Vokera AL71 Fault Code: Causes, Fixes & Repair Costs
What does the Vokera AL71 fault code mean?
The AL71 code appears on Vokera boilers when the control board detects a problem with the primary flow thermistor — the NTC (Negative Temperature Coefficient) sensor that continuously monitors the temperature of the water leaving the boiler's heat exchanger. When this sensor sends readings outside the expected range, or stops responding altogether, the boiler locks out and displays AL71 to prevent the system from operating with unreliable temperature data. This fault will not clear itself and requires investigation, as the boiler cannot safely regulate its output without accurate flow temperature information. AL71 is found across several Vokera models including the Unica HE range.
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
- Failed or degraded flow thermistor Common
The NTC thermistor itself has failed — either due to heat stress, age, or internal degradation — and is no longer producing accurate resistance values. This is the most frequent cause of AL71 and usually requires the sensor to be replaced outright.
- Loose or corroded wiring connections Common
The wiring connector at the thermistor or along the harness back to the PCB has worked loose, corroded, or suffered insulation damage. Even a partial loss of contact can cause the PCB to read the sensor as open-circuit or shorted, triggering AL71 without the thermistor itself being defective.
- PCB fault Sometimes
If the thermistor and all its wiring test correctly, the fault may lie with the PCB's sensor-monitoring circuit. A damaged input on the control board can misread a perfectly healthy thermistor. PCB faults are less common but more costly to resolve.
- Age-related component wear Sometimes
On older Vokera boilers, multiple components can degrade simultaneously. The thermistor may sit in a pocket exposed to repeated thermal cycling over many years, eventually causing the sensor body or its seal to fail. General wear is more likely in boilers over ten years old.
How to fix it
- Check the boiler's gas supply is on DIY safe
Before anything else, confirm the gas supply to the property is active — check other gas appliances such as a hob or oven. If there is no gas supply, contact your gas supplier rather than resetting the boiler.
- Attempt a single boiler reset DIY safe
Locate the reset button on your Vokera boiler (often labelled R or indicated by a symbol). Press and hold it for 3–5 seconds until the fault code stops flashing, then release and allow the boiler to restart. On dial-operated models, turn the selector to the reset or OFF/RESET position and hold briefly. If the boiler fires and runs normally, monitor it over the next few hours. Do not reset the boiler more than two or three times — repeated resets without fixing the underlying fault can mask a developing problem and should prompt you to call an engineer.
- Inspect the thermistor and wiring — engineer task Gas Safe engineer
A Gas Safe engineer will locate the primary flow thermistor, disconnect the wiring plug, and use a multimeter to measure the sensor's resistance at its current temperature. The reading is checked against the manufacturer's resistance-temperature curve for the Vokera thermistor. A reading that is open-circuit, short-circuit, or significantly out of range confirms the sensor has failed.
- Check and repair the wiring harness — engineer task Gas Safe engineer
Even if the thermistor tests within specification, the engineer will inspect the wiring connector, pins, and loom for corrosion, heat damage, or loose terminals. A corroded or intermittent connection can mimic a failed sensor. Damaged connectors or sections of wiring will be repaired or replaced.
- Replace the primary flow thermistor if faulty — engineer task Gas Safe engineer
If the thermistor is confirmed defective, the engineer will drain down the relevant section of the boiler, remove the old sensor from its pocket on the flow pipework or heat exchanger, fit a new Vokera-compatible NTC thermistor, and reinstate the wiring connection. The system is then refilled, pressure-checked, and the boiler is run through a heat cycle to confirm the fault has cleared.
- Investigate the PCB if the thermistor and wiring are both healthy — engineer task Gas Safe engineer
If the sensor and all connections test correctly yet AL71 persists, the PCB's sensor input circuit may be at fault. The engineer will assess whether the board can be repaired or needs replacement. Given the cost of a new PCB, this assessment will also factor in the age and overall condition of the boiler.
- Call a Gas Safe registered engineer if the fault persists or recurs Gas Safe engineer
If AL71 returns after a reset, or you have not been able to identify and fix the cause yourself, book a Gas Safe registered engineer. You can verify any engineer's credentials at www.gassaferegister.co.uk. Do not continue to operate the boiler in lockout or attempt to bypass the fault code.
Parts you may need
- Vokera primary flow NTC thermistor · from £25
- Thermistor wiring harness connector · from £12
- Vokera PCB (control 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 £100–£250, depending on the underlying cause.
Frequently asked questions
Can I fix Vokera AL71 myself without calling an engineer?
The one thing a homeowner can safely do is attempt a single reset of the boiler. Beyond that, diagnosing and replacing the flow thermistor — or tracing a wiring fault — requires the system to be partially drained and components tested with specialist equipment. This work should be carried out by a Gas Safe registered engineer. Attempting it yourself could invalidate your boiler warranty and, if anything goes wrong with the gas circuit, create a safety risk.
How much does it cost to fix a Vokera AL71 fault in the UK?
For the most common cause — a failed thermistor — expect to pay roughly £100–£250 in total, covering the part (around £15–£35) plus the engineer's labour and call-out charge. If the fault turns out to be the PCB rather than the sensor itself, repair costs can rise to £400–£600 or more depending on whether the board is repairable or needs full replacement. Always get a written quote before authorising PCB work, and consider the age of the boiler — on older units, replacement may be more cost-effective than a major repair.
Why does my Vokera boiler keep showing AL71 even after I reset it?
A reset only clears the lockout temporarily — it does not fix the underlying problem. If AL71 keeps returning after resets, the flow thermistor is almost certainly failing or has already failed, or there is a persistent wiring fault that needs physical repair. Repeated resets without addressing the root cause can stress other components, so a recurring AL71 is a strong signal to stop resetting and book an engineer promptly.
Which Vokera boiler models can show the AL71 fault code?
AL71 is associated primarily with the Vokera Unica HE range, where it is explicitly listed in the installation and servicing manual. However, similar thermistor fault codes appear across other Vokera models including the Unica, Mynute, and Excell series. If your specific model is not listed here, consult your boiler's installation manual or contact Vokera technical support to confirm the exact meaning of AL71 on your unit, as fault code definitions can vary slightly between generations.