Boiler Not Firing Up: Fault-Finding Guide
A boiler that will not light usually points at one of a handful of causes — no power, no gas, low pressure, a failed ignition or a control calling incorrectly. Working through them in order finds it fastest.
A boiler that will not fire is a common winter callout, and the cause is usually one of a small number of things: no electrical supply, no gas, system pressure too low, a failed ignition sequence, or a control that is not actually calling for heat. Diagnosis is about working through them logically rather than guessing.
This guide is a diagnostic walk-through of the common causes and the checks a Gas Safe registered engineer makes. It is written to help you understand and describe the fault — not to encourage unqualified work, because any intervention on the gas side of a boiler must be done by a Gas Safe registered engineer.
Key takeaways
- Work through the basics first — power, gas supply, system pressure and controls — before opening the appliance.
- Many combis lock out below about 0.5 bar; low pressure is one of the most common reasons a boiler won't fire.
- A fault code on the display is the fastest lead — read it against the manufacturer's manual before anything else.
- No gas at the boiler can mean a closed valve, a tripped meter, an empty LPG tank or a supply-wide issue.
- Ignition faults, gas valve faults and flame-detection faults are qualified-engineer territory, not DIY.
Start with the display and the basics
Before anything else, look at the boiler display. A fault code is the quickest route to the cause — every manufacturer publishes a code list, and a lockout code points you straight at ignition, flame loss, low pressure or a sensor. A blank display, by contrast, points at power.
Then check the obvious externals: is there power to the boiler and is any fused spur or switch on; is the room thermostat or programmer actually calling for heat and set correctly; and is the system pressure in the normal band. A surprising number of 'dead' boilers are a tripped switch, a flat thermostat battery or a programmer set to off.
Read the code first
A displayed fault code turns guesswork into diagnosis. Note it down and check it against the manufacturer's manual before you touch anything — it often tells you within seconds whether you are chasing ignition, pressure, a sensor or the gas valve.
Power and controls
A completely dead boiler with no display usually means no power: a tripped RCD or breaker, a blown fuse in the spur, or a fault on the supply. Restore power and the boiler should wake, though a persistent trip is itself a fault to investigate rather than repeatedly reset.
If the boiler has power but never fires, the demand may not be reaching it. Check the programmer is on and in a heating period, the room thermostat is set above the room temperature and its battery is not flat, and any wireless stat is paired. On systems with motorised valves, a stuck valve or its microswitch can also stop the boiler being told to fire.
Doing this on a job? Gas service & maintenance record — part of TradePlanr. Free to start, no card needed.
Low system pressure
Most combi and system boilers monitor pressure and will refuse to fire — often showing a low-pressure fault code — if it drops below roughly 0.5 bar. The gauge sitting near or on zero is a clear cause, and topping the system back to around 1–1.5 bar via the filling loop will usually let it fire again.
But low pressure is a symptom as well as a cause. If a boiler keeps dropping below the firing threshold, the underlying reason — a leak, a discharging PRV or a failed expansion vessel — needs finding rather than simply refilling each time.
Gas supply
No gas at the appliance is a common and easily overlooked cause. Check the appliance isolation valve is fully open, that other gas appliances in the property work (if the hob and fire are also dead, it is a supply-wide issue), and that a prepayment meter has not run out of credit or an emergency control valve been left off.
On LPG, an empty tank or cylinder, a frozen regulator in cold weather, or a changeover valve on the wrong bottle will all starve the boiler of gas. Any work on the gas supply, valves or regulator is for a Gas Safe registered engineer — confirming the supply is present is as far as diagnosis goes without qualification.
Is it just this appliance?
Before suspecting the boiler, check whether the hob, fire or other gas appliances work. If everything gas is dead, the fault is upstream — the meter, the ECV or the supply — not the boiler.
Ignition and gas valve faults
If power, controls, pressure and gas are all present and the boiler still will not light, the fault is inside the appliance: a failed ignition electrode or lead, a dirty or faulty flame-sensing electrode, a fan or air-pressure switch preventing the sequence, or a gas valve not opening. Many of these show as a specific ignition-lockout or flame-loss code.
These are diagnosed and repaired by a Gas Safe registered engineer with a manometer and analyser, working to the manufacturer's fault flow. This is the point where DIY stops and qualified work begins — the gas valve, combustion and flame-detection systems are not something to probe without the training and instruments.
From guidance to action
Frequently asked questions
Why won't my boiler fire up but has power?
Common reasons are low system pressure (below about 0.5 bar the boiler locks out), a control not calling for heat (programmer off, thermostat set low or a flat stat battery), no gas reaching the appliance, or an internal ignition fault. Check the display for a fault code first, then work through pressure, controls and gas.
Can low pressure stop a boiler firing?
Yes. Most combi and system boilers refuse to fire below roughly 0.5 bar and usually show a low-pressure fault code. Topping the system back to 1–1.5 bar via the filling loop normally clears it — but if it keeps dropping, the underlying leak or expansion-vessel fault needs finding.
Should I try to fix a boiler that won't ignite?
You can safely check power, controls, pressure and whether other gas appliances work. Anything on the gas side — ignition, the gas valve, flame detection or the supply — must be diagnosed and repaired by a Gas Safe registered engineer. It is not safe or legal DIY territory.
What does a lockout mean on a boiler?
A lockout is the boiler shutting itself down safely after failing a step in its start sequence — commonly failing to detect a flame after ignition. It usually shows a fault code. A reset may clear a one-off, but a boiler that repeatedly locks out has a fault that needs a Gas Safe engineer.
Related guides
Boiler Keeps Cutting Out: Causes and Fixes
Why a boiler fires then shuts down — low pressure, overheating, frozen condensate, flame loss and short cycling — with how to diagnose each.
Why Your Boiler Keeps Losing Pressure
The common causes of a boiler losing pressure — leaks, a discharging PRV and a failed expansion vessel — and how to diagnose each.
Boiler Fault Finding: A Systematic Approach
A structured method for diagnosing common boiler faults — no ignition, lockout and no heat.
