Vokera A02 Fault Code: Limit Thermostat Fault
The A02 code on a Vokera boiler means the safety limit thermostat has triggered a protective shutdown. This thermostat continuously monitors internal water temperature, and when it senses that the boiler is running hotter than it safely should, it cuts operation to prevent damage. The result is a loss of both heating and hot water until the underlying cause is identified and resolved. On newer Vokera Evolve models, the same issue may appear as code E020 rather than A02.
General guidance only — not a substitute for professional advice. 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.
What does the Vokera A02 fault code mean?
The A02 code on a Vokera boiler means the safety limit thermostat has triggered a protective shutdown. This thermostat continuously monitors internal water temperature, and when it senses that the boiler is running hotter than it safely should, it cuts operation to prevent damage. The result is a loss of both heating and hot water until the underlying cause is identified and resolved. On newer Vokera Evolve models, the same issue may appear as code E020 rather than A02.
Common causes
- Trapped air in the heating system Common
Air pockets in the radiators or pipework disrupt water circulation, forcing the boiler to work harder to distribute heat. The resulting hot spots inside the boiler can cause the limit thermostat to trip. Bleeding radiators is often the first and most effective remedy.
- Sludge or scale in the heat exchanger Common
Over time, magnetite sludge and limescale accumulate inside the heat exchanger, narrowing the passages through which water flows. Restricted flow means heat builds up locally, triggering the limit thermostat. Systems without inhibitor or a magnetic filter fitted are particularly susceptible.
- Faulty or sluggish circulating pump Common
The pump is responsible for keeping water moving around the system. If it has seized partially, worn out, or is running below the correct speed, water stagnates and the boiler overheats. A pump fault is a frequent cause of repeated A02 trips even after bleeding and pressure correction.
- Low system pressure Sometimes
If pressure drops below around 1 bar, there is insufficient water volume circulating, which can cause localised overheating and trigger the limit thermostat. This is especially common in older systems or those with a small leak.
- Faulty limit thermostat Sometimes
The limit thermostat itself can develop a fault and trip at temperatures lower than it should, producing an A02 code even when the boiler is not genuinely overheating. Component ageing, a poor electrical connection, or physical damage can all cause this.
- Blocked or partially closed radiator valves Sometimes
If thermostatic radiator valves or lockshield valves are closed off across too many radiators simultaneously, the system has nowhere to push water, causing pressure and heat to build inside the boiler.
How to fix it
- Switch the boiler off and let it cool for at least 15 minutes DIY safe
Do not attempt to reset a hot boiler immediately. Allowing it to cool fully gives the limit thermostat a chance to reset and makes subsequent checks safer and more accurate.
- Check and correct the system pressure 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 via the filling loop (usually two interlocked valves under the boiler) until it reaches around 1.2 bar, then close the valves. If pressure is already correct, move on to the next step.
- Bleed all radiators to release trapped air DIY safe
Starting from the radiator furthest from the boiler and working back towards it, use a radiator bleed key to open each bleed valve slightly until water (not air) flows out, then close it. After bleeding, check the boiler pressure again and top up if it has dropped, as releasing air often causes a small pressure loss.
- Check that all radiator valves are open DIY safe
Walk around and confirm that thermostatic radiator valves are not turned fully off across most of the radiators, and that lockshield valves (the plain-capped valves on the other end) are open. Closing off too many radiators restricts circulation and can cause overheating.
- Reset the boiler and monitor it closely DIY safe
Once the boiler has cooled and you have completed the checks above, perform a reset according to your Vokera model's manual (typically holding the reset button for a few seconds). If the boiler fires up and runs normally, monitor it over the next hour or so. If the A02 code returns promptly, do not keep resetting — this masks a fault that needs professional attention.
- Call a Gas Safe registered engineer if the fault persists or returns Gas Safe engineer
If the A02 code comes back after your basic checks, a Gas Safe engineer needs to investigate further. They will assess pump operation, test the limit thermostat, inspect the heat exchanger for sludge or scale, and carry out a full system check. Depending on findings, they may recommend a power flush, pump replacement, or thermostat replacement. Repeatedly resetting without fixing the root cause risks component damage and could void any remaining warranty.
Parts you may need
- Circulating pump · from £95
- Limit thermostat (high limit safety thermostat) · from £25
- Magnetic system filter (e.g. Magnaclean) · from £65
- Central heating inhibitor (1 litre) · from £15
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–£400, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting my Vokera boiler when A02 appears?
A single reset after carrying out the basic checks — bleeding radiators, correcting pressure — is perfectly reasonable. However, if the A02 code keeps coming back, resetting repeatedly is not a fix and could mask a serious problem such as a failing pump or a badly sludged heat exchanger. At that point you need a Gas Safe engineer to find and fix the root cause.
How much does it cost to repair a Vokera A02 fault in the UK?
For most households the repair falls between £120 and £400 depending on what is found. Simple jobs like a thermostat replacement or bleeding and system inhibitor top-up sit at the lower end. Pump replacement typically costs £200–£350 including parts and labour. A power flush for heavy sludge build-up is usually £400–£600 on its own. In rare cases where the PCB is implicated, replacement can reach around £500–£600, though this is uncommon for an A02 code specifically.
What is the difference between Vokera fault codes A02 and E020?
They point to the same underlying problem — a limit thermostat fault causing a safety shutdown due to overheating. A02 appears on older and mid-range Vokera models, while E020 is the equivalent code used on newer Evolve series boilers. The causes, checks, and repair approach are the same for both.
Could low boiler pressure cause an A02 fault?
Yes, indirectly. If system pressure drops below around 1 bar there is less water circulating around the system, which can cause localised overheating inside the boiler and trigger the limit thermostat. Checking and correcting the pressure via the filling loop is one of the first DIY steps worth trying before calling an engineer.