BoilerFaultCodes S.03 Medium Lockout
Your boiler is trying to start up to provide heating, but it keeps failing to light the flame correctly.
Book a service
This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.
Technical description: Heating mode – ignition sequence error
What causes this fault?
This fault occurs when the boiler attempts to start the combustion process but fails to detect a stable flame after several attempts. It is usually caused by a worn-out ignition spark plug that can no longer bridge the gap, or a failing gas valve that isn't providing the correct amount of fuel into the burner.
Typical repair cost £120-350 (including labour)
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like S.03 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.