Worcester Bosch 1017 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch 1017 fault code mean?
The 1017 W fault code appears on Worcester Bosch boilers — most commonly the Greenstar 8000 series — when the system water pressure has dropped below the acceptable operating threshold. The 'W' suffix indicates a warning rather than a full lockout, so the boiler may continue to run but at reduced efficiency, or it may shut down to protect itself. You may also see this condition flagged as F0, E9, or EF on older or alternative Worcester Bosch displays; all refer to the same underlying problem. Normal operating pressure for a Worcester Bosch boiler should sit between 1 and 1.5 bar — if it falls below this, the pressure sensor triggers the 1017 W alert and a warning light will flash on the control panel.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Radiators bled without repressurising afterwards Common
Bleeding a radiator releases a small amount of water along with the trapped air, which lowers the overall system pressure. Many homeowners bleed their radiators without realising they need to top the system back up using the filling loop afterwards. This is the single most common trigger for the 1017 W code.
- Slow leak somewhere in the heating system Common
Even a minor drip from a radiator valve, a compression fitting, or a section of pipework hidden behind a wall or under floorboards can cause pressure to drop gradually over days or weeks. If you find yourself needing to repressurise more than once or twice a year, a hidden leak is the most likely culprit.
- Faulty or weeping pressure relief valve (PRV) Sometimes
The PRV is a safety device designed to vent water when pressure rises dangerously high. If it has started to weep or is stuck partially open, it will release water intermittently — causing repeated pressure loss even when no visible leak is present. Check for a damp or dripping discharge pipe outside your property.
- Failed or under-charged expansion vessel Sometimes
The expansion vessel absorbs the increase in water volume as the system heats up. If its internal diaphragm has split or the pre-charge pressure has fallen, the PRV is forced to release water on every heat cycle, steadily reducing system pressure. The boiler may repressurise fine but drop again quickly.
- Closed or partially closed isolation valves Sometimes
If a service valve or isolation valve on the pipework has been inadvertently left closed or partially closed — perhaps after recent plumbing work — it can restrict water flow and give a misleading low-pressure reading at the sensor.
- Faulty water pressure sensor Rare
If the boiler holds pressure correctly when tested with a separate gauge but still displays 1017 W, the pressure sensor itself may have failed or developed a poor electrical connection, causing it to report inaccurate readings to the PCB.
How to fix it
- Check the pressure gauge on the boiler DIY safe
Look at the pressure gauge on your boiler's display or front panel. If it reads below 1 bar, low pressure is confirmed. If it reads between 1 and 1.5 bar and the fault is still showing, the sensor or its wiring may be the issue — skip ahead to calling an engineer.
- Top up the system pressure using the filling loop DIY safe
Locate the filling loop — a braided flexible hose with one or two small valves, usually found beneath the boiler or connected nearby. Slowly open the valve(s) and watch the gauge rise. Stop once it reaches around 1.2–1.5 bar, then close the valve(s) firmly. Reset the boiler using the reset button. If the fault clears, monitor the pressure over the next few days.
- Check that all system valves are fully open DIY safe
Walk around your home and check that the lockshield and thermostatic valves on each radiator are open. If any isolation valves on the pipework were recently touched — for example during plumbing or boiler servicing — make sure they are returned to the fully open position.
- Inspect for visible leaks around radiators, valves, and pipework DIY safe
Look under radiators and around pipe joints for water stains, drips, or corrosion marks. Check towel rails and any exposed pipework in the airing cupboard. If you spot an active drip or a damp patch, note the location to report to an engineer. Do not attempt to tighten compression fittings on live pressurised pipework yourself.
- Check the PRV discharge pipe outside DIY safe
Find the boiler's overflow or discharge pipe — it usually exits through an external wall and may drip over a drain or gulley. If it is continuously dripping or damp when the boiler has not recently been overpressurised, the PRV may be weeping and will need professional attention.
- Reset the boiler and monitor pressure DIY safe
After topping up, press and hold the reset button (usually marked with a flame symbol or 'Reset') for a few seconds until the boiler fires. If the pressure drops again within a day or two, do not keep repressurising — this is masking an underlying problem that needs a Gas Safe engineer to diagnose.
- Call a Gas Safe registered engineer if the fault persists or pressure keeps dropping Gas Safe engineer
If the 1017 W code returns after repressurising, or if you cannot find an obvious cause, contact a Gas Safe registered engineer. They will pressure-test the system to locate hidden leaks, check the expansion vessel charge, test the PRV, and inspect the pressure sensor and its wiring. Replacing any of these components must be carried out by a qualified engineer.
Parts you may need
- Water pressure sensor · from £35
- Pressure relief valve (PRV) · from £25
- Expansion vessel (external) · from £55
- Filling loop assembly · from £20
- Expansion vessel diaphragm kit · from £15
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£380, depending on the underlying cause.
Frequently asked questions
Can I fix the 1017 W fault myself by topping up the pressure?
Yes — if the pressure has simply dropped due to normal system losses or because you recently bled a radiator, topping up via the filling loop is a straightforward DIY task and will usually clear the fault immediately. However, if the pressure drops again within a few days, or if you cannot identify why it fell in the first place, you should stop repressurising and call a Gas Safe engineer instead. Repeatedly adding water to a leaking system can cause corrosion and damage over time.
How often should I need to repressurise my Worcester Bosch boiler?
Once or twice a year is considered normal for a healthy system. If you are topping up more frequently than that — say every few weeks or months — it is a reliable sign that water is escaping somewhere, whether through a slow leak, a weeping PRV, or a failing expansion vessel. Worcester Bosch themselves advise contacting technical support if repressurisation is needed more often than this.
What is the difference between fault codes 1017 W, F0, E9, and EF on Worcester Bosch boilers?
These codes all point to the same underlying condition — low system water pressure — but appear on different Worcester Bosch models or display generations. The 1017 W notation is used on newer Greenstar 8000 series boilers, while older or alternative Worcester Bosch displays may show F0, E9, or EF for the same pressure fault. The diagnosis and fix are identical whichever variant appears on your boiler.
Could a faulty expansion vessel be causing my 1017 W code, and how much does it cost to replace?
Yes — a failed expansion vessel is a common cause of recurring low pressure. When the internal diaphragm splits, the vessel can no longer absorb the expansion of water as the system heats up, forcing the PRV to release water on every heating cycle. Replacing a standard external expansion vessel typically costs £180–£350 including labour. An internal vessel inside a combi boiler requires more strip-down time and can cost £250–£450 or more. Note that in unusual cases where a cracked heat exchanger is responsible for repeated pressure loss, costs can be considerably higher — your engineer will advise if this is suspected.