Vokera AL28 Fault Code: Causes, Fixes & Repair Costs
What does the Vokera AL28 fault code mean?
The AL28 fault code signals that your Vokera boiler has detected an inverted temperature differential — in other words, the sensor monitoring the water returning to the boiler is reading a higher temperature than the sensor on the flow (the water leaving the boiler). Under normal operation this is physically impossible, so the boiler locks out to protect itself. When AL28 triggers, the pump runs for a 60-second post-circulation cycle and the red fault LED illuminates. The root cause is almost always related to circulation failure, a faulty temperature sensor, sludge restricting flow, or an incorrectly set pump speed — all of which prevent heat from being carried away from the heat exchanger efficiently.
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 flow or return thermistor Common
One or both of the temperature sensors (thermistors) may be giving inaccurate readings, making the boiler think the return is hotter than the flow even when it is not. A damaged, corroded, or ageing thermistor is the single most common trigger for AL28 on Vokera boilers.
- Circulation pump failure or seizure Common
The pump is responsible for moving hot water away from the heat exchanger and around the system. If it has seized, is running intermittently, or has failed altogether, heat builds up near the heat exchanger and the return temperature can appear to spike relative to the flow, causing the boiler to lock out.
- Sludge or scale restricting water flow Common
Magnetite sludge or limescale build-up inside the heat exchanger or pipework creates blockages that slow or stop proper circulation. This causes hot spots, uneven temperatures across the flow and return, and ultimately triggers the AL28 overheat protection.
- Incorrect pump speed setting Sometimes
If the pump is running on too low a speed setting, the temperature differential across the primary circuit becomes too large — particularly during domestic hot water mode. This is a known trigger for exactly the kind of inversion AL28 describes and is an easy adjustment for an engineer to make.
- Low system pressure Sometimes
Insufficient water pressure in the system reduces the volume of water available to carry heat away from the boiler. This can cause localised overheating and contribute to an apparent temperature inversion at the sensors.
- PCB fault Rare
In rare cases the main printed circuit board misreads or misinterprets signals from the thermistors, generating a false AL28. This is usually only suspected after sensors and pump have been ruled out.
How to fix it
- Check the system pressure gauge DIY safe
With the boiler cold, look at the pressure gauge — it should read between 1.0 and 1.5 bar. If it is below 1.0 bar, top up the system using the filling loop (usually a braided flexible hose beneath the boiler) until it reaches 1.2–1.5 bar, then close the filling loop valve fully. Low pressure alone can contribute to circulation problems.
- Reset the boiler DIY safe
Turn the mode selector dial to the OFF/RESET position, wait at least 10 seconds, then turn it back to your normal setting (e.g. Heating and Hot Water). Allow the boiler to restart and monitor it. Only attempt a reset two or three times at most — repeated resets without fixing the underlying fault can cause further damage.
- Monitor whether the fault returns immediately DIY safe
If AL28 clears and the boiler runs normally, keep an eye on it over the next hour. If the fault returns within minutes of resetting, stop resetting and call an engineer — the underlying problem needs diagnosis rather than repeated lockout cycles.
- Call a Gas Safe registered engineer to test the thermistors Gas Safe engineer
An engineer will use a multimeter to check the resistance values of the flow and return thermistors against the manufacturer's temperature-resistance curve. A sensor reading outside tolerance will be replaced. This is not a homeowner job — the sensors sit inside the boiler's waterway and require gas-safe competency to access safely.
- Have the engineer inspect and test the circulation pump Gas Safe engineer
The engineer will check whether the pump is running, spinning freely, and circulating water at the correct flow rate. A seized pump may sometimes be freed, but in most cases a replacement pump will be fitted. The pump speed setting should also be verified and adjusted if needed.
- Arrange a power flush or chemical clean if sludge is suspected Gas Safe engineer
If the engineer finds evidence of magnetite or scale restricting flow, a power flush using specialist equipment will clear the system. Adding a magnetic filter and inhibitor afterwards protects against future build-up. This work must be carried out by a qualified heating engineer.
- Request a PCB inspection if all other causes have been ruled out Gas Safe engineer
If the thermistors and pump are both functioning correctly and the fault persists, the engineer will assess whether the PCB is misreading sensor signals. PCB replacement is an engineer-only task.
Parts you may need
- Flow thermistor (NTC sensor) · from £25
- Return thermistor (NTC sensor) · from £25
- Circulation pump · from £95
- Magnetic system filter · from £55
- PCB (main circuit board) · from £180
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 £200–£450, depending on the underlying cause.
Frequently asked questions
Can I reset a Vokera AL28 fault myself?
Yes, a single reset is safe to try — turn the mode dial to OFF/RESET, wait ten seconds, then return it to your normal setting. If the AL28 comes back within a short time, stop resetting and call a Gas Safe engineer. Repeatedly resetting an overheat fault without fixing the cause risks damaging the heat exchanger.
How much does it typically cost to repair a Vokera AL28 fault?
Most people with an AL28 fault pay somewhere between £200 and £450 all in, which usually covers the engineer call-out, diagnosis, and either a thermistor replacement or pump replacement including labour. If a power flush is needed on top of the primary repair, expect to add £300–£600 depending on system size. A PCB replacement — which is rarely the cause — can push costs above £500, so it is worth getting a detailed quote before authorising that work.
What is a temperature differential inversion and why does it lock the boiler out?
Under normal operation, water leaves the boiler hot (the flow) and returns slightly cooler after giving up its heat to the radiators or hot water cylinder (the return). An inverted differential means the sensors are reporting the opposite — return hotter than flow — which cannot happen during healthy circulation. Rather than risk overheating the heat exchanger, the boiler shuts down and displays AL28 until a qualified engineer identifies and fixes the cause.
My Vokera boiler keeps getting AL28 — could sludge be the problem?
Absolutely. If your system has never had a power flush or a magnetic filter fitted, a build-up of magnetite sludge and limescale is a very common culprit. Sludge restricts water flow through the heat exchanger, causing hot spots and temperature imbalances that trigger AL28. An engineer can confirm this during diagnosis — look for discoloured water, cold spots on radiators, or a noisy pump as additional clues. Adding a magnetic filter after a power flush significantly reduces the chance of the problem recurring.