Boiler keeps locking out or won't light?

Checks before you reset

1. Look for a fault code on the display. If there is one, look it up here. Viessmann, Glow-worm
2. Check the gas: try the hob or another gas appliance, and check a prepayment meter has credit. Viessmann, Ideal
3. Check the power: look at your fuse box for a tripped switch. Viessmann, Vaillant, Ideal
4. Check the thermostat and timer. If the house is already at the set temperature, the boiler won't fire, which can look like a fault. Glow-worm, Vaillant
5. Check the pressure gauge. Below about 1 bar, many boilers won't light. Top it up as your manual describes; our pressure guide has your brand's figure. Ideal, Glow-worm, Baxi
6. In freezing weather, check the condensate pipe outside, from ground level only. If it's frozen, thaw it safely with warm water, never boiling, then reset. Worcester Bosch, Baxi, Ideal, Viessmann, Glow-worm

How many times can you reset it?

• Vaillant and Glow-worm both say that if an ignition fault won't clear after about three attempts, stop and call an engineer. Vaillant, Glow-worm
• Some boilers stop you resetting if you try too often. Ideal's L5 code means the boiler has been reset too many times in 15 minutes. Ideal

Call a Gas Safe engineer if

• The boiler locks out again soon after a reset, or the same code keeps coming back.
• The pressure drops again after you've topped it up.
• The condensate pipe is blocked rather than frozen.
• Anything to do with the ignition, gas valve, fan or circuit board. These aren't homeowner jobs.
• You smell gas at any point. Leave the property and call 0800 111 999.