Oil boiler not firing — step-by-step diagnosis

If your oil boiler has gone to lockout (the red reset button light is on), the burner has failed to ignite within the safety time — usually 30 seconds. First check the obvious: is there oil in the tank? Open the tank gauge or tap the dipstick — you need at least 15–20% to ensure a reliable supply.

Press the reset button once. If it fires and runs normally, the lockout was a one-off. If it locks out again within a few firing cycles, there is an underlying fault that will repeat.

Check the oil filter: there is usually a transparent bowl filter on the oil supply line between the tank and the boiler. If it is dark, brown, or has visible debris, the filter element needs replacing (£5–£15). A blocked filter starves the burner of oil and causes lockout.

Check the oil supply line for air locks — if the tank ran empty recently, air may have entered the supply pipe. Most oil burners have a bleed point on the pump. OFTEC engineers carry a bleed procedure, but this is a task for a professional if you are unfamiliar with oil systems.

A faulty or dirty nozzle is the most common cause of repeated lockouts. The nozzle atomises the oil into a fine spray — if it is partially blocked, combustion fails. Nozzle replacement (£5–£15 part) is a standard annual service item and requires the burner to be removed from the boiler.

If the boiler fires but cuts out on limit (gets to temperature then shuts down before the room is warm), check the heat exchanger for sooting — this is common if the boiler has been running with a sub-optimal oil/air mixture. Annual servicing is essential for oil boilers — every 12 months regardless of usage.