Is your Worcester Bosch boiler showing an A1 code? Don't panic. The A1 fault points to a problem with the pump, and it's often something you can sort quickly. Below is what causes it and how to get your heating and hot water back.
If your boiler's showing a different code, our guide to the full list of Worcester Bosch fault codes covers the rest.
What the A1 error code means
The A1 code flags a pump that's jammed, running dry, or full of air. The pump pushes warm water round to your radiators and hot taps. When it struggles, you'll notice tepid water at the taps or radiators that heat unevenly. Your boiler handbook has model-specific detail, but the three causes below cover most cases.
What causes an A1 fault
Seized or worn pump
Pump bearings and the spindle wear over time and can eventually seize. The warning signs are a strained or grinding noise, a hot pump casing, and noticeable vibration through the system. A pump fitted at an awkward angle wears faster too — they run best mounted horizontally.
Fix: Replacing the worn bearings, or the whole pump, is usually cheaper than a new boiler, but the final cost depends on the pump type, boiler model, access, and labour required. A Gas Safe engineer can confirm whether the pump is fitted correctly, repairable, or beyond saving before giving a current quote.
Trapped air (air lock)
Trapped air stops the pump circulating water — an air lock. Too much air also drops your pressure, and the boiler won't run properly below about 1.2 bar. Check the gauge on the front. A slow drop over time is normal; a sudden one usually means a leak that needs finding. Air gets in through a leaky pipe or component, or after certain repairs when the system needs re-pressurising.
Fix: Bleeding your radiators regularly clears excess air before it locks the pump. If a radiator is cold at the top and hot at the bottom, it needs bleeding. Banging pipes can also point to trapped air in the system.
Sludge blocking the pump
Rust and debris break down inside your radiators and pipes and form a thick sludge. It circulates through the system, slows the water down, and clogs the pump. Older systems, and any without a filter fitted, are most at risk.
Fix: An engineer can find a blockage quickly. Cleaning the magnetic system filter is often enough to get things moving again, as long as the bearings and shaft aren't damaged. Ask for a filter clean during your next annual service. If the system's badly sludged, they may recommend a power flush — pumping the sludge out under pressure. Cost depends on the number of radiators, system condition, and the repair approach agreed. Fitting a magnetic filter can help catch particles before they reach the pump, but the final cost should be confirmed with a current quote.
Can you fix an A1 fault yourself?
Sometimes. Bleeding a radiator or topping up pressure is a safe DIY job. But a seized pump, a hidden leak, or a sludged system needs an engineer — and anything inside the boiler casing should be left to one. If the code keeps coming back after you've bled the radiators and checked the pressure, book it in.
When to call an engineer
If the pump's seized, the pressure keeps dropping, or the system needs flushing, get it looked at. Serviceteam is a Worcester Bosch accredited installer, so your repair is in safe hands. We diagnose the fault and try to fix it on the first visit — and if a new part's needed, you get a fixed-price quote before any work goes ahead.
If the pump's seized or the system needs flushing, book a boiler repair and we'll diagnose it on the first visit.
Still getting the A1 code?
Our engineers cover A1 pump faults across London. We'll diagnose the fault and try to fix it within the first hour. If a part's needed, you get a fixed-price quote before we start — no surprises. Book a boiler repair with Serviceteam today.
