Vokera A09 Fault Code: Causes, Fixes & Repair Costs
What does the Vokera A09 fault code mean?
The A09 code on a Vokera boiler signals one of two things depending on the model and generation. Most commonly, it points to a fault with the flue thermistor — the sensor that monitors the temperature of exhaust gases leaving the boiler — or indicates that an internal condensing-hours counter has reached a threshold, telling you the boiler needs attention. On some Vokera models, A09 can alternatively mean the control board has detected a flame signal at a moment when no flame should be present (known as false flame detection), which may point to an electrical fault, a faulty flame sensor, or flue gas recirculation. In either case, the boiler will typically lock out to protect itself, and a reset alone may not resolve the underlying problem.
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 failed flue thermistor Common
The flue thermistor is a small temperature sensor positioned in the exhaust path. If it develops a high resistance, short-circuits, or its wiring corrodes or comes loose, the boiler control board receives an out-of-range or missing signal and triggers A09. This is the single most frequent cause across Vokera models displaying this code.
- Condensing-hours counter limit reached Common
Vokera boilers on certain firmware versions record the cumulative time spent operating in condensing mode. Once that counter hits a pre-set limit, the boiler raises A09 as a service prompt, indicating it needs a full inspection and reset by an engineer rather than a simple user reset. If your boiler is overdue a service, this is very likely the trigger.
- Blocked or partially obstructed flue Sometimes
Leaves, bird nesting material, ice, or physical damage around the flue terminal can restrict exhaust flow, causing flue temperatures to rise abnormally and triggering a thermistor-related fault. Even a partial blockage that is not obvious at a glance can be enough to cause A09.
- False flame detection or flue gas recirculation Sometimes
On models where A09 relates to false flame, the PCB detects an ionisation signal from the flame sensor when the burner is off or during pre-purge. This can be caused by a leaking gas valve allowing a residual flame, contamination on the flame sensor rod, flue gases being drawn back into the combustion chamber, or electrical interference within the appliance.
- Frozen or blocked condensate pipe Sometimes
In cold spells, a frozen external condensate pipe can cause back-pressure that disrupts boiler operation and triggers a range of fault codes including A09. This is most likely if the fault appears overnight or early morning during freezing weather.
- PCB fault or wiring issue Rare
If the printed circuit board is failing or there is a damaged wire between the PCB and the flue thermistor, the board may misread the sensor signal entirely. This tends to be a diagnosis of exclusion — reached after the thermistor and wiring have been checked and found to be serviceable.
How to fix it
- Check the flue terminal for visible blockages DIY safe
Go outside and look at where the flue exits the building. Check for obvious obstructions such as leaves, debris, bird nests, or ice covering the terminal grille. Do not attempt to dismantle or reposition the flue — this is gas work. If something is visibly blocking the opening from the outside, contact an engineer.
- Thaw a frozen condensate pipe if the weather is freezing DIY safe
If outdoor temperatures are at or below zero, your external condensate pipe (usually a white or grey plastic pipe running outside, often at the side or rear of the property) may be frozen. Pour warm — not boiling — water gently along its length to thaw it. Once thawed, try resetting the boiler. Lagging the pipe afterwards will help prevent a repeat.
- Check and top up system pressure DIY safe
Look at the pressure gauge on the boiler. It should read between 1.0 and 1.5 bar when cold. If it reads below 1.0 bar, use the filling loop (two small valves beneath the boiler connected by a short braided hose) to slowly bring the pressure up to around 1.2–1.5 bar, then close both valves firmly.
- Reset the boiler DIY safe
On most Vokera models, turn the mode selector to the OFF/RESET position, hold briefly, then return it to your desired setting (heating and hot water, or hot water only). Allow the boiler a few minutes to complete its startup sequence. If A09 clears and the boiler runs normally, monitor it over the next day. If the code returns, do not continue resetting — proceed to call an engineer.
- Have a Gas Safe engineer inspect the flue thermistor and wiring Gas Safe engineer
An engineer will use a multimeter to check the thermistor's resistance against the manufacturer's specification and inspect the sensor wiring harness for corrosion, chafing, or loose connections. A faulty thermistor is straightforward to replace once diagnosed. This step is required if the reset did not clear the fault.
- Have the condensing-hours counter checked and reset (if applicable) Gas Safe engineer
If the boiler is due a service and the A09 relates to the internal condensing counter, the engineer will carry out a full service — cleaning heat exchanger surfaces, checking combustion, inspecting seals — and then reset the counter using the boiler's diagnostic interface. A user reset alone will not clear a counter-triggered A09.
- Have the flame detection circuit inspected if false flame is suspected Gas Safe engineer
Where A09 on your model indicates false flame, the engineer will inspect the flame sensor rod for contamination and test the gas valve for internal leakage. They will also check for flue gas recirculation by examining flue routing and terminal position. The PCB may be tested last if all other components prove sound.
- Call a Gas Safe registered engineer if the fault persists or returns Gas Safe engineer
If A09 returns after completing the checks above, the boiler needs professional diagnosis. Do not attempt further resets. Contact a Gas Safe registered engineer and quote the fault code so they can arrive prepared with the likely parts. You can verify any engineer's Gas Safe registration at gassaferegister.co.uk.
Parts you may need
- Vokera flue thermistor sensor · from £25
- Thermistor wiring harness · from £18
- Flame sensor ionisation rod · from £20
- Boiler PCB (control board) · from £180
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 a Vokera A09 fault myself?
The preliminary checks — looking at the flue terminal for blockages, thawing a frozen condensate pipe, topping up system pressure, and resetting the boiler — are all safe for a homeowner to carry out. However, replacing the flue thermistor, working on the PCB, inspecting the gas valve, or resetting a service counter all require a Gas Safe registered engineer. Attempting gas-related work without registration is illegal in the UK and can be dangerous.
How much does it cost to fix a Vokera A09 fault code in the UK?
For the most common repair — flue thermistor replacement including labour — expect to pay roughly £120 to £250. If the fault turns out to be a full service requirement linked to the condensing-hours counter, a service visit typically costs £80 to £150. If the PCB is the root cause, parts alone can reach £180 to £250 plus labour, pushing the total to £300 or more. Most households with an A09 fault end up spending in the £120–£350 range.
Why does my Vokera A09 code keep coming back after I reset it?
A reset only clears the lockout — it does not fix the underlying fault. If A09 returns repeatedly, the most likely explanations are a thermistor that is failing intermittently, a condensing-hours counter that has genuinely hit its limit and needs a full service reset, or a wiring issue that appears and disappears with vibration. Repeated resets without fixing the root cause can mask a developing problem, so book an engineer rather than continuing to reset.
Is A09 the same fault on all Vokera boiler models?
Not quite. The A09 code carries slightly different meanings across Vokera's range. On most models it refers to a flue thermistor or flue thermistor counter fault. On some older or alternative Vokera models it can indicate false flame detection — where the PCB sees an ionisation signal when no flame should be present. If you are unsure which interpretation applies to your specific boiler, check the model number on the appliance data plate and refer to your installation manual, or ask your engineer to confirm before ordering parts.