A boiler that starts its fan, clicks through an ignition sequence, then locks out without producing heat is usually reporting a boiler flame failure. On many appliances this appears as an ignition or flame-loss fault code. The boiler has attempted to light, or has lit briefly, but has not received the flame signal it needs to continue operating.
That safety response is doing its job. A boiler must not continue releasing gas if it cannot prove stable combustion. For an engineer, the priority is to establish where the sequence has stopped, rather than replacing ignition-related parts on assumption. For a landlord or homeowner, the priority is knowing which basic checks are safe and when to stop.
What boiler flame failure actually means
A modern gas boiler follows a controlled sequence before it supplies heat. It checks demand, runs the fan where fitted, proves the air and flue route, starts ignition, opens the gas valve and looks for a flame signal at the electrode. If that signal is missing, weak or lost, the PCB closes the gas valve and places the appliance into lockout.
The wording of the fault varies by manufacturer and model. A code may refer to failed ignition, no flame detected, loss of flame or burner fault. These descriptions are closely related, but they do not always point to the same failed component. A boiler that never lights needs diagnosing differently from one that lights for two seconds and then goes out.
Repeated lockout is not a nuisance to reset away. It is evidence that the appliance cannot complete or maintain safe combustion.
Common causes of boiler flame failure
The likely cause depends on the boiler model, its age, the fault code and what happens during the start-up sequence. These are the areas an engineer will normally consider.
Gas supply or gas valve problems
If there is no gas reaching the burner at the required pressure, ignition cannot take place. The cause can be as simple as a closed emergency control valve or an issue affecting the property gas supply. It can also involve a faulty boiler gas valve, a wiring fault to the valve or incorrect settings.
Gas pressure and gas valve checks require suitable test equipment and competence. They are not homeowner checks. An engineer will confirm inlet and burner pressure against the manufacturer’s instructions before condemning a valve.
Worn ignition or flame-sensing electrodes
The ignition electrode creates the spark, while the flame-sensing electrode confirms that a flame is present. On some boilers, one combined electrode performs both functions. Over time, electrodes can become worn, cracked, contaminated or incorrectly positioned. Their leads and connections can also deteriorate.
A visible spark does not automatically mean the ignition side is sound. Spark strength, electrode gap, burner earth path and correct positioning all matter. Equally, a burner may ignite properly but the flame sensor may fail to prove it, causing an immediate shutdown.
Burner, combustion or condensate issues
A dirty burner, blocked injector, poor combustion settings or a compromised air supply can prevent reliable ignition. Condensing boilers may also lock out where the condensate trap, pipework or terminal arrangement creates an operational problem. Frozen external condensate pipework is particularly common during cold weather.
These faults need careful assessment because they affect combustion and flue safety. Do not remove a boiler case, clean a burner or interfere with flue components unless you are appropriately qualified and working within the manufacturer’s requirements.
Fan, air-pressure and flue-proving faults
Before ignition, many boilers must prove that the fan and flue system are operating correctly. A faulty fan, pressure sensor, air-pressure switch, damaged sensing tubes or restricted flue route can interrupt the sequence before the gas valve opens.
The symptoms can look like a straightforward ignition fault: the boiler starts, the fan may run, then it stops and reports lockout. This is why fault-code interpretation alone is not enough. Live readings, continuity checks and a visual inspection of approved serviceable components may be needed.
PCB, wiring and earthing faults
The PCB coordinates the ignition sequence. If it does not provide the correct output to the fan, ignition unit or gas valve, the boiler cannot light. It may also fail to recognise a genuine flame signal. Damaged wiring, loose plugs, moisture ingress and poor appliance earthing can produce similar symptoms.
PCB replacement should follow proper diagnosis. A board can be an expensive part, and the original fault may instead be a failed electrode, harness, fan or external electrical issue. Where a replacement is confirmed, match the boiler model and manufacturer part number precisely. Some PCBs require configuration, coding or set-up after fitting.
Safe checks before calling an engineer
There are a few checks a competent homeowner or site manager can make without opening the boiler casing. Start by noting the exact make, model and fault code. A photograph of the display can save time when booking a repair or identifying a replacement part.
Check whether other gas appliances are working, but do not use this as a substitute for a gas-pressure test. Confirm that the boiler has electrical power and that the external boiler isolation switch has not been turned off. If the manufacturer’s instructions permit it, perform one reset only. Repeated resets can delay diagnosis and are not a repair.
If the pressure shown on the boiler display is low, consult the appliance manual for the correct cold pressure and approved repressurising method. Low system water pressure more commonly causes a separate pressure fault, but it can prevent normal operation on some models. If an external condensate pipe is visibly frozen and safely accessible, thawing it gently with warm, not boiling, water may help. Take care around slippery surfaces and never use a flame or heat gun.
Stop immediately and seek urgent professional advice if you smell gas, suspect carbon monoxide, see scorch marks, notice damaged wiring, or the flue appears loose or damaged. Turn off the appliance if safe to do so, ventilate the area and follow the gas emergency procedure for your supplier where appropriate.
How an engineer narrows down the fault
A proper diagnosis follows the boiler’s sequence of operation. First, the engineer confirms the reported fault and checks the appliance condition, gas supply, electrical supply, polarity and earth. They then observe whether the fan runs, whether ignition occurs, whether the gas valve is energised and whether a stable flame is established.
Testing may include checking electrode condition and gap, inspecting leads and connectors, measuring gas pressures, verifying fan and air-proving operation, and assessing combustion with a calibrated analyser. The exact procedure depends on the manufacturer. A Vaillant, Baxi, Worcester Bosch, Ideal, Alpha or Biasi boiler may use different fault-code logic and component arrangements even where the symptoms appear similar.
This process matters for cost control. Replacing an ignition electrode may be sensible where it is clearly worn and the appliance has a weak spark. Replacing a gas valve, fan or PCB without proving the fault can create a costly return visit and leave the original problem unresolved.
Choosing the right replacement part
Once the failed item is identified, use the boiler’s GC number, full model name and existing part number where possible. Boiler ranges often contain visually similar components with different plugs, software versions, calibration requirements or mounting arrangements. A part that looks right is not necessarily compatible.
For repairable electronic components such as selected PCBs, refurbished options can be a practical way to control repair costs, provided the unit is supplied for the correct appliance and covered by a clear warranty. For safety-critical gas and combustion components, installation and commissioning must be carried out by a Gas Safe registered engineer.
Capital Boiler Parts can help customers identify genuine and compatible replacement components by boiler model and part number, reducing the risk of ordering an incorrect item when downtime matters.
When replacement is not the best answer
A flame failure can expose a wider issue with an older appliance. If the burner, heat exchanger, flue system and controls all need attention, repair costs may no longer be proportionate. That decision depends on the boiler’s condition, service history, parts availability and the value of restoring reliable heating quickly.
For a single confirmed component fault, a targeted repair is often the sensible route. Keep the fault code, boiler serial details and engineer’s diagnosis with the job record. It makes the next part order quicker, gives the installer a clear trail, and helps ensure the boiler is returned to service safely rather than merely persuaded to restart.
