Baxi E117 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E117 fault code mean?
The E117 code appears on Baxi boilers when the water pressure inside the central heating system climbs above approximately 2.9 bar — well beyond the safe operating window. At that level the boiler locks itself out as a protective measure, refusing to fire until the pressure is brought back into the normal cold-system range of 1.0–1.5 bar. Note: a small number of engineers and at least one trade source suggest that on certain Baxi models E117 can also relate to a circulation-detection issue rather than overpressure. If your pressure gauge reads within the normal range but the code keeps returning, mention this to your engineer so they can investigate the circulation side as well.
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
- Overfilled system Common
The most frequent trigger. If the filling loop was left open a fraction too long — whether recently or some time ago — the system simply holds too much water. As the water heats up and expands, pressure climbs past 2.9 bar and the boiler shuts down. Bleeding a radiator is usually enough to bring it back into range.
- Faulty or weeping filling loop Common
The filling loop's isolating valves can develop a slight internal leak over time, allowing mains water to trickle into the sealed system continuously. You may notice pressure that keeps creeping upward even after you've bled the system down. Closing and physically disconnecting the loop can confirm whether this is the source.
- Failed expansion vessel Common
The expansion vessel is designed to absorb the extra volume of water as it heats up. If the internal diaphragm has split or the air pre-charge pressure has dropped, there is nowhere for the expanding water to go and system pressure spikes instead. A ruptured diaphragm often shows as water weeping from the Schrader valve on the vessel.
- Faulty pressure sensor Sometimes
The pressure sensor that feeds the reading to the boiler's control board can give false high readings if it has accumulated limescale or suffered an electrical fault. The system pressure gauge on the boiler front may look normal while the sensor is telling the PCB otherwise, causing a phantom E117 lockout.
- Blocked or restricted system components Rare
Sludge or scale deposits in radiators, pipework, or the heat exchanger can restrict flow enough to create localised pressure spikes. More common in older systems or those that have never been power-flushed, particularly in hard-water areas of the UK.
How to fix it
- Switch the boiler off and check the pressure gauge DIY safe
Turn the boiler off at the programmer or room thermostat. Look at the pressure gauge on the boiler front panel — if it reads above 2.0 bar when the system is cold, overpressure is confirmed. Do not attempt to restart the boiler while pressure remains high.
- Make sure the filling loop is fully closed DIY safe
Locate the filling loop — usually a braided flexible hose with one or two blue or grey valves beneath the boiler. Both handles should be at 90° to the pipe (closed position). If either feels loose or is slightly open, close it firmly. A weeping loop can be temporarily disconnected if you are confident doing so.
- Bleed a radiator to release excess pressure DIY safe
Use a radiator bleed key on the bleed valve at the top of a radiator — ideally an upstairs one. Hold a cloth or small container underneath and open the valve slowly by a quarter turn. Let water trickle out until the pressure gauge drops to around 1.2–1.3 bar, then close the valve. Do not over-bleed; stop before the gauge falls below 1.0 bar.
- Reset the boiler once DIY safe
With pressure now reading 1.0–1.5 bar, press and hold the reset button for roughly 3 seconds until the display clears. Allow the boiler to complete its ignition sequence. If it fires successfully and holds pressure, monitor the gauge over the next day or two to check pressure is stable.
- Check the expansion vessel air pressure (engineer task if unsure) Gas Safe engineer
A Gas Safe engineer will isolate the system, release pressure, and use a gauge on the Schrader valve of the expansion vessel to check the air pre-charge — typically 0.75 bar for most domestic systems. If the charge is low, it can be topped up with a pump; if water escapes from the valve the diaphragm is ruptured and the vessel must be replaced.
- Have the pressure sensor tested or replaced if needed Gas Safe engineer
If the system pressure gauge reads within the normal range but E117 keeps appearing, the engineer should test the electrical signal from the pressure sensor and compare it against actual system pressure. A faulty sensor will need to be replaced; this is an internal boiler component and must be handled by a Gas Safe registered engineer.
- Call a Gas Safe registered engineer if the fault persists or returns Gas Safe engineer
If the E117 code comes back after bleeding and resetting, or if you suspect a failed expansion vessel, leaking filling loop internals, or a faulty sensor, do not keep resetting the boiler. Contact a Gas Safe registered engineer to carry out a full pressure diagnosis and carry out any required part replacements safely.
Parts you may need
- Expansion vessel (8–12 litre, domestic) · from £45
- Expansion vessel diaphragm kit · from £20
- Filling loop assembly (Baxi compatible) · from £25
- Pressure relief valve · from £18
- Pressure sensor / transducer · from £30
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 a Baxi E117 myself or do I need an engineer?
Some of the initial checks are perfectly safe for a homeowner: closing the filling loop, bleeding a radiator to bring pressure down, and resetting the boiler once. However, if those steps don't clear the fault — or if pressure keeps rising again — you'll need a Gas Safe registered engineer. Replacing an expansion vessel, pressure sensor, or filling loop internals involves either working inside the boiler casing or on sealed system components, none of which should be attempted without the correct qualifications.
Why does my Baxi boiler pressure keep rising and causing E117?
A repeatedly rising pressure almost always points to either a weeping filling loop or a failed expansion vessel. If the filling loop is seeping mains water into the system, pressure will creep up continuously. If the expansion vessel diaphragm has burst, the system has no buffer for hot-water expansion and pressure spikes every time the boiler fires. Both faults need an engineer to resolve properly.
How much does it cost to fix a Baxi E117 in the UK?
Most E117 repairs fall in the range of £120–£380 including parts and labour. Bleeding the system and checking the filling loop might cost only a call-out fee (£80–£120). Replacing an expansion vessel typically comes in at £200–£350 once labour is included. A pressure sensor swap is usually at the lower end. In rare cases where the fault is linked to a damaged heat exchanger or PCB, costs can be considerably higher, but those scenarios are uncommon for a pressure-related lockout.
What is the correct pressure for a Baxi boiler?
Baxi recommend a cold-system pressure of 1.0–1.5 bar. When the heating is running and the water is hot, it's normal to see the gauge rise slightly — up to around 2.0 bar is acceptable. E117 trips at approximately 2.9 bar. If your gauge regularly sits above 2.0 bar when the system is cold, have an engineer investigate before a lockout occurs.