Why Does My Boiler Keep Locking Out — The Real Cause
Why does my boiler keep locking out? The single most common cause is ignition fault. Repeated lockout = ignition electrode fouled or flame sense wire loose. Cheap fix pre-PCB replacement. Here is the diagnostic order UK plumbers follow so you don't spend money replacing the wrong part.
Step-by-step method
1. Rule out the free fixes first
Before spending anything: check system pressure, check the timer, replace room-stat batteries, and check all isolators are fully open. This clears 30% of callouts.
2. Confirm the symptom is what you think
Reproduce it deliberately. When exactly does it happen? Cold start, mid-cycle, only with heating, only with hot water? The pattern names the fault.
3. Test the likely cause
Repeated lockout = ignition electrode fouled or flame sense wire loose. Cheap fix pre-PCB replacement.
4. Try the cheap fix
Most root causes have a £5–£50 part fix. Do this before calling out an engineer — a plumber will do the same first-line diagnostic and charge £90–£140.
5. Call a professional if unresolved
If the cheap fix doesn't resolve within 24 hours, or the fault involves gas / a sealed unvented system, book a Gas Safe or G3 engineer. Have model number and serial ready.
Frequently asked questions
How urgent is ignition fault?+
Not immediate emergency unless water is actively escaping. Book within 3–5 days to avoid escalation.
Will insurance cover the repair?+
Most home insurance excludes wear-and-tear parts. Boiler cover (British Gas HomeCare etc) covers labour and parts on covered appliances.
DIY or plumber?+
Non-gas fixes are usually DIY-able with a spanner, PTFE tape and 30 minutes. Anything gas-side needs Gas Safe registration — legal requirement.