Vokera AL73 Fault Code: Causes, Fixes & Repair Costs
What does the Vokera AL73 fault code mean?
The AL73 code on a Vokera boiler signals that the boiler's control board has detected a problem with the return thermistor — the sensor that monitors the temperature of water coming back into the boiler from your central heating circuit. When this sensor reads an unexpectedly high or clearly incorrect temperature, the boiler triggers a safety lockout and stops operating until the fault is cleared. AL73 sits within a group of closely related temperature-sensor codes: AL72 covers the flow (primary) thermistor, AL74 points to a lack of water circulation, and AL79 indicates the flow temperature is too high or the temperature difference between the flow and return has become excessive. If you see AL73 on your display, the boiler has locked out and will not restart without either a successful reset or an engineer's attention.
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 return thermistor Common
The NTC return thermistor itself has failed electrically, producing a resistance reading that is outside the expected range (typically around 10 kΩ at normal operating temperature). This is the most direct cause of AL73 and usually means the sensor needs replacing.
- Damaged or disconnected wiring to the thermistor Common
Loose connectors, corroded terminals, or damaged insulation on the wiring between the return thermistor and the PCB can mimic a failed sensor. A visual and continuity check of the wiring loom is always part of an AL73 diagnosis.
- Poor circulation or pump fault Sometimes
If the central heating pump is failing or has seized, water does not circulate properly around the system. This can cause the return water temperature to rise abnormally, tripping the return thermistor alarm even when the sensor itself is healthy.
- System sludge or blockage restricting flow Sometimes
A build-up of magnetite or debris in the pipework or heat exchanger reduces water flow, creating large temperature differentials that can push the return temperature to a level the boiler interprets as a fault.
- PCB fault Rare
The printed circuit board processes all sensor signals. If the PCB has developed a fault on the return-thermistor input channel, it may generate AL73 even when the thermistor and its wiring are perfectly sound. This is the least common root cause but should be considered if all other checks are clear.
How to fix it
- Reset the boiler once or twice DIY safe
Turn the mode selector to the OFF/RESET position, wait a few seconds, then turn it back to your normal heating setting. A single fault can sometimes be a transient sensor glitch that clears on reset. If AL73 returns immediately or after a short time, do not keep resetting — a recurring lockout means the underlying fault is still present and needs investigation.
- Check your gas supply is on DIY safe
Confirm that other gas appliances in your home are working normally. If there is a wider gas supply issue, the boiler may not restart regardless of the fault code. If you suspect a gas leak, leave the property and call the National Gas Emergency Service on 0800 111 999.
- Check system pressure DIY safe
Look at the pressure gauge on the boiler. Normal operating pressure is typically 1–1.5 bar when the system is cold. If it has dropped below 1 bar, top it up using the filling loop according to your boiler's instructions. Very low pressure can affect circulation and contribute to temperature sensor alarms.
- Have a Gas Safe engineer test the return thermistor resistance Gas Safe engineer
An engineer will disconnect the return thermistor and measure its resistance with a multimeter, comparing the reading against the manufacturer's specification (approximately 10 kΩ). A reading that is significantly outside range confirms the thermistor has failed and needs replacing.
- Have the engineer inspect thermistor wiring and connectors Gas Safe engineer
The engineer will check the full wiring loom from the thermistor back to the PCB, looking for loose pins, corrosion, chafing, or heat damage. A wiring repair or connector replacement is often quicker and cheaper than a full thermistor swap.
- Have the engineer assess the pump and system circulation Gas Safe engineer
If the thermistor and wiring both test correctly, the engineer will check pump operation — verifying it runs freely, moves water at adequate flow rate, and is not air-locked. A power flush or magnetic filter clean may also be recommended if sludge is suspected.
- Arrange replacement of the faulty component Gas Safe engineer
Once the root cause is confirmed, the engineer will replace whichever component is at fault — most commonly the return thermistor itself, which is a relatively straightforward repair on Vokera models. If the pump or PCB is implicated, the repair scope and cost will be higher. Ensure any replacement parts used are compatible with your specific Vokera model.
Parts you may need
- Return NTC thermistor (Vokera compatible) · from £25
- Thermistor wiring harness / connector · from £20
- Central heating pump (replacement) · from £85
- PCB (printed circuit board) · 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 £120–£320, depending on the underlying cause.
Frequently asked questions
Can I fix the Vokera AL73 fault myself?
The only homeowner-safe steps are resetting the boiler (once or twice), checking the system pressure, and confirming the gas supply is on. Everything else — testing the thermistor, inspecting wiring, or replacing parts — must be carried out by a Gas Safe registered engineer. Attempting to work on the sensors or PCB yourself risks both personal safety and invalidating any remaining warranty.
How much does it cost to fix a Vokera AL73 fault?
If the return thermistor itself is the problem, most homeowners pay between £120 and £320 including parts and labour. If the pump is at fault, you should budget around £200–£400 for a replacement. A PCB fault is less common but significantly more expensive — typically £300–£500 or more. Mention the AL73 code when booking an engineer so they can bring the likely parts.
What is the difference between AL73, AL72, AL74, and AL79 on a Vokera boiler?
These four codes all relate to temperature monitoring in the water circuit. AL72 is a flow (primary) thermistor fault — the sensor on the water leaving the boiler. AL73 is a return thermistor fault — the sensor on the water coming back. AL74 indicates a circulation problem with insufficient water movement detected. AL79 means either the flow temperature has climbed too high, or the gap between the flow and return temperatures has become too large. Each code points an engineer towards a different component, though the underlying causes (pump, sludge, sensor failure) can overlap.
Will my Vokera warranty cover an AL73 repair?
If your Vokera boiler is still within its manufacturer's warranty period, a return thermistor fault may be covered at no cost. Check your warranty documentation or contact Vokera directly with your boiler's serial number before arranging an independent engineer. Using an unregistered engineer or attempting DIY repairs beforehand can void warranty cover, so always check first.