The burner control box runs an ignition sequence and expects the photocell to confirm a flame within a few seconds. If it does not, the box locks out and stops fuel. Pressing reset restarts the sequence. Each failed attempt sprays more unburnt oil into the chamber, which is why two attempts is the sensible limit.
What the control box is doing
When the boiler calls for heat, the burner control box runs a fixed sequence, and every step has a time limit.
- Pre-purge. The fan runs for a few seconds to clear the combustion chamber of any unburnt vapour before ignition. This is a safety step and it is why the burner makes a noise before it lights.
- Ignition. A high-voltage spark arcs between the two electrodes at the mouth of the blast tube.
- Fuel. The solenoid opens and the pump pushes oil through the nozzle, which atomises it into a fine cone of mist.
- Flame proving. The photocell must detect the light of the flame within a set safety time – typically a few seconds.
- Run. If the flame is proved, the sequence completes and the burner runs until the thermostat is satisfied.
If the photocell does not see a flame within the safety time, the box shuts the solenoid, stops the sequence and locks out. The reset button clears the lockout and starts the sequence again from the beginning.
Why two attempts is the limit
Every failed ignition attempt sprays a measured quantity of kerosene into the combustion chamber, where it does not burn. It pools in the base of the chamber and soaks into the chamber lining.
You now have a combustion chamber containing several doses of unburnt fuel. If the boiler then does light – because the fault was intermittent – it lights all of it at once. At best that is a loud thump, a cloud of black smoke from the flue and a chamber that needs stripping and cleaning. At worst it damages the appliance. The mess is entirely self-inflicted and it turns a small repair into a large one.
So: press it once. If it locks out again, press it once more. If it locks out a second time, stop and call somebody. The information you have at that point – that it fails twice in the same way – is genuinely useful to an engineer.
What the failure pattern tells you
| What happens | What it points at |
|---|---|
| Fan runs, no ignition attempt at all, immediate lockout | No fuel reaching the nozzle: empty tank, closed valve, tripped fire valve, blocked filter, or air in the line |
| Fan runs, you hear the spark, nothing lights | Fuel supply, a blocked nozzle, or ignition not reaching the fuel |
| Lights briefly, then locks out after a few seconds | Flame proving – a dirty or failing photocell, almost always |
| Lights, runs for minutes, then locks out | Flame loss during the run: air in the line, a failing pump, or a partly blocked filter |
| Locks out only in cold weather | Frozen condensate pipe, or waxing in an exposed fuel line |
| Locks out only when the weather is still | Occasionally a flue terminal issue |
| Locks out intermittently with no pattern, clears after bleeding | Air being drawn into the oil line at a joint or seal |
| No lockout light, nothing at all | Electrical supply, a control fault, or the boiler is not being called |
The intermittent ones are the most expensive to diagnose badly, because they frequently get resolved by replacing the control box – which is the wrong part, and the fault returns weeks later. Air in the fuel line is the usual culprit and it is a much cheaper fix.
The things to check before calling
- Oil in the tank. Gauges stick and lie. Check physically if the tank allows it.
- Fire valve. A thermal cut-off in the oil line, which can trip without a fire – a hot summer or a knock will do it. If it has tripped, no fuel reaches the burner and every reset will fail.
- Tank valve open. Particularly after any work on the tank.
- Electrical supply. Fused spur, RCD, boiler isolator.
- Room thermostat and programmer. Flat batteries present exactly like a dead boiler.
- Condensate pipe, in cold weather. A frozen external run shuts a condensing boiler down.
Between them, those six account for a substantial share of winter lockout calls, and all of them are free to check.
After a run-dry
If the tank has run empty, the boiler will not simply restart when oil arrives. The pump has drawn air into the fuel line, and air has to be bled out at the pump before the burner will prove a flame. Pressing reset repeatedly will not achieve it and will fill the chamber with oil once the air finally clears.
There is a second consequence. The bottom of a tank holds years of settled sludge and water, and running the level right down draws that straight into the fuel line. Filters block, and sometimes the nozzle and pump are affected. It is why a run-dry in December can become a filter change in January. See restarting an oil boiler after running out.
Locked out twice? Stop pressing and call. What you can tell us about the pattern usually identifies the cause before we arrive.
Questions we get asked about this
How many times can I press the reset button?
Twice. Each attempt puts more unburnt fuel into the combustion chamber, and the damage from repeated resets is entirely avoidable.
Where is the reset button?
Usually a red button with a light in it, either on the burner control box on the burner itself, or brought out to the boiler fascia. On an external boiler it is inside the casing.
Why does my boiler lock out every few weeks?
Intermittent lockouts that clear after bleeding are almost always air in the fuel line, drawn in at a joint, a filter seal or a high point in the run. It is a cheaper fix than the control box that frequently gets replaced instead.
The boiler locks out only in cold weather.
Most likely a frozen condensate pipe on a condensing appliance. Check the external plastic pipe before assuming anything else – see frozen condensate pipes.
Is a lockout dangerous?
No – it is the safety system doing its job. What is unwise is defeating it by repeatedly resetting, which is how unburnt fuel accumulates in the chamber.
There is a smell of oil after several resets.
That is unburnt fuel in the combustion chamber. Stop, ventilate the area, and do not reset again. It needs cleaning out before the boiler is fired.
You might also need
- Bunded or single-skin? How to tell which tank you are allowed
- Buying heating oil: order size, timing and tank management
- Eight signs an oil tank is near the end of its life
- Frozen condensate pipes: why it happens and how to stop it recurring
- Grant, Firebird, Warmflow, Worcester Danesmoor and Mistral, compared honestly
- Internal, external or boiler house: where an oil boiler should go
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.