Vokera Boiler Fault Codes: What A01, A04 and Others Mean
Vokera boiler showing a fault code or a red light? What the common A-codes and older AL-codes mean, the two checks you can safely make yourself, how to reset it and when a Gas Safe engineer is needed.
A Vokera fault code tells you why the boiler has stopped. On most current Vokera boilers the code starts with A: A01 is an ignition failure (no flame sensed), A04 is low system water pressure, A03 is a fan fault and A07, A08 and A09 are temperature sensor or overheat faults. Older models such as the Unica HE and Mynute show AL codes instead, where AL40 and AL41 point to low pressure. You can safely check the pressure (Vokera asks for 1 to 1.5 bar with the boiler cold) and reset once by turning the selector across the OFF/RESET position after two minutes. If the code comes back, the boiler needs a Gas Safe engineer.
Key Takeaways
- A04 (or AL40/AL41 on older boilers) is low water pressure, the one code most people can clear themselves.
- A01 means the boiler tried to light and did not sense a flame. Check the gas is on elsewhere in the home before calling.
- Vokera's manuals ask for 1 to 1.5 bar on the gauge with the boiler cold, and say it may lock out below 0.5 bar.
- Wait two minutes, then turn the mode selector across OFF/RESET. Reset once, not repeatedly.
- A code that returns after a reset, or any fan, sensor or overheat code, needs a Gas Safe registered engineer.
A fault code on a Vokera boiler is the boiler telling you why it has stopped. On most current Vokera models the code starts with an A: A01 means it failed to light, A04 means the water pressure is too low, and the rest point to the fan, the temperature sensors or overheating. Two of these you can often sort yourself in five minutes. The others need a Gas Safe engineer. This guide covers what each code means, the checks that are safe to make and when to stop and book a repair.
Which Vokera code system you have
Vokera has used more than one set of codes over the years, so start by matching the display to your boiler:
- A-codes (A01 to A11, A77) appear on the Vision range and many other current Vokera combi and system boilers.
- AL-codes (AL10 to AL79) appear on older models such as the Unica HE and Mynute.
- E-codes appear on the Evolve range.
The model name is printed on the front of the boiler or on the data badge underneath. The user booklet for your exact model is the final word, and Vokera publishes most of them on its website.
Vokera A-codes explained
These descriptions come from Vokera's own fault code table for the Vision range.
| Code | What Vokera says it means | What to do |
|---|---|---|
| A01 | Ignition failure, flame not sensed, internal fault | Check the gas supply, reset once, call an engineer if it returns |
| A02 | Limit thermostat fault | Reset once, call an engineer if it returns |
| A03 | Fan tacho signal fault | Reset once. A fan fault needs an engineer |
| A04 | Insufficient system water pressure | Top up to 1 to 1.5 bar and reset |
| A06 | Configuration fault | Engineer |
| A07 | Primary (flow) thermistor fault or over temperature | Reset once, then engineer |
| A08 | Return thermistor fault, over temperature or poor circulation | Reset once, then engineer |
| A09 | Flue thermistor fault or flue over temperature | Engineer |
| A11 | False flame (flame sensed when it should not be) | Engineer if it keeps appearing |
| A77 | Low temperature thermostat fault (underfloor circuits) | Engineer |
Two display messages are not faults: ADJ is calibration mode and ACO is service operation, both used by engineers. _St (autostop) and _CL (call for service) mean the boiler wants an engineer.
A01: the boiler will not light
A01 is the most common Vokera lockout. The boiler has tried to light the burner and has not detected a flame. Before anything else, check whether the gas is on. Try the hob, and if you have a prepayment meter, check it has credit. If the whole home has no gas, the problem is the supply, not the boiler. In cold weather, also check the white plastic condensate pipe outside, because a frozen pipe can stop the boiler firing. Our guide to a blocked or frozen boiler flue and condensate pipe explains how to thaw it safely.
If the gas is on and the pipe is clear, reset once. If A01 returns, the cause is almost always inside the case: the ignition or flame sensing electrode, the gas valve, the PCB or a flue problem. That is Gas Safe work.
A04: low water pressure
A04 means the pressure in the heating system has dropped too low for the boiler to run. Vokera's manuals ask for 1 to 1.5 bar on the gauge with the boiler cold, and say the boiler may lock out below 0.5 bar. Top it up using the filling loop (see below) and reset. If A04 comes back within days, water is escaping somewhere, typically a leaking radiator valve, the pressure relief valve or a failing expansion vessel. Our article on why a boiler keeps losing pressure walks through each cause.
A07, A08 and A09: temperature and overheat faults
These codes come from the thermistors, the small sensors that measure water and flue temperature. They can mean a sensor or its wiring has failed, or that the water really is getting too hot because it is not circulating. Poor circulation often comes from a sticking pump, sludge in the system or closed radiator valves. Vokera's action for A08 includes checking the pump and making sure there is enough circulation around the heating circuit. You can check that radiator valves are open, but the sensor and pump checks are for an engineer.
