ATAG 117 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 117 fault code mean?
Fault code 117 on an Atag boiler means the system water pressure has climbed above 3 bar, triggering a safety lockout. Under normal operating conditions your boiler should sit between 1 and 1.5 bar when cold — the green zone on your pressure gauge. Once pressure pushes past 3 bar the boiler shuts itself down and displays code 117 to prevent damage to internal components and pipework. Until the underlying cause is resolved and pressure is brought back into range, the boiler will not fire.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. 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
- Filling loop left open or passing water Common
The filling loop — the braided hose used to top up system pressure — should be firmly closed after use. If either valve is left slightly open, or if the internal seal is worn, mains-pressure water trickles continuously into the system and pushes pressure above 3 bar. Check that both handle valves on the filling loop are fully closed and that the hose itself feels firm rather than pressurised.
- Faulty or waterlogged expansion vessel Common
The expansion vessel absorbs the increase in water volume as your heating warms up. If its internal diaphragm has split or it has lost its nitrogen pre-charge, the vessel can no longer buffer that expansion. Water pressure then spikes sharply every time the boiler fires, causing repeated 117 lockouts and often water discharge from the pressure relief valve pipe outside. A Gas Safe engineer can test the vessel charge and either recharge or replace the unit.
- Passing pressure relief valve (PRV) Sometimes
The PRV is a safety device designed to release water if pressure gets dangerously high. Over time the valve seat can wear or become partially blocked with debris, causing it to stick open or to pass water even when pressure is not extreme. Conversely, a PRV that never opens when it should can allow pressure to build unchecked. Either condition needs a Gas Safe engineer to inspect and replace the valve.
- Trapped air affecting pressure readings Sometimes
Pockets of air in radiators or pipework can cause erratic pressure gauge behaviour, giving falsely high or fluctuating readings. Bleeding your radiators removes trapped air and can help stabilise the reading on the gauge, though it will not fix an underlying mechanical fault.
- Faulty pressure sensor Rare
If the pressure sensor inside the boiler has drifted out of calibration or failed, it may report a falsely high pressure reading to the boiler control board, triggering a 117 lockout even when actual system pressure is normal. This is less common but should be considered if pressure readings on an external gauge look fine while the fault persists.
How to fix it
- Check and close the filling loop DIY safe
Locate the filling loop — usually a grey or silver braided hose beneath the boiler with two small valves. Both handles should sit at 90° to the pipe (closed position). If either feels loose or can be turned further, close it firmly. If the hose itself feels under pressure when it should not, the internal valve may be passing and the loop will need replacing by an engineer.
- Bleed your radiators to reduce pressure DIY safe
If pressure is above 3 bar you need to release a small amount of water from the system. Using a radiator bleed key, open the bleed valve on one or more radiators — start upstairs — until water (not just air) trickles out. Watch the pressure gauge throughout and stop when it falls to around 1.2–1.5 bar. You may need to bleed approximately 2 litres from the system in total. Do not overshoot and take pressure below 1 bar.
- Reset the boiler DIY safe
Once pressure is back in the green zone (1–1.5 bar cold), press and hold the reset button on the boiler for a few seconds as described in your user manual. The boiler should attempt to restart. Limit resets to two or three attempts — repeated resets without resolving the root cause can mask a worsening problem.
- Monitor pressure over the next 24–48 hours DIY safe
Keep an eye on the pressure gauge over the following day or two. If pressure climbs again without you having used the filling loop, this points to a fault with the expansion vessel or PRV that requires a Gas Safe engineer. Note down how quickly the pressure rises and whether water is dripping from the discharge pipe outside — this information will help the engineer diagnose the fault faster.
- Have a Gas Safe engineer inspect the expansion vessel Gas Safe engineer
A registered engineer will use a pressure gauge to test the nitrogen pre-charge in the expansion vessel. If the charge has dropped, the vessel can often be re-pressurised at low cost. If the internal diaphragm has ruptured, the vessel will need replacing. This is the most common cause of recurring high-pressure faults and should not be left unresolved as it places sustained stress on other boiler components.
- Have a Gas Safe engineer inspect or replace the PRV Gas Safe engineer
If water is discharging from the overflow pipe on the outside wall, or the PRV is visibly weeping, a Gas Safe engineer needs to assess whether the valve is operating correctly or needs replacing. A PRV that has opened repeatedly under high pressure often fails to reseat properly afterwards and will need to be renewed.
- Have a Gas Safe engineer check the pressure sensor Gas Safe engineer
If system pressure looks normal on the external gauge but code 117 keeps returning, the engineer can test the boiler's internal pressure sensor and replace it if it is sending inaccurate readings to the control board.
- Call a Gas Safe registered engineer if the fault persists or recurs Gas Safe engineer
If reducing pressure and resetting the boiler does not clear code 117, or the pressure rises again within a short period, call a Gas Safe registered engineer. Persistent high pressure indicates a component fault that cannot be safely resolved without professional diagnosis and repair.
Parts you may need
- Expansion vessel (internal or external) · from £60
- Expansion vessel Schrader valve / recharge kit · from £8
- Pressure relief valve (PRV) · from £25
- Filling loop assembly · from £18
- Water pressure sensor · from £35
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 Atag fault code 117 myself?
Partially. Homeowners can safely close the filling loop, bleed radiators to bring pressure back below 1.5 bar, and reset the boiler. If those steps clear the fault and pressure stays stable, no engineer visit is needed. However, if pressure climbs again, or if you suspect the expansion vessel or PRV is faulty, you will need a Gas Safe registered engineer — working on sealed heating system components without registration is unsafe and may void your Atag warranty.
Why does my boiler keep going back to high pressure after I bleed the radiators?
If pressure returns to over 3 bar repeatedly, the most likely culprit is a failed expansion vessel — either its diaphragm has split or it has lost its nitrogen pre-charge. A healthy vessel absorbs the extra volume created when water heats up; a failed one cannot, so pressure spikes every time the boiler fires. A Gas Safe engineer can test and recharge or replace the vessel, typically for £150–£350 all in. A passing filling loop is the other common cause, so double-check that both valves are fully closed.
Is water dripping from my outside pipe related to fault code 117?
Yes, almost certainly. The pipe on the outside of your property that occasionally drips water is the discharge pipe from the pressure relief valve. When system pressure exceeds 3 bar the PRV opens automatically to protect the system — which is exactly what triggers code 117. Occasional drips are a safety feature working correctly, but if water is running continuously or the fault keeps returning, the PRV may have failed to reseat properly and will need replacing by a Gas Safe engineer.
Will fault code 117 affect my Atag warranty?
Atag boilers come with a 10-year manufacturer warranty, during which Atag's own engineers handle repairs. However, the warranty requires the boiler to be serviced annually by a qualified engineer. Faults caused by neglect or system issues (such as an un-maintained expansion vessel) may not be covered, and labour is not always included. Check your warranty documentation and contact Atag directly if your boiler is still within the warranty period before booking an independent engineer.