Main E118 Fault Code: Causes, Fixes & Repair Costs
What does the Main E118 fault code mean?
The E118 code appears on Main boilers (and the closely related Baxi and Potterton ranges, which share the same fault code system under the BDR Thermea group) when the system water pressure drops below 1 bar. At that point the boiler locks out to protect itself from running without adequate water circulation. Normal operating pressure should sit between 1.0 and 1.5 bar when the system is cold. The E118 fault is not an immediate safety emergency, but leaving it unaddressed can lead to component wear and repeated lockouts. In most cases, topping up the pressure via the filling loop clears the fault — though a recurring drop points to an underlying problem that needs professional attention.
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
- Gradual system leak Common
Small drips from radiator valves, pipe joints, or connections beneath floorboards slowly bleed water from the system over days or weeks until pressure falls below 1 bar. This is the most frequent reason the E118 code returns after repressurising. Check around radiator tails, towel rail fittings, and any visible pipework for damp patches or white limescale deposits indicating a past drip.
- Radiators bled without topping up afterwards Common
Bleeding a radiator releases trapped air but also expels a small amount of water, nudging the system pressure downward. If the pressure was already close to 1 bar before bleeding, even one radiator can trigger E118. Always check the pressure gauge immediately after bleeding and repressurise if needed.
- Expansion vessel failure or low pre-charge pressure Sometimes
The expansion vessel contains a pressurised air charge that accommodates water expansion as it heats up. If the rubber diaphragm inside perishes or the air charge drops, the vessel can no longer buffer pressure swings properly, leading to pressure fluctuations and eventual low-pressure lockouts. A Gas Safe engineer can test and re-charge or replace the vessel.
- Pressure relief valve (PRV) weeping Sometimes
The PRV is designed to open and discharge water if pressure becomes dangerously high. Over time — especially in hard-water areas — scale or debris can prevent the valve from seating fully, causing a continuous slow drip from the discharge pipe (usually outside or to a drain). This mimics a system leak and progressively reduces pressure. A weeping PRV needs replacement by an engineer.
- Faulty pressure sensor Rare
The sensor that monitors system pressure can develop scale build-up or an electrical fault, causing it to report a falsely low reading even when actual pressure is adequate. If pressure appears normal on the boiler gauge yet E118 still triggers, a defective sensor is a possible explanation. This requires engineer diagnosis.
- Frozen condensate pipe (winter months) Rare
A blocked condensate pipe can cause the boiler to behave erratically, and in some circumstances associated faults — including pressure-related codes — appear alongside a condensate blockage. If E118 appears during a cold snap and the boiler is also showing condensate-related symptoms, check the condensate pipe first.
How to fix it
- Check the pressure gauge reading DIY safe
Locate the pressure gauge on the boiler's front panel or beneath the casing. If the needle reads below 1 bar, low pressure is confirmed as the immediate cause. If the gauge reads 1 bar or above and E118 is still showing, do not attempt to add more water — move on to resetting or call an engineer.
- Repressurise the system using the filling loop DIY safe
Find the filling loop — typically a braided or silver flexible hose with two valves, located beneath the boiler. Slowly open both valves (quarter-turn levers or knurled knobs) and listen for water entering the system. Watch the pressure gauge rise and close both valves firmly once the needle reaches 1.5 bar. Never overfill above 2 bar. If you cannot locate the filling loop or are unsure, check the boiler manual or call an engineer rather than guessing.
- Reset the boiler DIY safe
Once pressure is restored to the correct range, press and hold the reset button (usually marked with a flame or reset symbol) for around 3 seconds until the boiler attempts to restart. Allow a few minutes for it to complete its ignition sequence. Do not reset more than two or three times in quick succession.
- Monitor pressure over the following days DIY safe
Check the gauge daily for at least a week. A one-off drop after bleeding radiators is normal. If pressure falls below 1 bar again within 24–48 hours without any obvious explanation, stop repressurising and treat it as a leak — continuing to top up simply masks the problem and can introduce excess oxygen into the system, accelerating internal corrosion.
- Inspect visible pipework, radiator valves, and the PRV discharge pipe DIY safe
Walk around the property looking for damp patches near radiators, under the boiler, or along visible pipe runs. Check whether the PRV discharge pipe (a copper or plastic pipe leading outside or to a drain) is dripping — a weeping PRV is a common culprit. If you find evidence of a leak, note its location to report to the engineer.
- Thaw a frozen condensate pipe if relevant DIY safe
If E118 has appeared during freezing weather and the condensate pipe (usually a white plastic pipe exiting through an outside wall) is frozen solid, carefully pour warm — not boiling — water over the external section or wrap it with a warm cloth until it clears. Once thawed, reset the boiler. This step only applies if a frozen condensate pipe is suspected.
- Call a Gas Safe registered engineer Gas Safe engineer
If E118 returns after repressurising, if you cannot identify a leak, or if the boiler gauge reads normal pressure but the fault persists, you need a Gas Safe registered engineer. They can pressure-test the system to find hidden leaks, inspect and re-charge or replace the expansion vessel, test the pressure relief valve, and check the pressure sensor. Do not attempt to work on the PRV, expansion vessel, gas valve, or internal boiler components yourself.
Parts you may need
- Expansion vessel (8–12 litre, combi boiler) · from £55
- Pressure relief valve (3-bar PRV) · from £18
- Water pressure sensor / pressure transducer · from £30
- Filling loop assembly (replacement) · from £15
- Expansion vessel diaphragm / bladder · from £25
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 £80–£320, depending on the underlying cause.
Frequently asked questions
Can I fix the Main E118 fault myself without calling an engineer?
Often, yes — if the fix is simply repressurising the system. Topping up via the filling loop and resetting the boiler is a straightforward DIY task and costs nothing. However, if pressure keeps dropping, or if you cannot locate the filling loop, the fault lies with a component such as the expansion vessel, PRV, or a hidden leak, and a Gas Safe engineer is required. Never attempt to work on internal boiler components yourself.
Why does my Main boiler keep losing pressure and showing E118?
Recurring pressure loss almost always points to one of three things: a slow leak somewhere in the system (radiator valves, pipe joints, or beneath floorboards), a weeping pressure relief valve that drips water continuously to the drain, or a failed expansion vessel that can no longer buffer pressure changes properly. Repeatedly topping up the pressure without finding the root cause can introduce excess oxygen into the pipework and speed up internal corrosion, so it is worth getting an engineer to investigate if the fault returns within a day or two.
How much does it cost to fix the E118 fault on a Main boiler?
If repressurising the system solves the problem, the cost is nothing. If an engineer is needed, a standard callout plus repressurise and check typically runs £80–£150. Replacing a pressure relief valve or re-charging an expansion vessel usually falls in the £100–£250 range, including labour. A full expansion vessel replacement tends to cost £150–£320 depending on the model and labour rates in your area. Leak detection and repair for a hidden leak can cost more and varies considerably — worth checking whether your home insurance or boiler cover includes trace-and-access.
Is the E118 code the same on Baxi and Potterton boilers?
Yes. Main, Baxi, and Potterton are all part of the BDR Thermea group and share the same fault code system. E118 means low water pressure on all three brands, and the diagnosis and repair steps are identical across them.