Boiler keeps locking out or won’t fire up (Ideal, Vaillant, Vokera, Glow-worm & Potterton)
⚠️ This needs a qualified professional
This fault involves gas or mains electrical work. Don't attempt a DIY repair — UK law requires gas work to be carried out by a Gas Safe registered engineer. Below is what to check and describe to them, not step-by-step repair instructions.
If your boiler shows a fault light, displays an error code, or simply won’t ignite when you call for heat or hot water, most modern combi boilers use electronic ignition rather than a standing pilot light — so “the pilot’s gone out” isn’t usually the cause on anything installed in the last 10–15 years.
What’s safe to check yourself
- Check the boiler pressure gauge — low pressure is one of the most common reasons a boiler refuses to fire, and many boilers will display a specific low-pressure fault code rather than trying to run. See our separate guide on repressurising if this is the cause.
- Check there hasn’t been a power cut recently that reset the boiler’s programmer/timer.
- Check the condensate pipe (a plastic pipe running from the boiler, often to an outside drain) isn’t visibly frozen in cold weather — this is extremely common in UK winters and causes a specific lockout. If accessible and safe to reach, pouring warm (not boiling) water over a visibly frozen section can sometimes clear it — but if you can’t safely access it, this is a job for an engineer.
- Try the boiler’s reset button once, per your manual — if it locks out again immediately, stop and call an engineer rather than repeatedly resetting it.
Do not attempt to diagnose or open the boiler beyond checking pressure and the condensate pipe. If you smell gas at any point, turn off the gas supply at the meter if it’s safe to do so, don’t operate any switches, ventilate the area, and call the National Gas Emergency Service on 0800 111 999.