Baxi E118 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E118 fault code mean?
The E118 code appears on Baxi boilers when the central heating system pressure has fallen to 0.5 bar or below — well beneath the minimum needed for safe operation. At this point the boiler locks out automatically to prevent damage to the heat exchanger and pump. The boiler will not fire or provide hot water until adequate pressure is restored and the unit is reset. In most cases the fix is straightforward: repressurise via the filling loop and reset. However, if pressure drops repeatedly, an underlying leak or a failing component is almost certainly to blame and a Gas Safe engineer should investigate.
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
- System water leak Common
A slow leak anywhere in the pipework, radiators, valves, or boiler connections is the single most frequent reason pressure falls far enough to trigger E118. Leaks are often small — a weeping joint or pinhole — and may only be visible as a damp patch, limescale staining, or corrosion mark. If pressure returns after repressurising but drops again within a day or two, a leak is the most likely explanation.
- Recent radiator bleeding Common
Bleeding radiators releases trapped air but also removes a small volume of water from the system, which lowers pressure. If the system wasn't topped up afterwards the pressure can fall below the 0.5 bar threshold and trigger E118. This is a very common reason for the code appearing after routine maintenance.
- Faulty or weeping pressure relief valve (PRV) Sometimes
The PRV is designed to open if system pressure climbs dangerously high, then reseat. Over time the valve can fail to fully reseat, allowing a slow trickle of water to escape via the discharge pipe outside. The tell-tale sign is a constantly damp or dripping overflow pipe on the exterior wall. A Gas Safe engineer must replace a faulty PRV.
- Waterlogged or under-charged expansion vessel Sometimes
The expansion vessel contains a pressurised air charge that accommodates the natural expansion of water as it heats. If the vessel's internal diaphragm fails or the air charge bleeds away, the vessel can no longer do its job — the system may vent excess pressure through the PRV, gradually losing water. A Gas Safe engineer can re-pressurise or replace the vessel.
- Faulty pressure sensor or loose wiring Rare
If the pressure gauge reads normal (1–1.5 bar) but E118 is still showing, the pressure sensor that feeds the PCB may be sending an incorrect low-pressure reading. A loose wiring connector to the sensor can cause the same symptom. This is less common but worth mentioning when all other causes have been ruled out — diagnosis and repair require an engineer.
How to fix it
- Check the pressure gauge DIY safe
Look at the pressure gauge on the boiler's front panel. E118 is triggered below 0.5 bar, but normal operating pressure should be 1.0–1.5 bar when the system is cold. If the needle is noticeably low, repressurising is almost certainly required. If the gauge shows a normal reading yet E118 persists, the fault may be with the pressure sensor itself — skip to the engineer step.
- Repressurise via the filling loop DIY safe
Locate the filling loop — a short flexible braided hose (sometimes with two inline valves) connected beneath or beside the boiler. Slowly open both valves to allow mains water into the system and watch the pressure gauge rise. Stop when the needle reaches 1.0–1.2 bar (do not exceed 1.5 bar). Close both valves fully. If you cannot locate the filling loop or the boiler has an internal filling key, consult your Baxi manual for the specific procedure for your model.
- Reset the boiler DIY safe
Once pressure is restored, press and hold the reset button (usually marked with a flame or 'R' symbol) for around 3 seconds until the display clears. The boiler should attempt a new ignition sequence. Allow a couple of minutes for it to complete its start-up checks. Do not reset more than two or three times — repeated failed resets can mask a worsening fault.
- Monitor pressure over the next 24–48 hours DIY safe
Keep an eye on the pressure gauge over the following day or two. If it stays steady around 1.0–1.5 bar, the issue was likely a one-off caused by bleeding radiators or normal minor variation. If pressure falls again within 48 hours — or if you find yourself repressurising more than once every couple of months — stop and call an engineer. Continuing to top up without finding the source of the loss will not resolve the underlying problem and may cause damage.
- Inspect visible pipework and radiators for signs of a leak DIY safe
Walk around the property and look for damp patches on floors or ceilings beneath radiator valves, wet marks on pipework joints, or white limescale deposits indicating a previously dried leak. Check the boiler's external overflow pipe (usually discharging outside a ground-floor wall) — a dripping or damp pipe suggests a weeping PRV. Note any findings to pass on to your engineer; do not attempt to repair leaking joints or valves yourself.
- Call a Gas Safe registered engineer if the fault persists or recurs Gas Safe engineer
If repressurising and resetting does not clear E118, if pressure drops repeatedly, if you can see a visible leak you cannot safely address, or if the PRV or expansion vessel is suspected, a Gas Safe registered engineer must carry out the repair. They can pressure-test the system to locate hidden leaks, recharge or replace the expansion vessel, fit a new PRV, or test and replace the pressure sensor. Never attempt to work on gas components, the PRV, or the PCB yourself.
Parts you may need
- System pressure sensor · from £25
- Pressure relief valve (PRV) · from £20
- Expansion vessel (boiler-mounted, 8–12 litre) · from £55
- Filling loop assembly · from £15
- Expansion vessel diaphragm / bladder · from £18
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 £100–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Baxi E118 fault myself?
If the cause is simply low pressure — which it often is — yes. Repressurising the system via the filling loop and resetting the boiler is a straightforward task a homeowner can do safely. However, if pressure keeps dropping, if you can see a leak, or if the fault returns after a reset, the underlying repair (fixing a leak, replacing a PRV, recharging an expansion vessel) must be carried out by a Gas Safe registered engineer.
How often should I need to repressurise my Baxi boiler?
A small, gradual loss of pressure over several months is normal — topping up once or twice a year is not unusual. If you find yourself repressurising more than once every two months, or if pressure drops noticeably within 24–48 hours of topping up, the system has a leak or a component fault. At that point, call a Gas Safe engineer rather than continuing to add water, as repeatedly repressurising without fixing the root cause can accelerate corrosion inside the system.
What does it cost to fix a Baxi E118 fault?
If repressurising resolves the issue yourself, there is no cost at all. When an engineer is needed — for example to fix a leak, replace a pressure relief valve, or recharge the expansion vessel — most homeowners pay in the region of £100–£350, depending on the fault and time on site. These are the most common repairs. More involved jobs such as replacing an expansion vessel or hunting down a hidden pipework leak can push costs higher. In rare cases where a faulty pressure sensor or PCB fault is diagnosed, a Gas Safe engineer will advise you on likely costs before proceeding.
Why does my Baxi boiler keep losing pressure even after I top it up?
Recurring pressure loss almost always points to one of three things: a leak in the system (which can be as subtle as a weeping radiator valve or a dripping PRV discharge pipe), a failed expansion vessel diaphragm (allowing the PRV to vent water each heating cycle), or, less commonly, a faulty filling loop valve that isn't sealing fully after repressurising and is allowing water to back-flow. A Gas Safe engineer can carry out a pressure-retention test to pinpoint the exact cause.