Vokera 02 Fault Code: Causes, Fixes & Repair Costs
What does the Vokera 02 fault code mean?
Fault code 02 — displayed as A02 on many Vokèra models and E020 on newer Evolve variants — indicates that the boiler's high-limit safety thermostat has tripped due to excessive heat building up inside the unit. When water temperature climbs dangerously high, this thermostat cuts in as a last line of defence, forcing the boiler into a protective lockout. Until the root cause is addressed, heating and hot water will remain unavailable. The code is the same across the display formats: whether your boiler shows 02, A02, or E020, it is pointing to the same overheat-lockout condition.
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
- Sludge or scale restricting water flow Common
Over time, iron oxide sludge and limescale accumulate in the heat exchanger and pipework, reducing the flow of water through the boiler. With less water able to carry heat away, temperatures rise rapidly until the limit thermostat trips. This is the single most common reason for a recurring 02 fault, particularly in older systems or properties with hard water that have never been power-flushed.
- Circulating pump failure or poor performance Common
The pump is responsible for pushing heated water around the system. If it is running slowly, seized, or failing intermittently, not enough cool return water reaches the boiler and heat builds up. Engineers often check pump speed and amperage draw as one of the first diagnostics for this fault.
- Air trapped in the system Common
Air locks in radiators or pipework reduce the effective volume of water circulating, which can cause localised hot spots in the boiler. Radiators that are cold at the top or make gurgling noises are a tell-tale sign of trapped air contributing to the problem.
- Blocked or fouled plate heat exchanger Sometimes
In combi boilers the domestic hot water plate heat exchanger can become partially blocked with scale or debris, causing flow restriction and rapid temperature spikes — especially during hot water draws. At around 92 °C the limit stat activates to prevent damage.
- Faulty limit or safety thermostat Sometimes
The limit thermostat itself can develop a fault and trip prematurely even when water temperature is within normal range. Similarly, a faulty domestic hot water stat may send an incorrect signal. In these cases the boiler is not genuinely overheating but the component is misreading conditions.
- Failed non-return valve in the plate heat exchanger circuit Rare
A non-return valve that sticks open or fails can allow hot water to recirculate incorrectly within the heat exchanger circuit, causing temperature to build quickly and trigger the safety stat.
How to fix it
- Turn the boiler off and let it cool for at least 15 minutes DIY safe
Switch the boiler off at its controls and leave it to cool fully before doing anything else. Attempting to reset a boiler that is still hot often results in the fault code reappearing immediately.
- Bleed your radiators, starting from the highest floor DIY safe
Use a radiator bleed key to release trapped air from each radiator, beginning upstairs and working your way down. Hold a cloth under the bleed valve and open it slowly until water (not air) trickles out, then close it. Air in the system reduces water circulation and can contribute to the boiler overheating.
- Check and restore system pressure to 1–1.5 bar DIY safe
Look at the pressure gauge on the boiler — it should read between 1 and 1.5 bar when cold. If it is below 1 bar, top it up using the filling loop (usually a braided flexi-hose with a valve beneath the boiler). Open the valve slowly, watch the gauge, and close it when pressure reaches about 1.2 bar. Do not overfill above 2 bar.
- Reset the boiler once it has cooled DIY safe
On most Vokèra models, press and hold the reset button (labelled R) for 3–5 seconds until the fault code stops flashing. On dial-operated models, turn the selector to the reset or OFF/RESET position and hold briefly. If the boiler fires up and runs normally, monitor it over the next hour. Avoid resetting more than two or three times in total — repeated resets without a fix can mask a worsening problem.
- If the fault returns, do not keep resetting — call a Gas Safe registered engineer Gas Safe engineer
A returning 02 fault means a deeper issue such as sludge, a failing pump, a blocked plate heat exchanger, or a faulty thermostat component needs professional diagnosis. An engineer can measure pump performance, carry out a power flush if needed, inspect the plate heat exchanger, and test or replace the limit thermostat or safety stat. These tasks involve pressurised heating components and must only be carried out by a Gas Safe registered engineer.
Parts you may need
- High-limit safety thermostat · from £35
- Circulating pump (Grundfos or equivalent) · from £85
- Plate heat exchanger · from £120
- Non-return valve · from £18
- Inhibitor/system cleaner (e.g. Fernox F1) · from £20
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–£450, depending on the underlying cause.
Frequently asked questions
What is the difference between fault code 02, A02, and E020 on a Vokèra boiler?
They all mean the same thing — the boiler's high-limit safety thermostat has tripped due to overheating. The way the code is displayed simply varies by model range. Older and mid-range Vokèra boilers typically show 02 or A02 on a simple display, while the newer Evolve range uses the E020 format. All three point to the same overheat-lockout condition and are diagnosed and repaired in the same way.
Can I fix a Vokèra 02 fault myself?
There are a few safe checks you can do yourself: bleed the radiators to remove trapped air, top up system pressure to around 1–1.5 bar via the filling loop, allow the boiler to cool, and then try a single reset. If the fault clears and does not return, great — but keep an eye on it. If it comes back, stop resetting and call a Gas Safe registered engineer. The underlying causes (pump failure, sludge, a faulty thermostat, or a blocked plate heat exchanger) all require professional tools and expertise to diagnose properly.
How much does it cost to repair a Vokèra A02 fault in the UK?
Most homeowners with a returning A02 fault pay somewhere between £150 and £450, depending on what the engineer finds. A thermostat or limit stat swap typically sits in the £100–£250 range, and pump replacement usually costs £200–£400 including parts and labour. A power flush to clear heavy sludge is a separate job and tends to cost £400–£600. If the plate heat exchanger needs replacing, costs can climb significantly higher — which is why it is worth mentioning in an FAQ rather than the headline estimate. Getting your boiler serviced annually reduces the risk of sludge-related faults and can keep these costs at bay.
Why does my Vokèra boiler keep showing the 02 fault code after I reset it?
Repeated resets that clear the code temporarily but allow it to return are a strong sign that a physical fault is present — most commonly restricted circulation from sludge, a weakening pump, or a failing thermostat component. Each time the boiler overheats without the root cause being fixed, you risk damaging the heat exchanger or other internal components. Rather than continuing to reset, arrange a visit from a Gas Safe engineer who can run a full diagnosis and carry out the appropriate repair.