.	You — bleed the radiators, then top pressure back up.
Stuck or seized pump	Pressure is fine but A1 returns; grinding or vibrating noise; pump body hot to the touch.	Gas Safe registered engineer — free or replace the pump.
Sludge build-up	Older system, no magnetic filter; uneven radiators; A1 keeps coming back even after a new pump.	Gas Safe registered engineer — power flush and fit a filter.
Underlying leak	Pressure drops again within a day or two; damp patches, drips or pooling near valves, pipes or under the boiler.	Gas Safe registered engineer — trace and fix the leak.
Flow temperature / pressure sensor fault	A1 with no obvious circulation symptoms; pressure and flow seem normal.	Gas Safe registered engineer — test and replace the sensor.

What are the symptoms of the A1 error?

A1 rarely arrives out of the blue — there are usually warning signs in the days before the boiler locks out. These are the things people describe most often, and spotting which ones you've got helps point to the cause.

Tepid water from the hot taps, radiators that warm unevenly, and heating or hot water dropping out altogether are the classics. Gurgling or bubbling noises from the boiler or pipes, and radiators cold at the top but warm at the bottom, point towards air in the system. A grinding or vibrating noise, or a pump that feels hot to the touch, points towards a failing pump. And water pooling under the boiler, drips under radiator valves or a towel rail, or damp patches on walls all point to a leak that's driving the pressure down.

Can you fix the A1 fault on a Worcester Bosch yourself?

Some of it, yes — and it's worth trying before you pay anyone, because the cheapest cause is also the most common. The two safe homeowner jobs are checking and topping up your pressure, and bleeding your radiators. Neither involves any gas work. Everything past that is for a Gas Safe registered engineer.

The honest way to think about it: do the free checks first. If low pressure was the cause and topping up holds, you're done for nothing. If the pressure drops again within a day or two, you've got a leak. And if the pressure is fine but A1 still comes back, that's the bad-news branch — circulation or pump — and you want an engineer before repeated resets damage the pump further.

Safe to do yourself

Check the pressure gauge and top up to 1.0–1.5 bar cold via the filling loop. Bleed the radiators, starting downstairs, then re-check and re-top the pressure. Have a look around the boiler, radiator valves and pipework for damp, drips or pooling. Reset the boiler once.

Leave to a Gas Safe registered engineer

Freeing or replacing a seized pump, bleeding or priming the pump itself, a power flush and fitting a magnetic filter, tracing a hidden leak, and any sensor, gas-valve or heat-exchanger work. Tapping the pump to free it might buy you a day, but it's a bodge, not a fix — and a quick word of warning: DIY pump tapping or flushing can void your Worcester guarantee, so if your boiler is still under warranty, go through Worcester or a registered engineer.

The free A1 check — pressure and bleed, step by step

Do this with the boiler switched off and the system cold. It costs nothing and clears the most common cause of A1.

Switch the boiler off at the controls and let the system go cold. Find the pressure gauge or pressure menu so you can watch the reading.
Check the gauge. If it's sitting below about 1 bar, low pressure is almost certainly behind your A1.
Find the filling loop under the boiler — usually a silver braided hose with a valve at each end (some Greenstars use a keyed or keyless filling link instead).
Open the valves slowly and watch the gauge climb. Stop the moment it reaches about 1.0–1.5 bar when cold, then close the valves firmly and detach an external loop.
If your radiators are cold at the top, bleed them with a radiator key (start downstairs), then go back and top the pressure up again, as bleeding lets pressure drop.
Turn the boiler back on and reset it once. If A1 clears and the pressure holds, low pressure or air was the whole problem — a free fix.
If A1 comes straight back, or the pressure drops again within a day or two, stop there. Don't keep resetting — book a Gas Safe registered engineer, because it's now circulation, the pump, or a leak.

How do I reset the A1 error, and can I keep resetting it?

Reset the boiler once — no more. On most Greenstars you press and hold the reset button (often the flame or reset symbol) for a few seconds; the exact button and hold time vary by model, so check your manual. A single reset after topping up the pressure is fine.

What you must not do is keep resetting an A1 that won't clear. Every reset on a stuck or dry pump runs it again with no water moving through it, and that's exactly how you turn a pump that needed freeing into a pump that needs replacing. Worcester and engineers are blunt about this: never repeatedly reset a boiler that keeps returning the same fault code without finding the cause. If A1 returns after one reset, stop and get it looked at.

Is the A1 fault dangerous?

No. A1 is a circulation and pump fault, not a safety lockout for gas. It isn't a gas leak, it isn't a carbon monoxide risk, and it isn't a 999 emergency. The boiler shuts down to protect its own heat exchanger from running with no water moving through it.

Because of that, the National Gas Emergency line — 0800 111 999 — is not the number for A1. That line is only for a smell of gas or a carbon monoxide alarm. If you ever smell gas, leave everything else, call 0800 111 999 straight away, open the windows, turn the gas off at the meter, don't touch any electrical switches, and leave the house.

