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Worcester Bosch A1 / 281 fault code explained
An A1 / 281 lockout on a Worcester Bosch Greenstar means the boiler cannot detect water circulating properly. It is usually caused by low pressure, an airlock or a seized pump. There is one safe check you can do yourself first, and a clear point at which it becomes an engineer's job.
Quick answer
A1 / 281 on a Worcester Bosch boiler is a water-circulation lockout: the boiler senses that water is not moving through the system as it should — typically because the pump is running dry, has air in it, or is stuck. The most common trigger is low pressure or an airlock, so the safe first step is to check the pressure gauge and, if it reads below about 1 bar, repressurise to 1–1.5 bar using the filling loop, then bleed your radiators and reset the boiler once.
If the pressure is already correct, the fault keeps returning, or you can hear the pump but the radiators stay cold, stop there and call a Gas Safe registered engineer — the likely cause is a seized pump, a stubborn airlock or a sensor fault that only an engineer should diagnose. Do not keep resetting a boiler that locks out again.
What does A1 / 281 mean on a Worcester Bosch boiler?
The A1 fault on a Worcester Bosch Greenstar combi or system boiler is a water-circulation (or "no flow") lockout. The number shown with it — here 281 — is the cause code that narrows down why.
With 281, the boiler's control board (PCB) has decided that water is not circulating properly: the pump may be running with air in it, running dry against low pressure, or stuck and not turning at all.
In plain terms, the boiler will not fire its burner unless it is confident water is moving past the heat exchanger. If it cannot confirm that, it locks out and shows A1 / 281 to protect itself from overheating. The display may alternate between "A1" and "281".
You may see closely related codes on different Greenstar ranges — for example EA 338, which typically flags too many short burner starts with the pump not detecting water, again usually linked to low pressure.
These sit in the same circulation family and share most of the same first checks, so the advice below applies broadly. The exact code shown can vary by model and software version, so always cross-check against your own boiler's manual.
Common causes of A1 / 281
- Low system pressure — if the gauge has dropped below about 1 bar, the pump can run dry and trip the lockout. This is the single most common cause.
- An airlock — trapped air in the radiators or pump stops water moving freely.
- A seized or failing pump — pumps can stick after a long idle spell (often at the end of summer) or wear out with age, so they spin slowly or not at all. An engineer can sometimes free a seized pump rather than replace it.
- A slow leak — a small leak on a radiator, valve or pipe joint lets pressure fall and air creep in.
- A blocked or sludgy system — corrosion debris restricts flow through the heat exchanger or pump.
- A faulty flow sensor or wiring fault — less common, but the boiler can misread circulation if a sensor or its connection has failed.
One safe job only. Topping up the pressure via the filling loop, bleeding radiators and a single reset are the only parts of an A1 / 281 fix a homeowner should attempt.
Anything involving the pump, sensors, gas, the burner, the gas valve, the PCB or opening the sealed boiler casing is Gas Safe registered engineer work — it is illegal and dangerous for an unregistered person to do it.
If you ever smell gas, leave the property and call the National Gas Emergency line on 0800 111 999.
How to fix A1 / 281 (the safe homeowner steps)
- Check the pressure gauge. A healthy cold system usually sits around 1–1.5 bar. See what your boiler pressure should be if you are unsure where yours should read.
- Repressurise if it is low. If the needle is below 1 bar, open the filling loop slowly until the gauge reaches about 1–1.5 bar (cold), then close it firmly. Our guide on boiler pressure that is too low walks through this step by step.
- Bleed your radiators. Releasing trapped air can clear an airlock that is choking circulation. Re-check the pressure afterwards and top up again if needed, as bleeding usually drops it.
- Reset the boiler once. Follow how to reset a Worcester Bosch boiler (or our general how to reset your boiler guide) and allow it to attempt a normal start. Do not reset repeatedly — a lockout that returns is telling you there is an underlying fault.
- Watch it. If the fault clears and pressure holds, you are likely sorted. If pressure keeps falling, or A1 / 281 returns within a day or two, you have an underlying fault — move on to an engineer.
