If your boiler stopped during a cold snap, the white plastic condensate pipe outside has probably frozen. You can usually thaw it yourself in a few minutes.
Find the condensate pipe: the white plastic pipe that runs from your boiler to a drain outside.
Feel along it for the frozen part.
Warm it using a hot water bottle, a microwaveable heat pack, a cloth soaked in hot water, or by pouring warm (not boiling) water along it. These are the methods Worcester Bosch, Baxi, Ideal, Vaillant, Viessmann and Potterton recommend between them.
Once it's flowing, reset the boiler as your manual describes.
Baxi warns that water you pour can freeze on the ground, so watch your footing.
Call a Gas Safe engineer if
The pipe isn't frozen and the boiler still won't light.
The boiler locks out again after thawing.
You smell gas. Call 0800 111 999 straight away.
Common questions
How do I know if my condensate pipe is frozen?
In freezing weather your boiler stops and shows a lockout code. Vaillant says you may also hear gurgling or bubbling from the boiler. The pipe is the white plastic pipe running from the boiler to a drain outside.
What fault code does a frozen condensate pipe show?
It varies by brand: Worcester Bosch EA, Vaillant F.28, Ideal L2, Baxi and Potterton E133 (shown as E1 then 33), Glow-worm F.28 or F.29, and Viessmann F4.
Can I use boiling water to thaw a condensate pipe?
No. Manufacturers say warm or hot, but not boiling, water.
How do I stop the condensate pipe freezing again?
Vaillant suggests keeping the heating on a timer during cold spells and insulating the outside pipe with lagging from a DIY shop.