The one risk with A1 is to the boiler itself, not to you — running a dry or seized pump on repeat damages it. So it's safe to do the free pressure and bleed checks, but don't sit there resetting it over and over. If it won't clear, switch it off and book an engineer.

How much does it cost to fix the A1 fault?

If it turns out to be low pressure or trapped air, it costs you nothing — that's normal maintenance you do yourself. The cost only comes in when the pump, sludge or a leak is behind it, and that's where A1 can sting.

As a rough guide using typical UK industry ranges (not our prices, and worth confirming with a local quote): an engineer's call-out and diagnosis is often around £50–£65 an hour. Bleeding or venting air from the system is commonly £60–£120. Freeing a jammed pump with no new part is often £80–£150. A full circulation pump replacement is commonly around £250–£450 fitted — older or discontinued Worcester pumps cost more because they're harder to source, and a boiler with no isolation valves costs more again because the engineer has to drain the whole system. A power flush to clear sludge is often £300–£600, and a flow temperature sensor is usually £100–£200 fitted.

It's worth weighing that against your boiler's age. On a unit under ten years old a pump replacement is almost always worth doing. Past fifteen years, with parts getting hard to find and efficiency well below a modern boiler, a big repair bill is a fair prompt to think about replacement — the rough rule of thumb is to lean towards a new boiler once a repair tops about half the cost of replacing it. That pump-cost uncertainty is exactly where ongoing cover earns its keep — more on that next.

Pump trouble? Here's how Smart Plan helps

A1 is the fault where the gap between best case and worst case is widest. Best case, you top the pressure up and it's free. Worst case, it's a seized pump or a sludged-up system and you're looking at a few hundred pounds, fitted, on an older boiler. That uncertainty is the bit nobody enjoys.

Smart Plan is a service plan, not insurance. When the free checks run out and it's clearly the pump or circulation, you've got two easy options.

Need it fixed now?

Book a one-off repair and we'll come and fix it for you. A Gas Safe registered engineer diagnoses whether it's a stuck pump, air, sludge or a leak, and gets your heating and hot water back on — before repeated resets do any more damage to the pump.

Want cover for next time?

Take out a Smart Plan boiler module and a pump or circulation failure isn't a surprise bill to absorb. Parts and labour are included up to your cover limit, and an annual service helps catch a tired pump or sludge before they turn into a no-heat A1 lockout on the first cold morning.

Cover is modular, so you pick only 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 A1 won't clear, we'll come and fix it for you.

Worcester Bosch A1 fault FAQs

What does A1 281 mean on a Worcester Bosch boiler?

A1 is the pump/circulation fault and 281 is the cause code — together they mean the pump is running dry or with air in the system, which almost always traces back to low system pressure. Check your pressure gauge first: if it's below about 1 bar, top up to 1.0–1.5 bar cold and reset once.

Is the A1 fault an emergency?

No. A1 is a circulation and pump fault, not a gas or carbon monoxide risk and not a 999 emergency. The boiler shuts down to protect its heat exchanger. The National Gas Emergency line, 0800 111 999, is only for a smell of gas or a CO alarm — not for A1.

Can I fix the A1 fault myself?

The free checks, yes — topping the pressure up to 1.0–1.5 bar cold and bleeding the radiators are safe homeowner jobs with no gas work. But if the pressure is fine and A1 keeps returning, it's the pump, sludge or a leak, and that needs a Gas Safe registered engineer. Never attempt pump or gas work yourself.

Can I keep resetting the A1 fault?

No. Reset only once, and only after topping up the pressure. Repeatedly resetting an A1 runs a stuck or dry pump over and over, which can turn a pump that just needed freeing into one that needs replacing. If A1 comes back after one reset, stop and book a Gas Safe registered engineer.

How much does it cost to replace the pump on a Worcester Bosch?

A fitted circulation pump replacement is commonly around £250–£450 as a typical UK industry range (not our price), and older or discontinued Worcester pumps cost more because they're harder to source. Freeing a jammed pump without a new part is cheaper, often £80–£150. Always get a local quote, and weigh a big repair against the boiler's age.

How do I know if it's the pump or just air in the system?

Check the pressure first. If it's low, top up and reset — air or low pressure is likely, and bleeding the radiators usually clears it. If the pressure is fine but A1 returns, listen and feel: a grinding or vibrating noise, or a pump body that's hot to the touch, points to a failing pump. Either way, if topping up doesn't hold it, book a Gas Safe registered engineer to confirm.

Would a boiler service have prevented the A1 fault?

Often, yes. An annual service can spot a tired pump, low inhibitor levels and sludge build-up before they cause an A1 lockout — and fitting a magnetic filter helps keep sludge out of the pump in the first place. It won't prevent every failure, but it catches the slow, predictable ones, which A1 frequently is.

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. Boiler cover runs up to £500 a year if your boiler's under 7 years old, or up to £200 if it's older, with a £95 call-out fee.

Pressure fine but A1 won't clear? We'll come and fix it for you.

Book a one-off repair or set up an ongoing Smart Plan boiler module so a pump or circulation failure isn't yours to absorb. A service plan, not insurance — parts and labour included up to your cover limit.