Vokera AL79 Fault Code: Causes, Fixes & Repair Costs
What does the Vokera AL79 fault code mean?
AL79 is a safety lockout fault on Vokera boilers. It activates when the boiler's internal monitoring detects that either the water leaving the boiler (the flow) has reached a dangerously high temperature, or that the gap between the flow temperature and the returning water temperature is wider than it should be. Both situations indicate that heat is not being distributed around the heating circuit properly. Because this is classed as a 'final' lockout, the boiler will remain shut down and will not fire again until it has been manually reset — and importantly, until the root cause has been identified and resolved. Simply resetting without addressing the underlying problem will almost always result in the fault returning.
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
- Weak or failing circulation pump Common
If the pump is not moving water around the heating circuit at the correct rate, heat builds up rapidly in the heat exchanger and the flow temperature spikes. The temperature difference between flow and return also widens well beyond the safe threshold, triggering AL79. The official Vokera service guidance for this fault specifically calls out checking the pump and ensuring adequate circulation, making this the most likely culprit.
- Sludge, limescale, or internal blockage Common
Over time, central heating systems accumulate magnetite sludge and limescale deposits that narrow pipework and restrict the flow of water. Even if the pump is functioning correctly, a partially blocked system can cause the same overheating pattern as a failing pump. Older systems without a magnetic filter are particularly susceptible.
- Faulty flow or return thermistor Sometimes
The boiler relies on temperature sensors fitted to the flow and return pipework to monitor heat levels. If either sensor develops a fault or drifts out of calibration, it can report an abnormally high temperature even when the actual water temperature is acceptable. The closely related code AL73 points specifically to a return thermistor fault, so if you have seen both AL73 and AL79 appear, a faulty return thermistor is a strong candidate.
- Airlock in the heating circuit Sometimes
Trapped air pockets within the system can prevent water from circulating evenly. Sections of the circuit effectively run dry or at very low flow, causing localised overheating and an uneven temperature differential between the flow and return pipes.
- Loose wiring or poor sensor connections Rare
Intermittent or corroded electrical connections to the flow or return thermistors can cause erratic temperature readings, leading the boiler's control board to log a false AL79 event. This tends to produce faults that come and go rather than a persistent lockout.
How to fix it
- Reset the boiler following the Vokera procedure DIY safe
Turn the mode selector switch to the OFF/RESET position, then immediately move it to either the 'heating and hot water' or 'hot water only' setting. Give the boiler a few minutes to attempt a restart. If it fires up and runs normally, monitor it closely over the next hour or so. If AL79 returns promptly or the boiler locks out again within a short period, do not keep resetting — move on to the next checks.
- Check the system pressure gauge DIY safe
With the boiler cold, the pressure gauge should read between approximately 1.0 and 1.5 bar. Vokera boilers commonly operate best around 1.2 bar when cold. If pressure is low, use the filling loop to top up slowly until the gauge reaches around 1.2 bar, then reset the boiler. Low pressure alone is unlikely to cause AL79, but it can compound circulation problems.
- Bleed the radiators to release trapped air DIY safe
Starting from the ground floor and working upwards, use a radiator bleed key to open each bleed valve slightly until water (not air) emerges, then close it again. After bleeding all radiators, recheck the system pressure as it will typically drop slightly, and top up again if needed. This addresses airlocks that may be restricting circulation.
- Check that all radiator valves and zone valves are open DIY safe
Ensure manual radiator valves (TRVs and lockshield valves) are open on most or all radiators, particularly in the same zone as the boiler. A system with too many closed valves can create excessive resistance against the pump, reducing circulation and driving up the flow temperature.
- Call a Gas Safe registered engineer to inspect the pump and circulation Gas Safe engineer
An engineer will test the pump to confirm it is operating at the correct speed and generating sufficient flow. They will check for signs of seizure, cavitation, or wear. If the pump has failed or is underperforming, replacing it is a straightforward repair for a qualified engineer and is one of the most common resolutions for AL79.
- Have the thermistors tested and replaced if necessary Gas Safe engineer
The engineer will use diagnostic tools to read the resistance values from the flow and return thermistors and compare them against the manufacturer's specifications. A sensor that is out of range or producing erratic readings will be replaced. This is a relatively straightforward and affordable repair when it is the root cause.
- Arrange a power flush if sludge or blockage is suspected Gas Safe engineer
If the engineer finds evidence of restricted flow, heavy sludge, or significant limescale in the system, a power flush uses specialist equipment to force water and cleaning chemicals through the circuit at high velocity to clear the blockage. A magnetic system filter should then be fitted to prevent sludge from accumulating again.
- Have the wiring and PCB connections inspected Gas Safe engineer
If no mechanical fault is found with the pump or sensors, the engineer will check all wiring harnesses and connections to the thermistors and control board for signs of corrosion, damage, or intermittent contact. In rare cases, a fault on the PCB itself may be responsible, though this is an unusual cause of AL79.
Parts you may need
- Circulation pump (Vokera compatible) · from £85
- Flow thermistor · from £20
- Return thermistor · from £20
- Magnetic system filter · from £55
- PCB (control board) · from £200
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 £120–£380, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the boiler when AL79 appears?
You can attempt a reset two or three times to see if the fault clears, but repeated resets without fixing the underlying problem will not resolve AL79. The boiler locks out for a genuine safety reason — the water is overheating or not circulating properly — and continuing to force restarts risks damage to the heat exchanger. If the fault keeps returning after one or two resets, stop and call a Gas Safe registered engineer.
Is AL79 related to AL73 on Vokera boilers?
Yes, these two codes are linked. AL73 is a fault specific to the return thermistor, while AL79 relates to excessive flow temperature or an abnormally large difference between the flow and return temperatures. If you have seen both codes appear on your Vokera boiler — either at the same time or on separate occasions — there is a strong chance a faulty return thermistor is contributing to the AL79 lockout. An engineer can confirm this quickly with a resistance test on the sensor.
How much does it typically cost to fix a Vokera AL79 fault?
For the most common repairs — pump replacement or thermistor replacement — most homeowners pay somewhere between £120 and £380 including labour and parts. A thermistor swap tends to sit at the lower end of that range (roughly £100–£200), while a pump replacement is typically £200–£350. If a full power flush is needed to clear a sludge-blocked system, that is a separate job usually costing £300–£600 depending on system size. In rare cases where the PCB is involved, parts alone can push the total higher — worth discussing with your engineer before proceeding.
Will an annual boiler service help prevent AL79 from occurring?
Yes, significantly. During a service a Gas Safe engineer will check the pump operation, inspect the thermistors, test system pressure, and look for early signs of sludge or limescale build-up. Catching a sluggish pump or a drifting sensor early is far less disruptive and expensive than dealing with a full lockout in the middle of winter. Vokera recommends annual servicing, and fitting a magnetic filter at the same time will help keep the system clean between visits.