Vokèra A02 fault codewhat it means and how to fix it
The safety limit thermostat has tripped on overheat. In practice this nearly always means water has stopped circulating properly.
Take care
What A02 means
A02 is the limit thermostat doing its job. This is a separate safety device from the normal temperature control, wired so that it cuts the burner independently if the primary circuit exceeds a temperature it should never reach. The natural reading is that the appliance is running too hot, but that is rarely what has happened. Far more often the burner is producing heat at exactly the rate it should while circulation has failed, so the heat has nowhere to go and accumulates until the limit trips.
Everything on the cause list below is therefore a flow problem rather than a heat problem. The reason to take this seriously rather than simply resetting it is that the heat exchanger is the most expensive component in the appliance, and repeatedly firing it without adequate flow through it is the single most reliable way to destroy one.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Pump seized or weak | Very common after a summer standing idle. The rotor sticks with deposit and either will not turn or turns without moving useful volume. | Common |
| Air in the primary circuit | An air pocket breaks circulation entirely. Look for radiators cold at the top and a knocking or gurgling noise from the appliance. | Common |
| Sludge in the heat exchanger | Magnetite settles in the narrow passages of the exchanger and restricts flow. Radiators cold at the bottom but hot at the top is the tell. | Occasional |
| Zone valve or service valve closed | A motorised valve failed shut, or an isolation valve left closed after work on the system. The pump runs into a dead end. | Occasional |
| Limit thermostat itself failed | Gone open circuit and reporting an overheat that is not occurring. Confirmed by testing continuity, not by inference. | Rare |
What to try yourself
Leave the appliance switched off for half an hour. A limit thermostat that is still hot will not reset, and you will otherwise conclude the appliance is beyond help when it is merely warm.
Check the pressure gauge reads 1.0 to 1.5 bar cold. Low pressure and poor circulation very often turn up together.
Open all thermostatic radiator valves fully. Check the service valves on the pipes below the appliance are aligned with the pipe, not across it.
Run the heating for twenty minutes and feel each radiator top and bottom. Cold at the top is air and you can bleed it. Cold at the bottom is sludge and you cannot.
Reset once. If A02 comes back, stop.
Each reset fires the burner into a heat exchanger that is not being cooled. This is precisely how a £300 pump job turns into a £900 heat exchanger job.
Get a Gas Safe registered engineer to look at your Vokèra A02
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What it costs to fix
Typical repair cost
£150–£950
Time on site
40 minutes
A pump is £220 to £420 fitted. A chemical clean with a filter is £400 to £750. A limit thermostat on its own is £140 to £220.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Central heating circulating pump
CH-PUMP-15-60£90–£180Plumbing merchant
Price checked 28 September 2026
Magnetic system filter, 22mm
MAG-22£65–£120Plumbing merchant
Price checked 28 September 2026
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Related Vokèra fault codes
- A07Get it checkedThe primary flow sensor is reading out of range or has failed. The appliance stops because it can no longer tell how hot it is running.Related: overheat and circulation
- A01Get it checkedThe burner failed to light and the appliance has locked out. Check the gas supply and, in cold weather, the condensate pipe before calling anyone.Related: same brand
- A04Usually simple to fixSystem water pressure is below the minimum the appliance will run at. Topping the system back up to 1.2 bar normally clears it straight away.Related: same brand
- A03Get it checkedThe appliance cannot confirm the fan is running correctly, so it will not permit the burner to light. This needs an engineer.Related: same brand
- A11Stop — call an engineerThe appliance is detecting a flame when there should not be one. Turn it off at the fused spur and get a Gas Safe engineer out.Related: same brand
- A09Get it checkedThe flue gas sensor has recorded a temperature above the safe limit, or the sensor itself has failed. Either way the appliance stops.Related: same brand
Own a Vokèra boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.