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Baxi E118 Fault Code: What It Means and How to Fix It
An E118 on your Baxi boiler almost always points to one thing: the water pressure in your heating system has dropped too low for the boiler to fire safely. The good news is that this is one of the few fault codes a homeowner can often clear themselves.
Quick answer
E118 means low system water pressure. Your Baxi has detected that the primary heating pressure has fallen too low (Baxi typically flags this below around 0.5 bar) and has shut down to protect itself. Check the pressure gauge — if it reads under 1 bar, you can usually top it up yourself using the filling loop until it reaches 1 to 1.5 bar, then reset the boiler.
If the pressure keeps dropping after you refill it, that points to a leak, a failed expansion vessel or a faulty pressure relief valve — call a Gas Safe registered engineer rather than repeatedly repressurising.
What does E118 mean on a Baxi boiler?
E118 is Baxi's low water pressure fault code. Your heating system is a sealed, pressurised loop of water, and that water needs to sit within a safe band — normally between 1 and 1.5 bar when the system is cold.
When the boiler's pressure sensor detects that the pressure has fallen too low (Baxi generally defines this as below roughly 0.5 bar), it locks out and displays E118 rather than firing. This is a safety measure: running a boiler with too little water in the circuit can cause overheating and damage.
On many Baxi control panels the code is shown in two parts — an "E1" prefix followed by "18". You may also see the closely related code E119, which carries the same low-pressure meaning on many Baxi models such as the Duo-tec range.
Because the same code can mean slightly different things across Baxi platforms, always defer to the documentation for your exact model if the wording differs.
Check the pressure gauge first. Before doing anything else, look at the pressure gauge on or under your boiler. If the needle is in the red zone or below 1 bar, low pressure is confirmed and a top-up is the likely fix.
If the gauge reads a healthy 1 to 1.5 bar but E118 still shows, do not keep adding water — the sensor or another part may be at fault, and that needs an engineer.
What causes E118?
Low pressure has a handful of common causes, ranging from harmless to needing a repair:
- Gradual, natural pressure loss — all sealed systems lose a little pressure over months; an occasional top-up is normal.
- Recently bled radiators — releasing air from radiators lowers system pressure and often triggers E118 afterwards.
- A small leak — the most common reason pressure keeps falling. Leaks can be at radiator valves, pipe joints, or internal boiler components.
- A failed or waterlogged expansion vessel — when the vessel loses its air charge it can no longer absorb pressure changes, leading to pressure swings and loss.
- A weeping pressure relief valve (PRV) — a stuck or passing PRV slowly releases water, usually visible as dripping from an outside discharge pipe.
How to fix E118
Topping up the pressure via the filling loop is a homeowner-safe task. You are only adding mains water to the sealed circuit — you are not touching any gas component. Here is the safe sequence:
- Turn the boiler off and let it cool for a few minutes.
- Locate the filling loop — usually a braided silver or flexible hose underneath the boiler, with one or two valves or levers.
- Open the valve(s) slowly. You should hear water flowing into the system.
- Watch the pressure gauge rise. Close the valve(s) as soon as it reaches between 1 and 1.5 bar — do not overfill.
- Make sure both valves are fully closed, then reset the boiler and check that it fires.
If you are unsure where your filling loop is or how it operates, check your Baxi manual or our guide on boiler pressure that is too low and what your boiler pressure should be.
If the boiler still shows E118 after repressurising, try the steps in how to reset your boiler — but if pressure has not held, do not keep resetting; move straight to calling an engineer.
When to call a Gas Safe registered engineer
Topping up the pressure is where the safe DIY ends. If any of the following apply, it is time for a professional:
- The pressure drops again within hours or days of refilling — this signals a leak that must be traced and repaired.
- You can see water dripping from an outside pipe (a likely PRV or expansion vessel issue).
- The gauge already reads 1 to 1.5 bar but E118 persists.
