Vokera AL26 Fault Code: Causes, Fixes & Repair Costs
What does the Vokera AL26 fault code mean?
The AL26 fault code on a Vokera boiler means the boiler has detected that the water returning from the heating circuit is running at a dangerously high temperature. Because this poses a risk to key internal components, the boiler has shut itself down and locked out as a safety precaution. AL26 sits within a group of Vokera overheat and thermistor-related fault codes — alongside AL10, AL20, AL21, AL28, and AL34 — all of which relate to excessive heat detection or temperature sensor problems within the appliance. Until the underlying cause is identified and resolved, the boiler will not restart permanently.
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
- Circulation pump failure or seizure Common
The pump is responsible for keeping hot water moving around the central heating circuit. If it has seized, lost power, or is running too slowly, heat builds up in the system and the return water temperature rises sharply. This is one of the most common reasons a Vokera boiler throws an AL26 code.
- Sludge or scale restricting system flow Common
Over time, magnetite sludge and limescale can accumulate inside radiators, pipework, and the heat exchanger. This reduces how freely water can circulate, causing localised hot spots and elevated return temperatures. Older systems without a magnetic filter are particularly vulnerable.
- Airlocks in the heating circuit Sometimes
Pockets of trapped air can prevent water from flowing properly around parts of the system. The resulting poor circulation can cause the boiler to detect abnormal temperatures and lock out with AL26.
- Faulty return thermistor or temperature sensor Sometimes
The return thermistor monitors the temperature of water coming back to the boiler. If the sensor is degrading or has failed, it may report inaccurate high-temperature readings to the PCB, triggering a lockout even when actual water temperature is within normal limits.
- PCB fault Rare
The printed circuit board processes all sensor data and controls boiler operation. If the PCB develops a fault, it may misinterpret thermistor signals or fail to correctly manage the pump, leading to spurious AL26 lockouts.
How to fix it
- Allow the boiler to cool down completely DIY safe
Switch the boiler off and leave it for at least 20–30 minutes. Attempting a reset while components are still hot is unlikely to clear the fault and could mask a more serious issue.
- Check your system pressure DIY safe
Look at the pressure gauge on the boiler — it should read between 1.0 and 1.5 bar when cold. If it has dropped below 1 bar, top up using the filling loop (consult your boiler manual for the location of the filling loop valves). Avoid overfilling above 2 bar.
- Bleed your radiators to remove trapped air DIY safe
Using a radiator bleed key, open each radiator bleed valve in turn, starting on the ground floor. Allow any hissing air to escape until water flows steadily, then close the valve. Re-check the boiler pressure afterwards and top up if needed.
- Attempt a boiler reset DIY safe
On most Vokera models, move the mode selector switch to the OFF/RESET position, hold briefly, then move it to your preferred operating mode (heating and hot water, or hot water only). A steady green light indicates the reset has been accepted. Do not attempt more than two or three resets — repeated lockouts mean the fault is still present.
- Check that your gas supply is active DIY safe
Confirm that other gas appliances in your home (e.g. a gas hob) are working normally. If there is no gas supply to the property, contact your gas supplier rather than an engineer.
- Have the circulation pump inspected and tested Gas Safe engineer
A Gas Safe engineer will check whether the pump is running, test its speed settings, and confirm it is generating adequate flow. A seized or under-performing pump is a leading cause of AL26 and will need replacement if it has failed.
- Arrange a power flush if sludge is suspected Gas Safe engineer
If the engineer identifies restricted flow caused by magnetite sludge or scale, a power flush cleans the entire heating circuit using specialist equipment. A magnetic filter should be fitted at the same time to prevent the problem recurring.
- Have the return thermistor tested and replaced if necessary Gas Safe engineer
The engineer will measure the thermistor's resistance against expected values for the current water temperature. If readings are outside specification, replacing the sensor is a relatively straightforward repair that can resolve AL26 quickly.
- Request a PCB inspection if the fault persists Gas Safe engineer
If all other components check out, the PCB may be misprocessing sensor data. A Gas Safe engineer can run diagnostic checks and advise whether repair or replacement is the appropriate course of action.
- Call a Gas Safe registered engineer Gas Safe engineer
If the boiler relocks after resetting, or if you have not been able to identify a simple cause such as low pressure or trapped air, contact a Gas Safe registered engineer. You can verify an engineer's credentials at gassaferegister.co.uk. Do not continue resetting a boiler that keeps locking out — this can cause further damage and may mask a safety issue.
Parts you may need
- Circulation pump (Grundfos or equivalent Vokera-compatible) · from £85
- Return thermistor / NTC temperature sensor · from £25
- Overheat thermostat / high-limit stat · from £20
- Magnetic system filter (e.g. Adey MagnaClean Pro2) · from £75
- PCB (Vokera model-specific) · from £220
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–£450, depending on the underlying cause.
Frequently asked questions
Can I reset a Vokera AL26 fault myself?
Yes, you can attempt a reset once or twice after allowing the boiler to cool and checking basics like system pressure and bled radiators. Move the mode selector to OFF/RESET and back to your chosen operating mode. However, if the fault returns, resetting repeatedly is not advisable — it will not fix the underlying cause and may cause further wear. At that point you need a Gas Safe engineer to investigate.
How much does it typically cost to fix a Vokera AL26 fault?
Most homeowners pay somewhere between £150 and £450 to resolve an AL26, depending on what the engineer finds. A thermistor replacement sits at the lower end (parts and labour typically £100–£200), while pump replacement usually costs £200–£400 all in. A power flush to clear sludge is often £300–£500 depending on the size of the system. In rare cases where a PCB needs replacing, costs can rise to £400–£600, but this is not the typical outcome for this code.
Why does my Vokera keep showing AL26 only in cold weather?
Cold spells can cause system pressure to drop slightly as water contracts, and freezing temperatures can affect external condensate pipes, causing the boiler to shut down. These factors can sometimes coincide with or trigger an AL26 if the system is already marginal — for example, if there is some sludge build-up or a pump that is starting to wear. If the fault only ever appears in cold weather and clears after a reset, it is still worth having a service to rule out underlying issues.
What other Vokera fault codes are related to AL26?
AL26 belongs to a family of Vokera overheat and thermistor codes that includes AL10, AL20, AL21, AL28, and AL34. All of these relate to the boiler detecting abnormal temperatures or receiving suspect readings from its temperature sensors. The diagnostic approach is similar across the group — checking circulation, pump operation, sensor condition, and system cleanliness — though the specific sensor or limit that has triggered the code will vary.