What the A1 281 code means
On most Greenstar models, the display code A1 means there is a problem with the pump, and the cause code 281 tells you what kind. Worcester's installation manuals describe 281 as "pump stuck or running dry", and the Greenstar 4000 manual words it as the pump being stuck or running in air. In both cases the boiler has worked out that the pump is not moving water round the system as it should.
The pump is what pushes hot water from the boiler to your radiators and, on a combi, through the plate heat exchanger for hot water. Without proper circulation the boiler could overheat, so it stops and shows A1 281 rather than carry on. On the Greenstar i this is listed as a blocking fault, which means the boiler holds off while the problem is present.
The good news is that some causes are simple. Low pressure and trapped air can both make a healthy pump behave as though it is running dry, and you can safely check for those yourself.
Common causes of A1 281
Low system pressure
If the pressure has dropped well below 1 bar, there may not be enough water around the pump for it to work properly. This is the first thing Worcester suggests checking.
Air in the system
Air collects at high points, in radiators and sometimes in the pump itself. It often appears after radiators have been bled, after a system has been drained, or after new radiators have been fitted.
A seized or sticking pump
Pumps can stick after a long idle spell, such as over the summer, or when sludge builds up inside. A pump that hums but does not turn will trigger this code.
Restricted circulation
Closed valves, a blocked filter or heavy sludge in older pipework can stop water moving freely, even with a working pump.
Wiring or pump failure
A worn out pump motor, or a problem with the pump wiring and connectors, can also produce A1 281. Pumps in boilers that run for long hours every winter tend to wear sooner, and a failing pump often gets noisier before it stops.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Low pressure | Gauge below 1 bar when cold | You, via the filling loop |
| Air in radiators | Cold at the top, gurgling noises | You, by bleeding radiators |
| Air trapped in the pump | Noisy boiler, fault after draining down | Gas Safe engineer |
| Seized or sticking pump | Humming, fault after a quiet summer | Gas Safe engineer |
| Sludge or blocked filter | Cold patches at the bottom of radiators | Gas Safe engineer |
| Failed pump or wiring | Fault returns with good pressure | Gas Safe engineer |
Safe checks you can do yourself
- Check the pressure gauge with the heating off and cool. If it reads below 1 bar, top up to 1 to 1.5 bar using the filling loop. Our guide on how to repressurise a boiler explains how.
- Close both filling loop valves fully when you have finished.
- If any radiators are cold at the top or you can hear gurgling, bleed them with the heating off, starting downstairs, then check the pressure again and top up if needed.
- Check the fused spur and power supply to the boiler are on.
- Press and hold the reset button as your manual describes, then turn the heating on. Stop after two or three attempts if the code returns.
Bleeding radiators always lowers the pressure. Check the gauge straight afterwards so the fix for one problem does not lead to another, such as CE 207 low pressure.
Do not remove the casing to vent or free the pump yourself. The pump sits inside the boiler next to live electrical parts and gas components, so it is a job for a Gas Safe registered engineer.
When to call a Gas Safe engineer
If the pressure is right, the radiators are bled and A1 281 still comes back, call a Gas Safe registered engineer. You should also call one if you can hear the pump humming without anything moving, or if the pressure keeps dropping after you have topped it up.
The engineer will check the pump wiring and operation, vent any air from the pump, look at circulation through the system and check the filter if one is fitted. In older systems they may recommend cleaning out sludge to protect a new pump. When you book, tell them the pressure reading, whether you have bled radiators recently and whether the fault appeared after a quiet spell.
Typical repair costs in the UK
If topping up or bleeding radiators fixes it, there is no cost. Otherwise, these ranges are typical. Your final bill depends on the model, the parts and your area.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Venting or freeing a sticking pump | Typically £80 to £150 |
| Pump replacement | Typically £250 to £450 fitted |
| Magnetic filter fitted | Typically £150 to £250 |
| System clean or powerflush | Typically £400 to £700 |
How to stop A1 281 coming back
Check the pressure once a month and keep it between 1 and 1.5 bar when cold. Run the heating for a short while every few weeks over the summer so the pump does not stick. If you keep losing pressure, look for leaks around radiator valves and pipe joints, and see our guide to a boiler leaking water. A yearly service is a good time to have the filter cleaned and the pump checked, and our boiler service cost guide explains what is included.