- You cannot find or operate the filling loop safely.
By law, only a Gas Safe Register engineer may work on the gas supply, burner, gas valve, flue, sealed combustion circuit, the boiler PCB or the pressure relief valve. It is illegal and dangerous for an unregistered person to attempt this work.
An engineer will pressure-test the system to find the leak, check and recharge or replace the expansion vessel, inspect the PRV, and confirm the pressure sensor is reading correctly. If you ever smell gas, leave the appliance off and call the National Gas Emergency line on 0800 111 999.
Typical Baxi E118 repair cost
If a top-up fixes it, your only cost is your time. If an engineer is needed, the figures below are indicative 2026 UK ranges and will vary by region, boiler model and how easy the boiler is to strip down.
| Job | Indicative 2026 cost |
|---|---|
| Diagnostic / call-out | £70 – £120 |
| Trace and repair a minor system leak | £120 – £300 |
| Expansion vessel replacement (parts + labour) | £150 – £350 |
| Pressure relief valve (PRV) replacement | £120 – £250 |
| Pressure sensor replacement | £100 – £220 |
For a wider view of what heating repairs cost, see our boiler repair costs guide.
Related Baxi codes
If you are working through Baxi fault codes, these related guides may help:
- Baxi E119 — the sibling low-pressure code on many models.
- Baxi E110 — an overheat / high-temperature lockout.
- Baxi E133 — an ignition or gas supply fault.
You can browse the full list on our Baxi fault codes hub.
Will boiler cover pay for an E118 repair?
It depends on the cause, and the following is general information rather than financial or insurance advice. If E118 is just a simple top-up, there is nothing to claim — it is a routine homeowner task.
But where the underlying problem is a genuine fault, such as a failed expansion vessel, a faulty PRV or a leak inside the boiler, that is the kind of repair many boiler cover policies are designed to help with, often including parts, labour and the call-out.
The detail is always in the policy: most insurers exclude pre-existing faults and may not cover wear-and-tear items unless your boiler passed an initial inspection, so check the terms before assuming a repair is covered.
To understand what is typically included, read what boiler cover includes, and weigh up whether a policy makes sense for you with is boiler cover worth it.
Is the Baxi E118 fault dangerous?
In itself, no. E118 is a protective lockout — the boiler has shut itself down because the water pressure is too low, which actually prevents damage. The thing to watch is the cause: if pressure keeps falling, there is a leak that should be found and fixed. If you ever smell gas, that is a separate emergency — call 0800 111 999.
Can I fix E118 myself?
Often, yes. Topping up the pressure via the filling loop to between 1 and 1.5 bar is a homeowner-safe job that involves no gas work. If the pressure will not hold, or the gauge already reads normal but the code persists, that needs a Gas Safe registered engineer.
How much does it cost to repair a Baxi E118 fault?
A simple repressurise costs nothing but your time. If an engineer is needed, expect roughly £70 to £120 for a diagnostic, and £120 to £350 for common repairs such as fixing a leak or replacing the expansion vessel or PRV, based on indicative 2026 UK prices.
Will E118 clear if I reset the boiler?
Only if the pressure is back in the correct range. Resetting alone will not help while the pressure is still low — top up to 1 to 1.5 bar first, then reset once. If it returns soon after, the system is losing water and needs an engineer rather than another reset.
Why does my Baxi keep showing E118 even after I top it up?
Repeated E118 codes almost always mean the system is losing water faster than normal — typically a leak, a waterlogged expansion vessel, or a weeping pressure relief valve discharging through an outside pipe. Constantly refilling masks the problem and can introduce fresh oxygen that corrodes the system, so book an engineer to trace the root cause.
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Compare boiler coverThis article is general information, not gas-safety or financial advice. Always have gas appliances checked and repaired by a Gas Safe registered engineer. In a gas emergency, call the National Gas Emergency line on 0800 111 999. Costs are indicative UK guides for 2026.