Vokera 04 Fault Code: Causes, Fixes & Repair Costs
What does the Vokera 04 fault code mean?
The Vokera A04 fault code indicates that the water pressure in your central heating system has dropped below the minimum level required for safe operation. Vokera boilers need a system pressure of between 1 and 1.5 bar when the boiler is switched off and cold. If the pressure sensor detects a reading below this threshold, the boiler will lock out and display code 04 or A04 to protect internal components. Note that on a small number of older Vokera models this code has been associated with an overheat or fan-related fault, so if the guidance below does not match what you are seeing, cross-reference with your specific model's manual. For the vast majority of Vokera boilers, however, A04 means low system water pressure.
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 naturally too low Common
Over time, all sealed central heating systems lose a small amount of pressure. This is entirely normal, especially if it has been several months since the boiler was last checked. If the pressure gauge reads below 1 bar with the boiler cold, a straightforward top-up via the filling loop is likely all that is needed.
- Radiators recently bled Common
Bleeding radiators releases trapped air but also reduces the overall water volume in the system, causing the pressure gauge reading to drop. If you or an engineer have bled radiators recently and the A04 code has appeared shortly afterwards, a top-up is the likely fix.
- Slow leak in the system Common
A weeping joint, a pinhole in a radiator, a dripping radiator valve, or a leaking boiler component can cause pressure to fall repeatedly. If you find yourself topping up the system more than once every few months, a leak is the most probable cause and a Gas Safe engineer should carry out a full system inspection.
- Faulty or waterlogged expansion vessel Sometimes
The expansion vessel absorbs pressure changes as the system heats and cools. If the vessel's internal membrane has failed or the pre-charge pressure is too low, system pressure can drop sharply after heating cycles. This typically causes repeated A04 faults rather than a one-off occurrence.
- Pressure relief valve (PRV) weeping Sometimes
If the PRV is discharging water through the overflow pipe outside your property, it is releasing pressure from the system. A PRV that weeps constantly will cause the system pressure to fall steadily, triggering A04. This requires a Gas Safe engineer to assess and replace the valve if necessary.
- Faulty pressure sensor or flow switch Rare
If the system pressure gauge reads a healthy 1–1.5 bar but the A04 code is still present, the pressure sensor or flow switch that feeds data to the boiler's PCB may be giving an incorrect reading. An engineer would test continuity across the flow switch and replace the faulty component if needed.
- System not refilled after repair or drain-down Rare
Following any maintenance work that involved draining the heating circuit, the system must be refilled to the correct pressure before the boiler is restarted. If this step was missed or the circuit was only partially refilled, A04 will appear immediately on start-up.
How to fix it
- Check the pressure gauge on your boiler DIY safe
Locate the pressure gauge on the front or underside of your Vokera boiler. With the boiler switched off and fully cooled, the needle should sit between 1 and 1.5 bar. If it reads below 1 bar, the system needs topping up. If the gauge already reads within the correct range, skip ahead to step 4, as the issue may be a faulty sensor rather than genuinely low pressure.
- Top up system pressure using the filling loop DIY safe
Find the filling loop — this is typically a braided flexible hose with one or two valves located beneath the boiler. Slowly open the valve (or both valves if there are two) and watch the pressure gauge rise. Stop when it reaches approximately 1.2 bar. Close the filling loop valve(s) fully before doing anything else. Never overfill beyond 1.5 bar as this can cause the PRV to discharge.
- Bleed your radiators if they feel cool at the top DIY safe
Trapped air can contribute to pressure fluctuations and reduced heating efficiency. Using a radiator bleed key, open each bleed valve slightly until water runs out steadily with no air bubbling. Have a cloth ready to catch drips. Once all radiators are bled, recheck the boiler pressure gauge and top up again if it has dropped below 1 bar.
- Reset the boiler DIY safe
Once pressure is restored to the correct level, reset the boiler. On Vokera models with a selector dial, move the dial to the OFF/RESET position, wait a few seconds, then move it back to your preferred operating mode (hot water only, or heating and hot water). On models with a dedicated reset button, press and hold it for a few seconds until the boiler restarts. If the boiler fires up and runs normally, monitor the pressure gauge over the next week or so to check it remains stable.
- Check for visible leaks around the boiler and radiators DIY safe
If the A04 code returns within a short period, carry out a visual inspection of pipework joints, radiator valves, and the area beneath and around the boiler. Look for damp patches, white limescale deposits, or rust staining — all of which can indicate a slow weeping joint. Also check outside your property for the boiler's overflow pipe; if water is dripping from it, the PRV may be discharging. Do not attempt to repair any leaks yourself — this requires a Gas Safe engineer.
- Do not continue repeatedly topping up — call a Gas Safe registered engineer Gas Safe engineer
If pressure drops again within a few weeks of being topped up, or if the A04 code returns immediately after a reset, there is an underlying fault that needs professional diagnosis. This could be a leaking joint, a failed expansion vessel, a weeping PRV, or a faulty pressure sensor or flow switch. A Gas Safe registered engineer will carry out a full system pressure test, identify the source of the problem, and carry out any necessary repairs legally and safely.
Parts you may need
- System pressure sensor / pressostat · from £35
- Flow switch · from £40
- Pressure relief valve (PRV) · from £25
- Expansion vessel (boiler-mounted) · from £60
- Filling loop assembly · 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 £100–£380, depending on the underlying cause.
Frequently asked questions
Can I top up my Vokera boiler pressure myself?
Yes — re-pressurising via the filling loop is one of the few boiler tasks that is safe for a homeowner to carry out. Locate the filling loop beneath the boiler, slowly open the valve until the gauge reaches around 1.2 bar, then close the valve and reset the boiler. However, if the pressure drops repeatedly and the A04 code keeps coming back, stop topping up and call a Gas Safe engineer to find the root cause.
Why does my Vokera boiler keep losing pressure and showing A04?
If the fault code returns within a few weeks of being topped up, something is causing the system to lose water. The most common culprits are a slow leak at a pipe joint, radiator valve, or inside the boiler itself, a waterlogged expansion vessel that can no longer absorb pressure changes, or a pressure relief valve that is weeping and releasing water outside. A Gas Safe engineer can carry out a pressure test to pinpoint the source. Do not keep re-pressurising without investigating, as masking a leak can cause wider damage over time.
How much does it cost to fix a recurring A04 pressure fault on a Vokera boiler?
If a simple top-up resolves the issue, the fix costs nothing. For professional repairs, a PRV replacement typically costs around £100–£150 including labour, while replacing a faulty expansion vessel usually runs between £150 and £350. Leak repairs to pipework or radiator valves vary depending on access and the number of joints affected but commonly fall in the £100–£250 range. For rare cases where a water-damaged PCB needs replacing, costs can rise to £400–£500 — but this is uncommon for a straightforward pressure fault and would only arise if a significant leak had been left unaddressed for some time.
My Vokera boiler gauge shows the correct pressure but the A04 code is still on — why?
If the pressure gauge reads between 1 and 1.5 bar but A04 is still displayed, the pressure sensor or flow switch feeding information to the boiler's PCB is likely faulty and giving a false low-pressure reading. An engineer will check continuity across the flow switch and test the sensor. Either component can be replaced at a relatively modest cost compared to larger parts. It is also worth checking that the gauge itself is accurate, as mechanical gauges can stick or give misleading readings over time.