Boiler not working? Here’s what it usually means
A boiler that won't run usually comes down to one of five things: low system pressure, a frozen condensate pipe (in cold snaps), a tripped or faulty component showing an error code, a failed pump or valve, or no gas supply. Check the pressure gauge and the display code first — between them they identify most faults.
Likely causes
- Low pressure
- The gauge reads below 1 bar. Many boilers let you repressurise via the filling loop — your manual shows how. If it keeps dropping, there's a leak or a failing expansion vessel: that's a repair, not a top-up.
- Frozen condensate pipe
- In freezing weather, the white plastic waste pipe outside can freeze and lock the boiler out with a gurgling noise. Thawing it gently with warm (not boiling) water often restores it.
- Error code lockout
- The display code names the failed component — sensor, fan, ignition, PCB. One reset is fine to try; repeated resets can turn a cheap fault into an expensive one.
- No gas supply
- If the hob is also dead, check your meter and whether a prepay credit has run out. A smart meter in 'off' mode after a power cut is a surprisingly common culprit.
What not to do
- Don't remove the boiler casing — anything behind it is Gas Safe territory by law
- Don't keep force-resetting a boiler that locks out repeatedly
- Don't run the boiler if it's leaking water onto electrics
- If you ever smell gas: stop, ventilate, leave, and call 0800 111 999 before anything else
When to request help
If pressure top-up and one reset don't restore it, it needs a Gas Safe registered engineer. Same day if you have no heating in cold weather or vulnerable people at home.
Related questions
How much does a boiler repair cost in London?
Typically £150–£400 including parts, with common faults around £240. See our boiler repair cost guide for the full breakdown.
My boiler works for heating but not hot water — same problem?
That split usually points to the diverter valve, a classic repairable fault — and worth booking promptly, because it rarely fixes itself.
Reviewed by the London Heroes Operations Team. General guidance, not a substitute for an on-site professional assessment — and never for the gas emergency line (0800 111 999) if you smell gas.