How to Fix the 1017 W Error Code on a Worcester Bosch Boiler

A Worcester Bosch boiler showing a 1017 W error code is telling you one thing: the water pressure has dropped too low. The boiler's pressure sensor has spotted it and shut things down to protect itself. You'll usually see it alongside a flashing red light on the control panel. Some Greenstar models show it as H07-1017, which is the same fault.

The good news is that low pressure is often something you can sort yourself. The cause matters though, because a one-off top-up is very different to a slow leak. Here's how to work out which one you're dealing with.

What the 1017 W error code means

The 1017 W code means the water pressure in your heating system has fallen below the level the boiler needs to run. A sensor monitors that pressure constantly. When it drops too far, the boiler flags the fault and stops firing.

Your boiler should sit between 1 and 1.5 bar when the heating is off. If the gauge reads below 1 bar, that's your problem. The job now is finding out why the pressure dropped.

What causes the 1017 W error code

A water leak

A leak is the most common reason pressure keeps dropping. Check around the boiler, the pipework, and the radiators for damp patches or staining. Small leaks are easy to miss but they'll keep draining pressure no matter how often you top up. If you top up and the pressure falls again within a day or two, you've almost certainly got a leak.

Closed valves

Every valve leading to and from the boiler should be fully open. A closed or partly closed valve restricts water flow and pulls the pressure down. Check the isolation valves and the filling loop valves first.

A problem with the filling loop

The filling loop is the braided hose used to add water and raise pressure. If it isn't connected properly or the valves aren't open, you won't be able to re-pressurise the system. Check your boiler manual if you're not sure which part is the filling loop.

A faulty pressure relief valve

This valve releases excess pressure from the system. If it sticks open, pressure leaks out and the boiler reads it as low. A faulty relief valve needs replacing by an engineer, not just resetting.

A blockage in the system

Sludge or debris in the pipes or heat exchanger restricts flow and can drag pressure down. A central heating power flush clears it out. If you're seeing cold spots on radiators alongside the pressure fault, a blockage is likely.

How to fix a 1017 W error code

Check for leaks

Before you top anything up, look for leaks. Check the boiler, visible pipework, and radiator valves for damp or staining. If you find one, get it fixed before re-pressurising — otherwise you're just topping up a system that'll drop again.

Open the valves

Make sure every valve to and from the boiler is fully open. That includes isolation valves and the filling loop valves. A half-shut valve is an easy thing to miss and a quick fix.

Re-pressurise the boiler

If there's no leak, you can usually re-pressurise the boiler yourself in a couple of minutes. Open the filling loop valves slowly and watch the gauge climb to between 1 and 1.5 bar, then close them. Your boiler manual has the exact steps for your model.

Bleed the radiators

Trapped air can cause low pressure too. Bleeding the radiators releases it. Start with the lowest radiator in the house and work up, one at a time, until water runs out steadily with no hissing. You may need to top the pressure back up afterwards.

When to call an engineer

If the pressure won't hold or you've found a leak you can't reach, you can book a boiler repair in London and we'll send a Gas Safe engineer out.

If the pressure keeps dropping after you've topped it up, or you've found a leak you can't reach, it's time for a Gas Safe engineer. The same goes for a stuck pressure relief valve or a faulty sensor — both need replacing properly.

Don't take the boiler apart yourself. Anything past topping up the pressure and bleeding radiators should be left to a qualified engineer.

If your boiler is throwing a different code, our guide to other Worcester Bosch fault codes walks through what each one means.