Older AL codes and Evolve E-codes
On older Vokera boilers such as the Unica HE, the manual lists AL codes. The ones to know:
- AL40 and AL41: check the system pressure and refill if needed, then reset. These are the low pressure codes.
- AL52 and AL55: Vokera simply says call an engineer.
- AL10, AL20, AL21, AL26, AL28, AL34, AL60, AL74, AL79: reset the appliance and call an engineer if the fault comes back.
The Evolve range uses E-codes, with separate codes for flame lockout, fan and water pressure faults. Check the booklet for the exact meaning on your model.
What you can safely check
You should never take the front cover off a gas boiler. These checks are all outside the case:
- Gas supply. Is the hob working? Is the meter on and in credit?
- Pressure gauge. With the boiler cold, the needle should sit between 1 and 1.5 bar.
- Condensate pipe. In freezing weather, look for ice where the pipe leaves the wall.
- Power and controls. Is the fused spur switched on, and is the thermostat or programmer calling for heat?
- Radiator valves. For overheat codes, make sure a few radiators are open so water can circulate.
How to top up the pressure on a Vokera
Let the boiler cool
Switch the heating off and wait until the boiler and pipes are cool, so the gauge gives a true cold reading.
Find the filling loop
Vokera's instructions say the filling valve connections are usually beneath the boiler. On some models the loop is a flexible hose you connect between two valves.
Open the valve slowly
Open the filling valve until you hear water entering and watch the gauge rise.
Close at 1 to 1.5 bar
Close the valve when the gauge reads between 1 and 1.5 bar, then disconnect the loop if it is a removable one.
If you overfill, do not open the pressure relief valve yourself. Our guide on boiler pressure that is too high covers how to bring it down by bleeding a radiator.
How to reset a Vokera boiler
Vokera's instructions are to check the action required for the code first, then allow about two minutes before turning the mode selector knob across the OFF/RESET position and back to your usual setting. Some newer models have a dedicated reset button instead. Either way, reset once. Vokera's own manuals note that if the boiler needs resetting frequently, it may indicate a fault. For a general walkthrough across brands, see how to reset a boiler.
When to call an engineer
Book a Gas Safe registered engineer if:
- the same code returns after one reset;
- you see A01 and the gas supply is fine;
- you see a fan, sensor, flue or overheat code (A03, A06 to A09, A11, A77);
- A04 keeps coming back because the pressure keeps dropping;
- the boiler is making unusual noises alongside the code. Our guide to a noisy boiler explains what the common sounds mean.
When you call, tell us the boiler model, the exact code, the pressure reading and whether the code came back after a reset. That lets the engineer bring the likely parts. Our labour is £95 an hour, the £60 call-out is waived when the job runs over 2 hours, and we confirm the price before any work starts. For boilers that have stopped completely, see our boiler repair service and our guide to a boiler that is not working.
Vokera code that won't clear?
Tell us the model and the code. We'll arrange a Gas Safe engineer and confirm the price before starting.
📞 Call 020 7870 3200Or see our boiler repair service and pricing.
Frequently Asked Questions
What does A01 mean on a Vokera boiler?
A01 means the boiler tried to ignite but did not sense a flame, so it locked out. Check that the gas is on at the meter and that other gas appliances work, then reset once. If A01 returns, the cause is usually inside the boiler, such as the ignition or flame sensing electrode or the gas valve, and needs a Gas Safe engineer.
How do I fix A04 on my Vokera boiler?
A04 means the system water pressure is too low. With the boiler cold, top it up using the filling loop until the gauge reads between 1 and 1.5 bar, close the valves and reset the boiler. If the pressure drops again within days, there is a leak or a faulty component that needs finding.
How do I reset a Vokera boiler?
Vokera's instructions say to check the action for the fault code first, wait about two minutes, then turn the mode selector knob across the OFF/RESET position and back to your normal setting. Some newer models have a separate reset button. Reset once; if the fault comes back, call an engineer.
Why does my Vokera boiler keep locking out?
Repeated lockouts usually come from low pressure that keeps falling, an ignition problem, a blocked or frozen condensate pipe, or a sensor or fan fault. Vokera's own manuals say that needing to reset frequently may indicate a fault, so the boiler should be checked rather than reset again and again.
Can a plumber fix a Vokera boiler?
Anyone can top up the pressure or reset the boiler, but any work on the gas side or inside the sealed case must be done by a Gas Safe registered engineer. Check the engineer's Gas Safe ID card before they start.
Sources and further reading
Source links checked 27 September 2026. Scheme rules and manufacturer instructions can change; use the current official guidance for your account or exact appliance.