An oil boiler that won’t fire is usually protecting itself. More often than not the cause is on the supply side: a low tank, air in the oil line or a blocked filter. Faults inside the burner, such as a worn nozzle or a dirty photocell, come next, and genuine control box failures are rarer than most people fear.
That is little comfort when the house is cooling down, so this guide answers the questions people actually ask when it happens. Each answer starts with the short version, followed by the detail, so you can check the simple things yourself and know when to stop and call an engineer.
Why is my oil boiler not firing up?
The most likely reason is that oil isn’t reaching the burner in good condition. When that happens the burner tries to light, can’t find a flame, and locks out for safety. Here are the causes we see most often, roughly in order of how common they are.
| Cause | How it usually shows up | Can you check it? |
|---|---|---|
| Tank empty or very low | Lockout light on, gauge near the bottom | Yes, check the gauge or dip the tank |
| Air in the oil line | Lockout after a run-out, a delivery or new pipework | Partly, bleeding is often an engineer job |
| Blocked oil filter | Repeated lockouts, worse when the tank is low | No, it needs changing |
| Worn or blocked nozzle | Weak, smoky or noisy flame, then lockout | No |
| Dirty or failed photocell | Burner lights, then cuts out after a few seconds | No |
| Worn or badly set electrodes | Delayed ignition, a thud or bang on lighting | No, stop resetting |
| Water or sludge in the oil | Lockouts that return days after a repair | No, the tank needs checking |
| Closed valve or blocked tank vent | Tank full but no oil getting through | You can look, an engineer should clear it |
| No power or no call for heat | Totally silent, no lockout light | Yes |
Less often, the motor coupling seizes, the oil pump wears out, the solenoid valve sticks or the ignition transformer fails. Those give themselves away with specific noises, like a motor humming but not turning, or a fan running with no spark, and they always need an engineer.
What should I check before calling anyone?
Four things, and they take about five minutes. They also solve a surprising number of call-outs.
- Make sure the boiler has power. Check the fused spur or isolator switch and look for a blown fuse.
- Check the controls are asking for heat. Turn the room thermostat up, check the programmer is in an “on” period and replace the batteries in any wireless thermostat.
- Look at the oil tank gauge. If it is low, order oil before doing anything else.
- Look for the lockout light on the burner. If it is lit, press the reset button once and give the boiler a few minutes to run through its start-up.
If it fires and keeps running, keep an eye on it over the next day or so. If the lockout light comes back, leave it off and call for help.
Is it safe to keep pressing the reset button?
No. Reset once, and twice at the very most with a few minutes between. Each failed start can leave a little unburned oil in the combustion chamber, and repeated attempts can light it all at once with a loud bang. That can damage the boiler and, in a bad case, the flue. If one reset doesn’t sort it, the boiler is telling you something needs fixing.
Why won’t my oil boiler fire after I ran out of oil?
Because air got into the oil line when the tank ran dry, and the pump can’t push oil through an airlock. Refilling the tank doesn’t clear it on its own. The line needs bleeding, and the filter often needs changing too, because the last few centimetres of oil in any tank carry the most sediment. Our guide on how to bleed an oil boiler explains what that involves and when it is better left to an engineer.
Why did my boiler stop working straight after an oil delivery?
A delivery can stir up sludge and water sitting at the bottom of the tank, which then gets drawn towards the filter. If your tank was very low before the tanker arrived, you may also have air in the line. Neither means the boiler is broken. A new filter and a bleed usually bring it back, and it is worth asking the engineer to look at the tank while they are there.
Could the problem be my oil tank rather than the boiler?
Quite possibly. The tank is the most likely culprit if the boiler runs well for a few days after a repair and then locks out again, if the tank is old or steel, or if the lid or gauge cover is damaged and letting rain in. Water in the oil encourages sludge and bacteria, which clog the filter and nozzle.
OFTEC suggests most oil tanks have a working life of around 20 years. If yours is older than that, or shows cracks, bulging, rust or staining, have it assessed. We check the tank and oil line as standard when we fit a new boiler, and replace it if needed as part of an oil boiler installation
Why does my oil boiler only fail in cold weather?
Cold brings out a few specific faults. On condensing oil boilers the condensate pipe can freeze and shut the boiler down; thawing it with warm (not boiling) water and lagging the pipe usually solves it. On external boilers and above-ground tanks, kerosene can thicken in a hard frost and struggle through exposed pipework. And a tank running low in winter is more likely to draw up sediment. Homes in exposed spots, such as the Pennine villages around Holmfirth or the higher Dales above Skipton, tend to see these first.
Does the brand of boiler change the likely fault?
A little. The causes are mostly the same because most oil boilers use similar pressure-jet burners, but the warning lights and resets differ between makes. We have separate guides for a Grant Vortex that won’t fire, a Worcester Bosch oil boiler in lockout and a Firebird showing a red light.
How much does it cost to fix an oil boiler that won’t fire?
Most lockout repairs come down to inexpensive parts such as a filter, nozzle, electrodes or photocell, which are the same parts replaced at an annual service. Bigger bills come from pumps, motors and control boxes, or from cleaning out a contaminated tank. Booking a proper oil boiler service each year is the cheapest way to avoid most of these faults in the first place.
When is it time to replace rather than repair?
When the repairs keep coming. An older oil boiler that locks out every few weeks, needs parts that are hard to find or has a corroded heat exchanger is usually costing more to keep than to replace. A modern condensing oil boiler also burns less oil for the same warmth, which matters with kerosene prices as changeable as they have been. If you are weighing it up, our best oil boilers guide compares the current models, and the oil boiler replacement page shows fixed prices.
Some people use a breakdown as the moment to look at other options entirely. Our comparison of a heat pump vs an oil boiler is a good place to start if that is you.
Oil boiler not firing: quick answers
Is an oil boiler lockout dangerous?
A lockout itself is a safety feature, not a danger. The risk comes from resetting it over and over. Smell oil, see smoke or hear a bang on ignition and you should switch it off and leave it off.
Can I fix an oil boiler myself?
You can check the power, controls, tank and do a single reset. Anything involving the burner, oil line, filter or fire valve is for an OFTEC registered engineer.
How do I know if the fire valve has closed?
Usually the tank has oil, the burner has power and tries to start, but no oil arrives. The fire valve shuts the supply when it senses excess heat, so an engineer should find out why before reopening it.
My burner runs but the radiators stay cold. Is that a lockout?
No. If the burner is firing, the problem is circulation. A failed pump, a stuck zone valve, sludge in the system or closed radiator valves are the usual suspects.
How often should an oil boiler be serviced?
Once a year. Oil burns dirtier than gas, so filters, nozzles and electrodes wear faster, and most manufacturer warranties expect a yearly service record.
With years of experience in boilers and home improvements in general, Myles Robinson is a passionate advocate for making home heating simpler, safer, and more affordable for households across the UK. With years of experience in the heating industry, Myles combines hands-on knowledge with a focus on customer-first service, helping homeowners choose the right boiler, understand their systems, and keep their heating running efficiently all year round. His articles break down complex topics into clear, practical advice you can trust.