First: what a “lockout” actually means (in plain English)
A lockout is the boiler protecting itself. It’s basically saying: “I tried to run, something didn’t look right, so I’m stopping until you reset me.” That “something” could be small… or it could be the start of a bigger failure.
Real-life example: we had a call in East London where the customer had reset it six times in one evening. It would light, run for a bit, then fault out. The actual cause was intermittent ignition (you could see it on testing), and the repeated resets just stressed the system and wasted time. Once we diagnosed it properly, it was a straightforward repair.
What to check safely (no deep DIY, just the basics)
1) Power & controls
Is the boiler actually powered? Is the programmer calling for heat? Is the thermostat turned up? (If it’s a wireless thermostat, check batteries.)
2) Pressure (quick glance)
On many combis, low pressure can trigger lockouts. If the gauge is near 0, the boiler may refuse to run. If you top it up and it drops again soon, that’s not “normal” — it needs diagnosis.
3) If the house is cold right now
When the boiler is locking out, it often feels like “nothing works” — especially if you’ve got no heating and no hot water. Do the safe basics once (controls, pressure glance, obvious issues), then stop the reset loop.
4) Look (don’t touch) for damp or water
If there’s water under the boiler or damp around pipework, don’t keep trying to run it. Water + electrics is where things get spicy in the worst way.
Common causes of lockouts (what we usually find on diagnosis)
These are the patterns we see most when someone says “it runs then stops” or “keeps showing a code”. The exact fault varies by boiler brand, but the categories are pretty consistent.
Ignition / flame detection issues
Boiler tries to light, fails, then locks out. Can be ignition components, flame sensing, gas supply problems, or poor combustion. This is a “test properly” job, not a guessing job.
Overheating / circulation problems
If heat can’t move away from the boiler (sludge, pump issues, blocked parts, air), temperatures spike and safety sensors trip. Often you’ll see it behave, then stop, then behave again.
Sensors / wiring / intermittent electrical faults
These are the frustrating ones: the boiler works, then doesn’t, with no obvious pattern. It can be a sensor drifting out of range, a loose connection, or moisture getting where it shouldn’t. Diagnosis is about proving it with readings, not swapping random parts.
Night-time failures (real-world pattern)
A lot of lockout calls happen late — heating demand, colder air, and a fault that’s been “on the edge” all week finally trips. If it’s happening after-hours and you’re trying to work out what to expect, use: If it’s failing at night, read this.
What to write down (this speeds up the repair a lot)
If you want a proper engineer to move quickly, give them clean info. Not an essay — just the useful bits:
Make/model + the exact code
Photo of the boiler front badge and the display code is perfect.
What happens right before it locks out
Does it try to ignite? Any clicking? Any fan noise? Does it run for 2 minutes or 20?
Pressure reading + anything you’ve already tried
One reset? Topped up? Changed thermostat batteries? Good to know — but don’t keep repeating actions.
If you want us to handle it, this is the quickest route: book local boiler repairs or call +44 7727 154746.