When an oil boiler stops working, the cause is very often something simple: the tank has run low or empty, the burner has locked out and needs a single reset, the power has tripped, the system pressure has dropped or the controls are not calling for heat. Run through those five checks before you call anyone. If the boiler locks out again after one reset, smells of oil, smokes or makes an unusual noise, stop and call an OFTEC registered engineer, because repeated resets on a boiler with a real fault can flood the combustion chamber with oil.
This guide takes the checks in order, from the ones that take a minute to the ones that need an engineer, and explains what is happening inside the boiler at each stage.
1. Check the oil level
Look at the tank gauge, or dip the tank if it has no gauge. Oil boilers draw from the bottom of the tank and a tank that reads a few centimetres can still run out because of sludge or a sloping base. If the tank is empty, order oil, and expect the boiler to need bleeding of air from the fuel line after the delivery. Our guide on a boiler not working after an oil delivery explains why.
2. Check for a lockout and reset once
Most oil boilers have a red or orange lockout light on the burner control box, sometimes with a reset button on it. A lockout means the burner tried to light and failed, or lost its flame. Press the reset once and wait. If the boiler fires and runs, note the date; an occasional lockout after a windy night or an oil delivery is normal. If it locks out again, do not keep pressing. Each attempt sprays oil into the chamber, and a second failure means something needs attention. Our guide to resetting an oil boiler lockout goes into detail.
3. Check the power
Oil boilers need electricity for the burner, pump and controls. Check the fused spur beside the boiler is on and its fuse is intact, and that no trip has gone at the consumer unit. An external boiler has its own switch nearby. After a power cut, some boilers need a reset.
4. Check the pressure
On a sealed system, a pressure below about 1 bar stops the boiler. Top it up using the filling loop to around 1.3 bar. If it keeps dropping, there is a leak somewhere that needs finding. Our guide on how to repressurise a boiler shows the top-up.
5. Check the controls
Make sure the programmer is on and in a heating period, the room thermostat is set above room temperature and any wireless thermostat has good batteries. On a system with a cylinder, check the cylinder thermostat too. A boiler that is not being asked for heat is not broken.
Faults that need an engineer
- Repeated lockouts. A blocked nozzle, a dirty photocell, worn electrodes, a failing pump or air in the fuel line.
- Smell of oil. A leak on the fuel line, filter, pump or nozzle assembly. Isolate at the fire valve and call.
- Smoke or soot. Poor combustion from a blocked nozzle, wrong air setting or a sooted heat exchanger. Our guide to an oil boiler smoking or sooting explains the causes.
- Burner runs but no heat. Pump, diverter or motorised valve, or a sludge blockage in the system.
- Tank problems. Water or sludge in the tank, a blocked filter or a frozen fuel line in winter.
- Unusual noises. Rumbling, banging or a fan that screams.
What the engineer will do
An OFTEC engineer starts at the tank and works towards the boiler: oil level and quality, filter, fire valve, fuel line for air and leaks, then the burner. They check the nozzle, electrodes, photocell, pump pressure and air setting, then fire the boiler and test combustion with an analyser and a smoke test. Most oil boiler faults are found and fixed in a single visit, often as part of a service, because the parts that fail are the ones a service replaces.
Preventing the next one
Keep the tank above a quarter full, particularly in winter, have the boiler serviced every year with a new nozzle and filter, and have the tank checked for water and sludge. Most oil boiler breakdowns trace back to fuel quality or a missed service. Our guide on how often an oil boiler should be serviced explains what a service prevents.
How DCI Boilers can help
DCI Boilers repairs oil boilers across Cornwall and Devon with OFTEC registered engineers who carry the common parts and diagnose from the tank to the flue. Fixed prices are agreed before work starts, and our emergency boiler call-out runs around the clock for households with no heat. For anything less urgent, book boiler repairs and we will get you warm again.
Need help now? Call DCI Boilers on 07888 862 603 or get a free quote.
Frequently asked questions
Why does my oil boiler keep locking out?
Usually a blocked nozzle, a dirty photocell, air in the fuel line or low oil. One reset is fine; repeated lockouts need an engineer.
Can I reset my oil boiler more than once?
Reset it once. If it locks out again, stop, because each attempt puts unburnt oil into the chamber.
My oil boiler runs but the radiators are cold. Why?
The boiler is fine; the problem is the pump, a valve or sludge in the system. An engineer can diagnose which.
Is an oil boiler not working an emergency?
If you smell oil or see smoke, yes. If it is simply not firing and you have some heat, it can usually wait for a next-day visit.