A boiler can have power, show a demand for heat and run its fan, yet still lock out before the burner lights. To diagnose boiler ignition faults properly, follow the appliance’s firing sequence rather than assuming the fault code points to one failed part. That approach prevents unnecessary PCB, fan or gas valve replacements and gets to the cause faster.
For landlords and maintenance teams, an ignition lockout also needs a clear safety boundary. Checks involving the gas supply, burner, combustion, flue integrity or gas valve adjustment must be completed by a Gas Safe registered engineer. The practical checks below help establish where the sequence has stopped and what component or condition needs further investigation.
Understand the boiler ignition sequence first
Most modern boilers follow a similar order when there is a call for central heating or hot water. The PCB receives the demand, checks key safety circuits, starts the fan and waits for the correct air-pressure or fan-proving signal. It then energises the ignition circuit and opens the gas valve. Once a flame is established, the flame-sensing circuit confirms it. If that confirmation is not received within a few seconds, the boiler shuts the gas valve and locks out.
The exact sequence varies by manufacturer and model. A Vaillant, Baxi, Ideal or Worcester Bosch boiler may display a different fault code for the same stage of failure. The useful question is not simply, “What code is showing?” It is, “What was the last action the boiler completed before it locked out?”
A boiler that never starts the fan is in a different diagnostic area from one that sparks repeatedly but cannot prove a flame. Listening and observing the sequence from a safe position can save a repeat visit.
Safety checks before you diagnose boiler ignition faults
First, check for a gas smell. If there is one, do not operate the boiler, switches or any electrical equipment. Follow the gas emergency procedure, ventilate the area if safe to do so, and contact the emergency gas service.
If there is no gas smell, confirm that the boiler has electrical power, its isolation switch is on and the display is active. Check system pressure against the manufacturer’s normal cold-pressure guidance. Low pressure will commonly prevent operation, although it is not usually an ignition fault in itself. Also check that the condensate pipework is not frozen or visibly blocked, particularly during cold weather.
Reset the appliance once only after recording the displayed code. Repeated resets can obscure an intermittent fault and, where the boiler attempts ignition, do not deal with the underlying cause. If it locks out again, move on to systematic checks.
Check the fault code, but do not stop there
The manufacturer’s fault-code information is a starting point, not a diagnosis. Codes can identify a failed safety condition, a missing flame signal or an ignition failure, but they do not always distinguish between a component fault, a wiring issue, poor earth continuity or an external condition.
Record the boiler make, full model, GC number where available and the complete code shown. For trade engineers, fault history and live diagnostic values can be particularly useful. A code that occurs only on hot water demand, for example, may point towards a diverter issue, flow sensing or a condition linked to a particular firing demand rather than the ignition hardware itself.
Before ordering a replacement, check the appliance’s wiring connections for loose plugs, damaged insulation, signs of moisture and heat damage around the PCB, fan and ignition components. Isolate electrical power before handling any internal connections.
Trace the failed stage in the firing sequence
No fan operation or no pre-ignition checks
If there is a heat demand but no fan start, the fault may involve the PCB output, fan, wiring harness or a safety control preventing the sequence from beginning. A seized or failed fan is not the only possibility. Some boilers will not progress if their temperature sensors report an implausible value, if the condensate safety circuit is open, or if an internal overheat condition has not reset.
A competent engineer should verify supply and control voltages to the fan in line with the manufacturer’s test procedure. Replacing a fan without proving that it is being commanded is a costly guess, especially where PCB or wiring faults are possible.
Fan runs, but the boiler does not spark
When the fan runs but there is no audible ignition attempt, look at the fan-proving and air-pressure stage. Depending on the boiler, this can involve an air pressure switch, venturi, pressure tubes, fan speed feedback or an electronic sensor. A blocked, split, loose or water-filled pressure tube can stop the boiler moving to ignition. So can a blocked flue terminal, a contaminated venturi or a fan that turns but cannot achieve the required speed.
Flue checks must be carried out safely and in accordance with the appliance instructions. Never dismantle or alter a flue system to get the boiler running. Poor flue condition is a combustion safety issue, not just an inconvenience.
The boiler sparks but there is no flame
A visible or audible spark with no flame points towards the gas path, ignition electrode condition, gas supply or burner-related issue. Check the simple external causes first: is the emergency control valve open, is there gas supply to other gas appliances, and has a prepayment meter run out of credit? These checks do not replace proper gas-rate and inlet-pressure testing.
The ignition electrode can crack, become badly sooted, corroded or incorrectly positioned. Its lead may also break down under load even if it looks acceptable at rest. On some models, the spark generator is part of the PCB; on others it is a separate ignition unit. Exact part identification matters here, as similar-looking electrodes and leads are not necessarily interchangeable across boiler variants.
Testing the gas valve, inlet pressure, burner pressure and combustion is Gas Safe work. A gas valve should never be adjusted by trial and error. Incorrect settings can create unsafe combustion and damage the appliance.
Flame lights briefly, then the boiler locks out
If the burner lights and goes out within a few seconds, the boiler may not be receiving a reliable flame-proving signal. The flame-sensing electrode, its lead, burner earth path, PCB input and polarity can all be relevant. A contaminated flame sensor is a common cause, but cleaning or replacing it without checking earth continuity can result in the same fault returning.
Poor electrical earthing, reversed polarity or a compromised connection can affect flame rectification. This is why a replacement electrode is not always the complete answer. The engineer should follow the manufacturer’s testing values and inspect the burner and seals as part of the repair.
Common symptoms and likely areas to investigate
| What you observe | Likely stage to investigate |
| --- | --- |
| Boiler locks out with no fan noise | Electrical supply, PCB command, fan, safety controls |
| Fan runs but no spark follows | Air proving, pressure tubes, venturi, flue condition, fan feedback |
| Repeated sparking with no flame | Gas availability, electrode, lead, gas valve and burner checks |
| Flame appears then immediately disappears | Flame sensor, earth path, polarity, PCB flame detection |
| Fault occurs mainly in freezing weather | Condensate discharge, frozen external pipework, flue terminal conditions |
These are diagnostic directions, not definitive part numbers. The same symptom can have more than one cause, and manufacturer-specific procedures always take priority.
Avoid the parts-cannon approach
Ignition faults are often made more expensive by changing several components at once. A PCB may be blamed because it controls the sequence, but the PCB is often responding correctly to a failed fan-proving signal or missing flame detection. Equally, fitting a used or reconditioned component without confirming its exact compatibility can create a different fault altogether.
Match parts using the appliance’s full model and manufacturer part number where possible. Check whether the boiler has a revision change, as fans, PCBs, electrodes and gas valves can differ within the same named range. For a repairable item such as a PCB, refurbishment can be a cost-effective option when it is supplied with the correct specification and warranty, but only after the external cause of failure has been ruled out. A water leak, damaged harness or failed fan can damage the replacement part again.
Capital Boiler Parts can assist with identifying genuine and serviceable replacement components when you have the boiler model and part number to hand. For urgent breakdown work, that accuracy is usually worth more than ordering the first part that appears to match.
When the fault needs escalation
Escalate without delay where there are combustion concerns, recurring lockouts after repair, signs of overheating, water damage around electrical components, damaged flue parts or any uncertainty over gas pressures and valve operation. An analyser-based combustion check and the appliance manufacturer’s fault-finding procedure are essential after work affecting the burner, gas train, fan or sealed combustion case.
A clean diagnosis starts with the sequence, confirms the failed condition and only then selects the part. That gives engineers and property managers the best chance of a first-time repair - and helps ensure the boiler returns to service safely rather than simply firing once before failing again.
