Vokèra A04 fault codewhat it means and how to fix it
System water pressure is below the minimum the appliance will run at. Topping the system back up to 1.2 bar normally clears it straight away.
Before you start
Is there any sign of water anywhere around the system?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
Nothing visible anywhere
Most likely normal slow loss, or a leak somewhere concealed. Top up and monitor the interval before spending money on a leak trace.
Top up to 1.2 bar, note the date, and check the gauge weekly for a month.
A pipe outside the wall near the boiler is dripping or stained
That is the safety valve discharge, and it means the system is being over-pressurised while it heats. The usual reason is a failed expansion vessel rather than anything you have done wrong.
Top up to 1.2 bar so you have heating, but book an engineer to check the expansion vessel charge. Topping up alone will not fix this and you will be back within days.
Damp patch, staining, or a wet spot near a radiator or under the floor
You have found the leak. That is genuinely good news because a located leak is far cheaper to fix than a hunted one.
Photograph it, note where it is, and get it repaired before topping up again. Continued refilling accelerates corrosion throughout the system.
What A04 means
A04 is Vokèra reporting insufficient system water pressure, which is their own wording for it. The appliance monitors pressure continuously and stops the burner when it falls too low — the Vision manual gives 0.5 bar as the point at which the boiler may lock out, and the later Vision Plus displays a low pressure warning below 0.6 bar and a high one above 3.0 bar. That cut-out protects the heat exchanger from being fired without enough water passing through it. This is the most straightforward fault in the Vokèra range and the one most often resolved without an engineer.
The pressure itself is the thing worth thinking about, though. A sealed heating system is closed — the water in it has nowhere to go — so a falling gauge means water has physically left the system somewhere. Once or twice a year is normal ageing. Every few weeks is a leak, and the repeated refilling that comes with it introduces fresh oxygenated water, which is what generates the black magnetite sludge that eventually costs far more than the leak would have done.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Normal slow loss over months | Small amounts escape through valve glands and the automatic air vent. If you top up once a year, nothing is wrong. | Common |
| Radiators recently bled | Bleeding removes water as well as air. A round of bleeding across the house will drop the gauge noticeably. | Common |
| Leak on the system | Often a compression joint under floorboards or a slow weep at a radiator valve gland. Rate of loss is the diagnostic clue. | Occasional |
| Expansion vessel failed | Once the vessel loses its air charge, heating the system pushes pressure past the safety valve setting and water discharges outside. The gauge then reads low when everything is cold again. | Occasional |
What to try yourself
Read the pressure gauge with the system cold. On the Vision it is on the display; on older Vokèra appliances it is a small analogue dial low on the front of the case.
Locate the filling loop. Vokèra commonly fit an integral loop operated by a small lever or key underneath the appliance rather than an external braided hose, so look underneath before assuming there is not one.
Open it slowly and watch the gauge climb. Close it at 1.2 bar. The last part of the range fills quickly, so do not walk away.
Do not exceed 2.5 bar cold. Overfilling makes the safety valve discharge outside, which puts you back below minimum once the system cools.Close the loop completely and press reset for two to three seconds. On an integral loop, make sure the key is returned to the closed position and removed.
Note the date somewhere. The interval between top-ups is the single most useful piece of information if this becomes a recurring problem.
Get a Gas Safe registered engineer to look at your Vokèra A04
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What it costs to fix
Typical repair cost
£0–£450
Time on site
10 minutes
Nothing if it is a top-up. A pressure sensor replacement is £120 to £200 fitted, a leak trace £90 to £160, and an expansion vessel £180 to £450 depending on how awkward the appliance is to get into.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Expansion vessel (8 litre, sealed system)
EV-8L£35–£85Plumbing merchant
Price checked 28 September 2026
Pressure relief valve, 3 bar
PRV-3B£18–£42Plumbing merchant
Price checked 28 September 2026
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Related Vokèra fault codes
- A01Get it checkedThe burner failed to light and the appliance has locked out. Check the gas supply and, in cold weather, the condensate pipe before calling anyone.Related: same brand
- A02Get it checkedThe safety limit thermostat has tripped on overheat. In practice this nearly always means water has stopped circulating properly.Related: same brand
- A03Get it checkedThe appliance cannot confirm the fan is running correctly, so it will not permit the burner to light. This needs an engineer.Related: same brand
- A11Stop — call an engineerThe appliance is detecting a flame when there should not be one. Turn it off at the fused spur and get a Gas Safe engineer out.Related: same brand
- A09Get it checkedThe flue gas sensor has recorded a temperature above the safe limit, or the sensor itself has failed. Either way the appliance stops.Related: same brand
- A07Get it checkedThe primary flow sensor is reading out of range or has failed. The appliance stops because it can no longer tell how hot it is running.Related: same brand
Own a Vokèra boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.