Vokera A07 Fault Code: Causes, Fixes & Repair Costs
What does the Vokera A07 fault code mean?
The A07 code appears on Vokera boilers when the control board detects an abnormal reading from the primary flow thermistor — the temperature sensor mounted on the hot-water outlet side of the boiler. NTC thermistors work by changing their electrical resistance as temperature rises or falls; the PCB reads this resistance to judge whether the heating water is at the right temperature. When that signal looks wrong — either because the sensor itself has failed, the water is genuinely overheating, or the board is misreading the data — the boiler locks out and displays A07. This code relates specifically to the central heating flow circuit, not to the domestic hot water sensor.
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 NTC Flow Thermistor Common
The thermistor itself is the single most common culprit. Over time the sensor degrades and begins sending resistance readings that no longer match the actual water temperature. The PCB interprets this as an overtemperature condition and locks the boiler out. A straightforward resistance test with a multimeter will confirm whether the sensor is out of specification.
- Restricted Water Circulation or Pump Failure Common
If water cannot move freely around the system — due to a sluggish pump, partially closed valves, or a heavily sludged circuit — heat builds up at the flow sensor faster than normal. The thermistor then records a genuine overtemperature, triggering A07. A weak or seized pump is a frequent contributor, particularly in older systems.
- Limescale or Sludge Build-up Common
In hard-water areas, limescale can coat the thermistor pocket and the heat exchanger, insulating them from the surrounding water. The sensor then reads a higher temperature than the bulk of the water, causing nuisance lockouts. Black iron oxide sludge causes a similar problem by restricting flow passages and reducing heat transfer efficiency.
- Airlock in the Heating Circuit Sometimes
A trapped pocket of air can starve part of the circuit of water, causing localised overheating around the flow sensor or heat exchanger. The boiler locks out to protect itself. Airlocks can develop after draining down, adding inhibitor, or following a drop in system pressure.
- PCB Fault or Wiring Issue Rare
If the circuit board itself is damaged — most often by moisture ingress or a loose connector vibrating free — it may misinterpret a perfectly healthy thermistor signal and show A07 in error. This is less common than a sensor fault but worth investigating once all other causes have been ruled out.
How to fix it
- Check power, controls, and system pressure DIY safe
Before anything else, confirm the boiler has power (check the fused spur switch), that the programmer or thermostat is calling for heat, and that the pressure gauge reads between 1 and 1.5 bar when the system is cold. If the pressure is low, top it up via the filling loop following the guidance in your Vokera manual. Correct pressure alone occasionally resolves circulation-related lockouts.
- Bleed the radiators DIY safe
If you cannot remember the last time the radiators were bled, work through them from the ground floor upwards using a bleed key. Release air until water flows steadily, then re-check and top up pressure if needed. Clearing an airlock this way can sometimes resolve an A07 caused by localised overheating at the flow sensor.
- Attempt a single boiler reset DIY safe
Using the reset procedure described in your Vokera manual, reset the boiler once. If it fires up and runs normally, monitor it closely over the next few hours. If A07 returns promptly, do not keep resetting — repeated resets without fixing the underlying problem can mask a serious issue and risk damaging internal components.
- Arrange a Gas Safe engineer to test the flow thermistor Gas Safe engineer
An engineer will remove the boiler casing, locate the NTC thermistor on the flow pipework, and check its resistance against the manufacturer's temperature-resistance curve using a multimeter. A reading outside the expected range confirms the sensor has failed. Thermistor replacement is a relatively quick job and is the most cost-effective first repair to carry out.
- Have the engineer assess circulation and carry out a power flush if needed Gas Safe engineer
If the thermistor checks out, the engineer will test pump speed, check for partially closed valves, and evaluate whether sludge or limescale is restricting flow. A power flush combined with a dose of descaler and inhibitor can clear blockages, after which fitting a magnetic system filter will help prevent the problem returning.
- Have the engineer inspect the PCB and wiring harness Gas Safe engineer
If the thermistor and circulation are both satisfactory, the engineer will inspect the wiring loom for signs of corrosion or loose terminals, and test the PCB. In some cases a connector can be re-seated and the fault clears without a full board replacement. If the board has moisture damage it will need replacing.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
If the steps above have not resolved the A07 fault, or if you are unsure at any point, contact a Gas Safe registered engineer. You can verify an engineer's registration at gassaferegister.co.uk. Do not continue using a locked-out boiler by repeatedly resetting it.
Parts you may need
- NTC Flow Thermistor (Vokera compatible) · from £15
- Circulating Pump (replacement) · from £85
- Central Heating Inhibitor (1 litre) · from £12
- Magnetic System Filter · from £75
- PCB (model-dependent) · 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 £120–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Vokera A07 fault myself?
The safe DIY steps are limited to checking system pressure, bleeding radiators, and attempting one reset. Anything beyond that — including accessing the thermistor, testing the PCB, or bleeding the pump — requires the boiler casing to be opened, which must only be done by a Gas Safe registered engineer. Attempting internal boiler work without registration is illegal and dangerous.
How much does it cost to fix a Vokera A07 fault in the UK?
For the most common repair — replacing the NTC flow thermistor — expect to pay roughly £120 to £200 once parts and a call-out plus labour are included. If the system also needs a power flush to clear sludge, budget an additional £150 to £350 depending on the size of the system. PCB replacement and heat exchanger replacement are significantly more expensive (often £450 or more for parts alone) but are much less frequently the root cause of A07.
Why does my Vokera keep showing A07 after I reset it?
If A07 returns within a short time of resetting, the underlying fault has not been fixed. Repeated resets without a proper diagnosis can mask a genuine overtemperature condition, which risks cracking the heat exchanger. Stop resetting and arrange for a Gas Safe engineer to diagnose the cause — most likely a failing thermistor or a circulation problem.
Could limescale cause the A07 code on my Vokera boiler?
Yes, particularly in hard-water areas of the UK such as the South East and East of England. Limescale can coat the thermistor pocket so it no longer accurately reads the water temperature, and it can also foul the heat exchanger and restrict flow. An engineer can use a descaling agent as part of a system flush, and fitting a scale reducer or magnetic filter afterwards helps prevent recurrence. Annual servicing is the best long-term defence.