Vokera A77 Fault Code: Causes, Fixes & Repair Costs
What does the Vokera A77 fault code mean?
The A77 code is a safety-related fault that Vokera boilers use across several model ranges, and its precise meaning can vary slightly depending on which boiler you own. On many Vokera models it signals a reversible fan failure — the fan that manages combustion airflow and safe removal of exhaust gases has either stopped working correctly or is no longer able to reverse its direction as required. On other models, A77 points to a low-temperature safety thermostat fault, where an unusually low temperature reading or a faulty thermostat switch has triggered a protective shutdown. A third interpretation, seen in some Vokera documentation, is a flue thermostat fault — meaning the temperature within the flue has climbed above safe limits, typically because heat cannot escape properly. Regardless of which underlying cause applies to your specific model, A77 is always a safety shutdown and should not be ignored. Check your model's manual if you are unsure which interpretation applies, or ask a Gas Safe engineer to confirm.
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 or failing fan motor Common
The fan motor may have worn out, seized, or lost the ability to reverse its spin direction as the boiler's control logic demands. This is the most frequent root cause when A77 appears on models where it is defined as a reversible fan fault. The boiler shuts down to prevent unsafe flue gas spillage.
- Blocked or restricted flue and airway Common
A build-up of debris, a bird nest, or physical damage to the flue terminal can restrict airflow to the point where the fan struggles and the boiler's sensors trigger a safety lockout. On models where A77 is a flue thermostat fault, restricted airflow causing heat build-up is a primary driver.
- Low-temperature or flue thermostat fault Sometimes
The thermostat component itself may have failed electrically or mechanically, sending an incorrect signal to the PCB even when temperatures are within normal range. This causes a spurious A77 shutdown that persists after reset.
- Loose or damaged wiring to the fan or thermostat Sometimes
Vibration over time or a poor original connection can cause wiring harnesses connected to the fan motor or thermostat to become intermittent or open-circuit. The PCB then cannot confirm the component is operating correctly and raises the A77 fault.
- PCB fault Rare
If the printed circuit board has developed a fault, it may misinterpret signals from the fan or thermostat — or fail to send the correct commands — triggering A77 even when the physical components are in good condition. This is typically a diagnosis of exclusion once other causes have been ruled out.
How to fix it
- Check for an obvious cause before touching anything DIY safe
Note whether the boiler has any unusual smells, sounds, or visible signs of damage around the flue terminal outside your property. Also confirm that your gas supply is working normally (check other gas appliances). Do not attempt to open the boiler casing.
- Reset the boiler once or twice DIY safe
Press and hold the reset button as described in your Vokera manual (usually 3–5 seconds). If the boiler fires up and runs normally, monitor it closely. If A77 returns within a short time, do not keep resetting — repeated resets will not fix an underlying fault and could mask a safety issue.
- Check your room thermostat and programmer settings DIY safe
Confirm your room thermostat is calling for heat (set it above the current room temperature) and that your programmer or timer is in a heating period. An incorrect setting can occasionally cause unusual boiler behaviour, though it rarely causes A77 specifically.
- Arrange inspection of the fan and flue by a Gas Safe engineer Gas Safe engineer
A registered engineer will remove the boiler casing safely and inspect the fan motor, its electrical connections, and wiring harness. They will also check the flue run for blockages — including the external terminal — which requires proper tools and training.
- Have the thermostat components tested and replaced if necessary Gas Safe engineer
If the fan checks out, the engineer will test the low-temperature or flue thermostat with a multimeter to confirm whether it is reading correctly. A faulty thermostat is a relatively straightforward replacement once correctly diagnosed.
- Have the PCB assessed if all other components are serviceable Gas Safe engineer
If wiring, fan, and thermostat are all in good condition, the engineer may test the PCB for output faults. On older Vokera boilers, PCB replacement can be costly, so the engineer may also discuss whether a new boiler is a better long-term investment.
- Call a Gas Safe registered engineer to carry out a full diagnosis Gas Safe engineer
If the fault persists or you have not already done so, contact a Gas Safe registered heating engineer. This fault involves either the combustion fan or safety thermostat — both of which are integral to safe boiler operation and must only be diagnosed and repaired by a qualified professional.
Parts you may need
- Vokera boiler fan motor assembly · from £95
- Low-temperature safety thermostat · from £35
- Flue thermostat · from £30
- PCB (printed circuit board) · from £220
- Fan wiring harness · from £25
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–£350, depending on the underlying cause.
Frequently asked questions
Is the Vokera A77 fault dangerous — can I keep using the boiler?
Because A77 is a safety shutdown related to either the combustion fan or a thermostat, you should not continue running the boiler until the cause has been identified. If the fan is genuinely failing, exhaust gases may not be vented correctly, which poses a carbon monoxide risk. Switch the boiler off and book a Gas Safe engineer promptly rather than repeatedly resetting and using the heating.
Why does my Vokera show A77 but the boiler seems to run fine after a reset?
An intermittent fan motor, a loose wire, or a borderline thermostat can allow the boiler to run normally for a while before the fault recurs. Clearing it with a reset does not repair the underlying component. If A77 keeps coming back — especially within hours or a day or two — treat it as a persistent fault and arrange an engineer visit rather than continuing to reset.
How much does it typically cost to fix a Vokera A77 fault in the UK?
For the most common repairs — fan motor replacement or thermostat swap — most homeowners pay in the region of £150–£350 including parts and labour. Fan assemblies with fitting typically cost around £225–£275, while thermostat replacements usually come in between £100 and £200. PCB replacement, if needed, can push costs to £400 or beyond, and on older boilers this may make a new boiler more economical. Labour rates in London and the South East tend to run 15–25% higher than the UK average.
Will A77 affect my Vokera warranty?
Most Vokera boilers carry a five-year warranty, with some models such as the Evolve offering ten years. To keep the warranty valid, Vokera requires annual servicing by a Gas Safe registered engineer — you will need to retain proof of each service. If your boiler is still within its warranty period, contact Vokera or your installer before arranging independent repairs, as unauthorised work could invalidate cover.