Vokera AL40 Fault Code: Causes, Fixes & Repair Costs
What does the Vokera AL40 fault code mean?
The AL40 code appears on Vokèra Mynute boilers when the system water pressure has fallen below the minimum safe operating level. It is the persistent low-pressure lockout specific to the Mynute range — equivalent to the A40 code you may see displayed on other Vokèra models such as the Compact and Linea. When pressure drops too low for safe circulation, the boiler shuts itself down as a protective measure and will not restart until the fault is resolved. On most Vokèra boilers the target cold pressure is around 1.0–1.5 bar; anything below roughly 0.5–0.8 bar will typically trigger this code.
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
- System pressure topped up then leaked back down Common
The most frequent scenario is that the homeowner or an engineer has repressurised the boiler in the past, but a slow leak somewhere in the system — at a radiator valve, pipe joint, or internal component — has allowed pressure to gradually fall again. If AL40 keeps returning within days or weeks of repressurisation, a leak is the most likely culprit.
- Recent radiator bleeding Common
Bleeding air from radiators releases a small amount of water along with the trapped air. If several radiators were bled without subsequently topping up the system pressure, the AL40 code can appear shortly afterwards.
- Gradual natural pressure loss over time Common
All sealed central heating systems lose a small amount of pressure over months and years, even without any obvious leak. If the boiler has not been topped up for a long time, pressure can drift below the minimum threshold and trigger this code.
- Faulty or waterlogged expansion vessel Sometimes
The expansion vessel absorbs pressure changes as water heats and cools. If the vessel's internal diaphragm has failed or it has become waterlogged, pressure can fluctuate wildly and drop below safe levels, causing repeated AL40 faults even after repressurisation.
- Pressure relief valve (PRV) weeping or discharging Sometimes
A PRV that has started to weep or fully discharge will steadily bleed water from the system, causing pressure to fall. You may notice a small trickle of water from the overflow pipe outside the property. This requires an engineer to diagnose and replace the valve.
- Cold weather causing pressure contraction Rare
Water contracts slightly in very cold conditions, which can cause a marginal pressure dip. In unusually cold snaps this alone can tip a borderline system below the threshold. A frozen condensate pipe can also cause a separate boiler shutdown that is sometimes misread as a pressure fault.
How to fix it
- Check the boiler pressure gauge DIY safe
Most Vokèra boilers have a pressure gauge either on the front panel or visible through a small window. If the needle is below 1 bar (or the display reads below 1.0), low pressure is confirmed as the immediate cause. If the pressure reads within the normal range (1.0–1.5 bar), the fault may have been a transient dip — try a reset before going further.
- Locate the filling loop and top up the system pressure DIY safe
The filling loop is usually a short braided-metal or plastic hose connected underneath the boiler, with one or two manual valves. Slowly open the valve(s) and watch the pressure gauge rise. Stop when the needle reaches around 1.2–1.5 bar — do not overfill. Once done, close the filling loop valve(s) firmly and, if it is a removable type, disconnect and store the hose.
- Reset the boiler DIY safe
Press and hold the reset button (often labelled R, or indicated by a symbol on the control panel) for approximately 3–5 seconds until the AL40 fault stops flashing. Release the button and allow the boiler to go through its start-up sequence. If it lights successfully and the error clears, monitor the pressure gauge over the next few days.
- Monitor the pressure gauge over the following days DIY safe
Check the gauge every day or two. A healthy system should hold pressure at roughly the level you set it to — small drops of 0.1–0.2 bar are normal over weeks, but if the pressure falls back below 1 bar within a day or two of topping up, there is a leak or a component problem that needs an engineer's attention.
- Check for visible water leaks around radiators and pipework DIY safe
Walk around the property and inspect radiator valves, pipe joints, and anywhere you can see exposed pipework. Look for damp patches, rust staining, or small drips. If you find an obvious external leak, note the location so you can describe it to an engineer.
- Call a Gas Safe registered engineer if the fault persists or keeps returning Gas Safe engineer
If the AL40 code returns more than once or twice after repressurisation, or if you cannot identify a simple external cause, a qualified engineer needs to investigate. They will pressure-test the system to find hidden leaks, check the expansion vessel pre-charge pressure, inspect the PRV, and assess internal boiler components such as the heat exchanger and pressure sensor. Do not attempt to work on the gas valve, internal seals, the PRV, or any gas-carrying component yourself.
Parts you may need
- Expansion vessel (replacement) · from £60
- Pressure relief valve (PRV) · from £25
- Filling loop assembly · from £15
- Pressure sensor / transducer · from £30
- Radiator valve (replacement pair) · from £20
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–£400, depending on the underlying cause.
Frequently asked questions
Can I fix the Vokèra AL40 fault myself, or do I need an engineer?
Topping up the system pressure via the filling loop and resetting the boiler is straightforward and safe to do yourself — no tools or gas work are involved. However, if the pressure drops again within a day or two, that points to a leak or a failed component such as the expansion vessel or PRV, and a Gas Safe registered engineer will need to investigate. Repeatedly topping up without finding the underlying cause is not a long-term fix.
What pressure should my Vokèra boiler be at?
When the system is cold, the pressure should sit between 1.0 and 1.5 bar — 1.2 bar is a good target. It is normal for pressure to rise a little (sometimes up to 2.0–2.5 bar) when the heating is running and the water is hot, then fall back once the system cools. If the cold pressure consistently drops below 1.0 bar, the system needs topping up or an engineer needs to find out why it keeps losing pressure.
Why does my Vokèra boiler keep losing pressure and showing AL40?
Recurring pressure loss almost always means water is escaping the sealed system somewhere. The most common culprits are a weeping radiator valve or pipe joint, a failing expansion vessel (the internal diaphragm can split and allow water to fill it entirely), or a PRV that has started to discharge — you may spot a damp patch or trickle of water near the overflow outlet on an outside wall. A Gas Safe engineer can pressure-test the system to pinpoint the source. In rarer cases a faulty pressure sensor can give a false AL40 reading even when the actual pressure is fine.
How much does it cost to fix a Vokèra AL40 low-pressure fault in the UK?
If you can repressurised the system yourself, there is no cost at all. If an engineer is needed just to top up and reset, expect to pay roughly £60–£100 for the call-out. The most common paid repairs — tracking and fixing a small leak or replacing the expansion vessel — typically cost between £120 and £400 including parts and labour. More significant work such as replacing the heat exchanger or PCB can cost considerably more (£450–£800+), but these are uncommon causes of an AL40 fault and would normally only come to light after thorough diagnostics.