If your pressure keeps dropping after every top-up, you probably have a leak rather than a one-off air pocket, and chasing it with repeated top-ups will not solve it.
When to call a Gas Safe registered engineer
Book an engineer if the pressure is already correct, if A1 / 281 keeps coming back, if pressure falls repeatedly, or if you can hear the pump but the radiators stay cold. This is where safe DIY ends. A registered engineer can legally and safely work on the pump, sealed circuit, sensors and PCB; you must not.
Always check the engineer's ID on the Gas Safe Register before they start. A typical visit involves:
- Testing the pump — freeing or replacing it if it has seized (a noisy central heating pump is often an early warning sign).
- Power-flushing or removing an airlock the system will not shed on its own.
- Finding and fixing the leak behind a repeated pressure drop.
- Checking the flow sensor, wiring and PCB for faults.
For background on why boilers shut themselves down like this, see our explainer on boiler lockout, and the wider Worcester fault codes hub.
Typical Worcester Bosch A1 / 281 repair cost
Costs vary by region, model and pump availability. These are indicative 2026 UK ranges to help you judge a quote — not a fixed price.
| Work | Indicative 2026 cost (£) |
|---|---|
| Engineer diagnostic / call-out | £70–£120 |
| Repressurise & bleed (homeowner) | £0 |
| Clear airlock / minor leak repair | £90–£200 |
| Circulation pump replacement (parts + labour) | £200–£450 |
| Power flush (sludge / blockage) | £350–£600 |
| PCB replacement | £350–£650 |
See our wider boiler repair cost guide for context. If several major parts are failing on an older boiler, an engineer may advise that replacement is more economical than repair.
Related Worcester Bosch codes
- Worcester error codes — the full A-to-EA list, including EA 338.
- Worcester boiler no hot water — if the lockout has left you with cold taps.
- Worcester boiler repair — what a typical repair involves and costs.
Will boiler cover pay for an A1 / 281 repair?
It depends on your policy.
If the cause is a covered breakdown such as a failed pump or a leaking internal component, many boiler cover policies will pay for the engineer and parts, subject to your excess and the policy terms. What they generally will not cover is a boiler that was already faulty when you took the policy out, wear during an exclusion window, or sludge damage where the system was never properly maintained.
Always read your own policy wording, as cover varies between providers.
Because a simple pressure top-up costs you nothing, cover tends to earn its keep on the bigger jobs — pump replacement, leak repair or a new PCB. To weigh that up, read what boiler cover includes and our honest take on whether boiler cover is worth it. This is general information, not financial advice.
Is the A1 / 281 fault code dangerous?
The lockout itself is a safety feature, not a hazard — the boiler has shut down precisely to avoid running unsafely without water moving through it. The underlying cause (a leak or seized pump) needs fixing, but the boiler stopping is the system protecting itself.
Never bypass the lockout or attempt gas work yourself. If you ever smell gas, leave the property and call 0800 111 999.
Can I fix A1 / 281 myself?
You can safely do the first step: check the pressure and, if it is low, repressurise to about 1–1.5 bar, bleed your radiators and reset the boiler once. If that clears it and pressure holds, you are done. Anything beyond that — the pump, sensors, leaks inside the boiler or the PCB — must be left to a Gas Safe registered engineer.
Will A1 / 281 clear if I just reset the boiler?
A single reset may clear it temporarily, but if the real cause (low pressure, an airlock, a seized pump or a leak) is still there it will return. Resetting repeatedly without fixing the cause is not a solution and is not recommended. Correct the pressure and bleed first, then reset once.
How much does an A1 / 281 repair cost?
If it is just a pressure top-up, nothing. If an engineer is needed, expect roughly £70–£120 for a diagnostic, around £200–£450 for a pump replacement, and £350–£650 for a PCB. These are indicative 2026 UK figures — get a written quote.
What is the difference between A1 / 281 and EA 338?
They are closely related circulation faults. A1 / 281 is a no-flow lockout, while EA 338 typically flags too many short burner starts with the pump not detecting water, usually linked to low pressure.
The first checks are the same — pressure and air — but the exact code shown depends on your Greenstar model and software, so check your own boiler manual.
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Compare boiler coverThis 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.