What the A04 code means
Combi and system boilers run on a sealed heating circuit that needs to hold a certain water pressure. A pressure sensor or switch inside the boiler checks this all the time. If the pressure falls too low, the boiler cannot be sure the heat exchanger is full of water, so it stops firing and shows A04.
Across current Vokera ranges, including the Vision, Evolve, Easi-Heat Plus, Compact and Unica, the user manuals list A04 as insufficient system water pressure. The action given is to check and refill the system pressure, reset and check the boiler works. That makes A04 one of the few Vokera codes a homeowner can usually clear alone.
A04 is protection rather than a breakdown. The real question is why the pressure dropped. A one-off fall after bleeding radiators is normal. Pressure that keeps falling points to a leak or a faulty component.
Common causes of A04
Gradual pressure loss
Sealed systems lose a little pressure over months. If the boiler has not been topped up for a long time, a slow drift down may be all that has happened.
Bleeding radiators
Each radiator you bleed releases air and a little water, which lowers the pressure. Bleeding several in one go can easily push the system below the level the boiler needs.
A leak in the system
Small leaks at radiator valves, pipe joints or towel rails can go unnoticed. Look for damp patches, stains, rust marks or crusty deposits. Our guide on a boiler leaking water covers the signs.
Pressure relief valve or expansion vessel
If pressure rises too high when the system heats up, the pressure relief valve opens and lets water out through a pipe that usually runs outside. The system then ends up too low once it cools. This cycle often points to an expansion vessel that has lost its charge.
Faulty pressure sensor
Less commonly, the gauge shows normal pressure but the boiler still reports A04. That suggests the sensor or its wiring is at fault. The gauge on the front and the sensor the boiler reads are separate parts, so it is possible for one to be accurate while the other is not.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Normal pressure loss | Gauge low, no other symptoms | You, via the filling loop |
| Recently bled radiators | Pressure dropped straight after bleeding | You, via the filling loop |
| Leak in the system | Pressure keeps falling, damp or staining | Gas Safe engineer or plumber |
| Expansion vessel or relief valve | Pressure high when hot, drips from outside pipe | Gas Safe engineer |
| Faulty pressure sensor | Gauge reads normal but A04 persists | Gas Safe engineer |
Safe checks you can do yourself
- Turn the heating off and let the system cool. Vokera manuals say the gauge should read between 1 and 1.5 bar when the boiler is off and cold.
- Find the filling loop or filling valve, usually under the boiler. On some models it is a braided hose with two valves, and on others a built-in tap or lever. Your manual shows where it is.
- Open the filling valve slowly until you hear water entering the system, and watch the gauge.
- Close the valve when the gauge reads between 1 and 1.5 bar. Do not overfill, and make sure the valves are fully closed afterwards.
- Reset the boiler using the method in your manual, either the reset button or the selector dial on some models.
- Check radiator valves, exposed pipes and the area under the boiler for water.
- Note the reading and check it each day for a week.
Our step-by-step guide on how to repressurise a boiler has more detail if you have not done this before.
If you bleed radiators, do it with the heating off and cool, and top up the pressure straight afterwards. That avoids an A04 later the same day.
When to call a Gas Safe engineer
Topping up once is routine. Vokera's own advice is that a system which regularly needs topping up may have a leak, so contact an engineer if:
- The pressure keeps falling after you have topped it up.
- Water is dripping from the pipe outside or from under the boiler.
- The gauge reads normal but A04 will not clear.
- The pressure climbs close to 3 bar when the heating is on.
- The code returns after two or three resets.
Do not remove the boiler casing to look for a leak inside. Internal leaks, the expansion vessel, the pressure sensor and the relief valve must be dealt with by a Gas Safe registered engineer.
Typical repair costs in the UK
If a top-up solves the problem, A04 costs nothing. If there is an underlying fault, these typical ranges include parts and labour.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Fixing a leaking radiator valve or joint | Typically £80 to £200 |
| Recharging the expansion vessel | Typically £80 to £150 |
| Replacing the pressure relief valve | Typically £120 to £220 |
| Replacing the pressure sensor | Typically £120 to £220 |
How to stop A04 coming back
Check the gauge once a month, especially before winter. A system that holds steady between 1 and 1.5 bar when cold is in good shape. Top up after bleeding radiators, and always close the filling valves fully. A yearly service lets an engineer check the expansion vessel and spot small leaks early, and our boiler service cost guide explains what that involves. If the pressure has gone the other way, read our guide on boiler pressure that is too high.