Vokera AL74 Fault Code: Causes, Fixes & Repair Costs
What does the Vokera AL74 fault code mean?
AL74 is a final lockout alarm on Vokera Mynute boilers (including the Mynute VHE) indicating that the boiler's control board has detected insufficient or no water movement through the heating circuit. Because a lack of circulation can cause dangerous overheating inside the heat exchanger, the boiler shuts itself down completely and will not restart without a deliberate reset. The root issue can range from a seized pump through to trapped air or a blocked system — the steps below help you identify which applies to your situation.
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
- Faulty or seized circulation pump Common
The circulation pump is by far the most common reason for AL74. Over time the pump's impeller can seize — often accompanied by a buzzing or vibrating noise — or the motor itself can fail silently. Either way, water stops moving around the circuit and the boiler locks out to protect the heat exchanger.
- Airlock in the heating circuit Common
Air trapped in the system can create a pocket that blocks water flow past certain points. This is more likely after recent maintenance, a pressure loss event, or a system that has not been bled for some time. Symptoms often include cold spots on radiators alongside the AL74 fault.
- Magnetite sludge or debris blockage Sometimes
In older or poorly maintained systems, a build-up of magnetite sludge and debris can partially or fully restrict water flow through the pipework, heat exchanger, or pump. The boiler detects the reduced flow rate and trips into lockout.
- Closed or partially closed isolation valve Sometimes
An isolation valve inadvertently left closed — perhaps after a previous repair or service — can obstruct circulation enough to trigger AL74. This is worth checking before calling an engineer, as it is a quick and cost-free fix if found.
- Faulty PCB or wiring to the pump Rare
The PCB acts as the boiler's control centre and monitors pump operation. If the wiring between the PCB and pump is damaged or a relay on the PCB has failed, the boiler may lock out even if the pump itself is mechanically sound. This is the least common cause but one of the more expensive to resolve.
How to fix it
- Check that all isolation valves are fully open DIY safe
Trace the pipework around the boiler and any motorised or manual isolation valves in the system. A valve handle parallel to the pipe means open; perpendicular means closed. If you find a closed valve, open it slowly, then attempt a reset. This costs nothing and takes only a few minutes.
- Bleed the radiators to release trapped air DIY safe
Work from the ground floor upwards. Place a cloth under each bleed valve, insert a bleed key and open the valve slowly until water trickles out steadily rather than air hissing. Once done, check the system pressure on the boiler gauge — it should read between 1.0 and 1.5 bar when cold. If it has dropped below 1 bar, top up via the filling loop before attempting a reset.
- Top up system pressure if needed DIY safe
Locate the filling loop (a braided hose or lever valve usually found under the boiler). Open the valve(s) slowly and watch the pressure gauge. Stop when pressure reaches approximately 1.2–1.5 bar. Close the filling loop securely. Never over-pressurise beyond 2 bar.
- Reset the boiler DIY safe
On a Vokera Mynute, turn the mode selector dial to the OFF/RESET position, hold for 5–10 seconds, then return it to your normal operating setting (e.g. Heating and Hot Water). Allow the boiler a minute to restart. Do not reset more than two or three times in total — repeated resets without resolving the underlying fault can mask the problem and cause further damage.
- Listen for pump operation after the reset DIY safe
Stand near the boiler and listen carefully once it attempts to fire. You should hear a faint hum from the pump. A buzzing, grinding, or complete silence from the pump location suggests a seized or failed pump. Make a note of what you hear — this information will help the engineer diagnose the fault quickly.
- Have a Gas Safe engineer inspect and test the pump Gas Safe engineer
If the boiler locks out again with AL74, a qualified Gas Safe engineer needs to check the pump directly — testing voltage at the pump terminals, measuring flow rate, and assessing whether the impeller can be freed or whether the pump needs replacing. Typical pump replacement costs are in the region of £150–£300 for parts and labour combined.
- Ask the engineer to investigate for sludge and consider a power flush Gas Safe engineer
If the pump is found to be working but flow is still restricted, magnetite sludge build-up may be the cause. The engineer can test the water quality and, if necessary, carry out a power flush of the system to clear blockages. Adding a magnetic filter after the flush helps prevent recurrence.
- Have the PCB and wiring inspected if the pump checks out Gas Safe engineer
If the pump and system are both functioning correctly yet AL74 persists, the fault may lie with the wiring loom between the PCB and pump, or with the PCB itself. This requires component-level testing by an engineer and can be a more costly repair — get a written quote before proceeding.
- Call a Gas Safe registered engineer if the fault has not resolved Gas Safe engineer
If AL74 returns after your basic checks, do not continue resetting. Contact a Gas Safe registered engineer to carry out a full diagnosis. You can verify any engineer's credentials at gassaferegister.co.uk. Vokera's own customer care line is also available on 0344 391 0999 if you would like a manufacturer-approved repair.
Parts you may need
- Circulating pump (compatible replacement, e.g. Grundfos UPS or Wilo equivalent) · from £95
- Pump capacitor · from £12
- System magnetic filter (e.g. Fernox TF1 or Adey MagnaClean) · from £55
- PCB (Vokera Mynute control board) · from £220
- Motorised zone valve · from £40
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 £150–£380, depending on the underlying cause.
Frequently asked questions
Can I fix AL74 myself without calling an engineer?
The checks you can safely carry out yourself are: confirming isolation valves are fully open, bleeding the radiators, topping up system pressure to around 1.2 bar, and resetting the boiler once or twice. Beyond that — inspecting or replacing the pump, diagnosing wiring faults, or dealing with sludge — requires a Gas Safe registered engineer. Attempting to work on the pump or PCB yourself could invalidate your warranty and create safety risks.
How much does it typically cost to fix AL74 on a Vokera Mynute?
For the most common cause — a seized or failed circulation pump — expect to pay roughly £150–£300 including parts and labour, with Checkatrade averaging around £275 for a full pump replacement. If the system needs a power flush due to sludge build-up, that typically adds £300–£500. A PCB replacement is a less common but more expensive outcome, generally £300–£550 including parts and labour — if the engineer diagnoses this, ask for a written quote and compare it against the cost of a new boiler, particularly if the boiler is over 10 years old.
Why does AL74 keep coming back after I reset it?
If AL74 returns within a short time of resetting, the underlying cause has not been resolved. Repeated lockouts are most commonly caused by a pump that is on its way out, a heavily sludged system that keeps restricting flow, or a persistent airlock. Continuously resetting without investigation can damage the heat exchanger over time. If AL74 has returned more than twice, stop resetting and call a Gas Safe engineer for a proper diagnosis.
Is AL74 a gas-related fault — should I be worried about a gas leak?
No — AL74 relates to water circulation in the heating circuit, not to the gas supply or combustion side of the boiler. It is not a gas leak indicator. That said, the boiler is in lockout and your heating will not work until the fault is resolved, so it is still worth arranging an engineer promptly — particularly in cold weather. If you ever smell gas, leave the property, avoid using switches or open flames, and call the National Gas Emergency Service on 0800 111 999 immediately.