Vokera A04 Fault Code: Low System Water Pressure
The A04 code on a Vokera boiler indicates that the system water pressure has dropped below the minimum level required for safe operation. When cold and at rest, a Vokera boiler typically needs to read between 1 and 1.5 bar on the pressure gauge. If pressure falls significantly below this, the boiler will lock out and display A04 rather than continue running. Topping up the pressure via the filling loop will often clear the fault, but if pressure keeps dropping, an underlying leak or component fault needs professional attention.
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.
What does the Vokera A04 fault code mean?
The A04 code on a Vokera boiler indicates that the system water pressure has dropped below the minimum level required for safe operation. When cold and at rest, a Vokera boiler typically needs to read between 1 and 1.5 bar on the pressure gauge. If pressure falls significantly below this, the boiler will lock out and display A04 rather than continue running. Topping up the pressure via the filling loop will often clear the fault, but if pressure keeps dropping, an underlying leak or component fault needs professional attention.
Common causes
- Gradual pressure loss over time Common
All sealed heating systems lose a small amount of pressure naturally through micro-permeation and minor evaporation at joints. Over months or years this can drop the pressure gauge below the threshold that triggers A04, even without a visible leak anywhere in the system.
- Recent radiator bleeding Common
Bleeding air from radiators releases both air and a small volume of water, which can drop the overall system pressure noticeably. If you or an engineer bled radiators recently and no one topped up the pressure afterwards, this is very likely the cause.
- Leak in pipework, radiators, or fittings Common
A slow drip or pinhole leak — for example in copper pipework hidden beneath floorboards or behind walls — can cause pressure to fall repeatedly. The leak may not be visible at all, yet the boiler will keep losing pressure and returning to A04 after every top-up.
- Faulty pressure relief valve (PRV) Sometimes
The pressure relief valve is designed to open and discharge water if pressure becomes dangerously high. If the valve seat degrades or the valve is ageing, it can weep water continuously even at normal operating pressures, causing a steady pressure drop and triggering A04.
- Faulty auto air vent Sometimes
An auto air vent that is stuck open or has a worn seal will allow water to escape from the system slowly. This is often overlooked but is a recognised cause of persistent low-pressure faults on Vokera boilers.
- Cracked heat exchanger Rare
A hairline crack in the primary heat exchanger creates an internal leak that gradually reduces system pressure. This is a more serious fault — heat exchangers are rarely repairable and usually need full replacement, making this one of the costlier outcomes of a persistent A04.
How to fix it
- Check the pressure gauge reading DIY safe
Find the pressure gauge on the front or underside of your Vokera boiler. With the boiler off and the system cold, the needle should sit between 1 and 1.5 bar. If it reads below 0.8 bar or the needle is in the red zone, low pressure is almost certainly the cause of the A04.
- Repressurise via the filling loop DIY safe
Locate the filling loop — usually a short braided hose with one or two valves beneath the boiler. Slowly open the valve(s) and watch the gauge rise. Stop when the needle reaches around 1.3 bar (do not exceed 1.5 bar). Close the valve(s) fully, then reset the boiler using the reset button. The A04 should clear and the boiler should fire up normally.
- Monitor the pressure over the following 48 hours DIY safe
Keep an eye on the gauge over the next day or two. A healthy system should hold pressure without needing another top-up. If the needle drops back noticeably within 48 hours, you have a leak somewhere in the system and should not keep topping up — move on to the next step.
- Check for visible leaks around radiators, pipework, and the boiler DIY safe
With the heating running, inspect radiator valves, pipe joints, and any visible pipework for damp patches, staining, or drips. Also check around the boiler casing and underneath it. Even a slow drip will leave limescale marks or rust staining over time. If you find an obvious leak, note its location for the engineer.
- Do not keep topping up if pressure continues to fall DIY safe
Repeatedly repressurising the system without fixing the underlying cause is not a solution — it simply delays the repair while potentially allowing water damage to spread. If the boiler has needed topping up more than once in a short period, stop and call a professional rather than continuing to mask the fault.
- Call a Gas Safe registered engineer to locate and repair the leak Gas Safe engineer
A qualified engineer can carry out a pressure test to pinpoint hidden leaks, inspect the pressure relief valve and auto air vent, and check the heat exchanger for signs of cracking. Any component replacement — including the PRV, auto air vent, or heat exchanger — must be carried out by a Gas Safe registered professional. Do not attempt to open or modify any gas or sealed system components yourself.
Parts you may need
- Pressure relief valve (PRV) · from £25
- Auto air vent · from £15
- Filling loop assembly · from £20
- Primary heat exchanger · from £180
- System pressure gauge · 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 £120–£350, depending on the underlying cause.
Frequently asked questions
Can I top up the pressure myself to fix the Vokera A04 fault?
Yes — repressurising via the filling loop is a safe DIY task. Bring the cold pressure up to around 1.3 bar, close the valve, and reset the boiler. If the pressure holds and the A04 does not return, the issue was simply a gradual pressure drop that is normal over time. However, if the pressure drops again within a day or two, do not keep topping up. Repeated pressure loss points to a leak that needs a Gas Safe engineer to find and fix.
Why does my Vokera boiler keep losing pressure and showing A04?
If the A04 keeps returning after topping up, your system has a leak somewhere. This could be a dripping radiator valve, a weeping pressure relief valve, a faulty auto air vent, or in more serious cases a hairline crack in the heat exchanger. A hidden leak in pipework under floors or behind walls can also be the culprit. A Gas Safe engineer can pressure-test the system to find the source.
How much does it cost to fix a Vokera A04 fault?
For most households the repair falls between £120 and £350. Straightforward jobs like replacing a pressure relief valve or sealing an accessible pipe joint sit at the lower end of this range. If the fault turns out to be a cracked heat exchanger, costs can rise to £450–£550 for the part and labour, and a water-damaged PCB can cost a similar amount to replace. If your boiler is older and facing a major repair, it is worth comparing the repair cost against a new boiler installation.
Is the Vokera A04 fault dangerous?
A04 itself is not a gas safety emergency — it is the boiler protecting itself by shutting down when water pressure is too low. You are safe to top up the pressure and reset the unit as described. That said, do not ignore a persistent or recurring A04. A slow water leak left unattended can eventually cause water damage to electrical components such as the PCB, turning a minor repair into a much more costly one. If in any doubt, call a Gas Safe registered engineer.