Boiler won't turn on? What to check first

This guide is for a boiler that won't start at all. Hot water still working? Then see our guide to hot water but no heating. You can do every check below from the outside. If they don't work, you'll need a boiler repair from a Gas Safe engineer. This guide is part of our common boiler problems series.

What should I check first if my boiler won't turn on?

Check six things, in this order: power, gas, pressure, the thermostat and timer, the condensate pipe and any fault code. None of these checks need you to open the boiler. Work down the list and try the boiler again after each one.

1. Power. Is the boiler's display or light on? If not, check its fused spur. This is the switched plate on the wall that the boiler is wired into. Make sure it's on. Then look at your fuse box for a switch that has tripped, including the RCD. An RCD is a safety switch that cuts the power if electricity leaks.
2. Gas. Does your gas hob or gas fire light? If it does, the gas supply is fine. If you have a prepayment meter, check it has credit. Then check the lever on the gas pipe by your meter. This is the emergency control valve. It is on when the handle lines up with the pipe.
3. Pressure. Look at the gauge on the front of the boiler. With the heating off, it should usually read about 1 to 1.5 bar. If it's too low, many boilers won't start. See our guide to a boiler losing pressure for how to top it up.
4. Thermostat and timer. Set the thermostat a few degrees above the room temperature. Change the batteries if it has them. Check the timer is set to "on", and that its clock shows the right time.
5. Condensate pipe. On a freezing day, the small plastic pipe outside can ice up and stop the boiler. See how to thaw a frozen condensate pipe with warm water.
6. Fault code or lights. Write down any code or flashing light, and the boiler make and model. Your manual explains what it means. Try the reset button once, as the manual shows.

If your gas hob won't light either, the problem is with the gas supply, not the boiler. Your gas supplier can help with meter problems.

Why won't my boiler fire up?

A boiler won't fire up if it has no power, no gas or too little water pressure. It also stays off if the thermostat or timer isn't asking for heat. If all of those are fine, a part inside the boiler has usually failed.

What you can see often points to the cause. This table sums up the most common signs.

Does my boiler have a pilot light?

Probably not. Most boilers fitted since about 2005 don't have a pilot light. They light with an electric spark each time they need to heat. So there is nothing for you to relight.

Some older boilers still have one. A pilot light is a small flame that stays lit all the time. It lights the main burner when the heating comes on. You can often see it through a small window on the front.

• Only relight a pilot light by following the maker's instructions. These are often on the boiler or in the manual.
• Never try to relight it if you can smell gas. Follow the safety box above instead.
• If it won't light, or won't stay lit, stop and call a Gas Safe engineer.

A pilot light that keeps going out often has a worn safety part. On many older boilers, this is a thermocouple, a sensor that shuts off the gas if the flame goes out. Only an engineer should replace it.

No heating and no hot water at all?

If you have no heating and no hot water, the boiler itself has stopped. Start with power, gas and pressure, as these stop everything at once. If you have heating but no hot water, or the other way round, the boiler is usually running, and the fault is somewhere else.

• Nothing works: the boiler isn't firing at all. Work through the checks at the top of this page.
• Heating works but no hot water, or the other way round: the boiler can still fire. The fault is often a valve or a setting. Read our guide to heating but no hot water, or our guide to hot water but no heating.

On a combi boiler, try a hot tap as a quick test. A combi heats water as you use it. If the boiler fires up for the tap but not for the heating, check the thermostat and timer first.

Why is my boiler not working after a power cut?

Many boilers stop or lock out when the power goes off and comes back. A single reset and a check of the timer clock fix most of these. Give the boiler a few minutes to start up after the power returns.

1. Check the boiler's display is back on. If it isn't, check the fused spur and the fuse box.
2. If the boiler shows a fault code or flashing light, press the reset button once.
3. Check the timer clock. Many lose the time in a power cut, so the heating comes on at the wrong times.
4. Check your thermostat. Some wireless ones need setting up again. Your manual shows how.

If the whole house still has no power, see our emergency electrician page.

What faults inside a boiler stop it turning on?

If the simple checks don't help, a part inside the boiler has usually failed. Only a Gas Safe registered engineer should open the case to find it. By law, paid gas work must be done by someone on the Gas Safe Register.

These are the parts that most often stop a boiler from starting:

• The ignition. This makes the spark that lights the gas. If it fails, the boiler may click but never light.
• The gas valve. This opens to let gas into the burner. If it sticks, no gas gets through.
• The fan. This pushes the fumes out through the flue, the pipe that carries them outside. Most boilers won't light if the fan isn't working.
• The PCB. This is the printed circuit board, the boiler's main control board. It tells every other part what to do.
• The pump. This pushes hot water round your radiators. If it fails, the boiler may shut down to stop overheating.

Don't keep pressing reset. Each time, the boiler tries to start and then shuts down for safety. That won't fix the fault, and it can make some faults worse.

When to call a Gas Safe engineer

Call a Gas Safe engineer if the checks above haven't helped, or the boiler locks out again after one reset.A boiler that won't start after these checks usually has a fault inside the case.

• power, gas and pressure are all fine, but the boiler still won't start
• it clicks but won't light, or it locks out again after a reset
• a pilot light won't light or won't stay lit
• the same fault code keeps coming back
• you smell burning, or see soot or black marks round the boiler

The engineer first runs a thorough check of the boiler. That finds the fault and the parts needed. See our boiler repair page for the faults we fix. We repair all makes.

If an older boiler keeps breaking down, a repair may not be the best value. We'll tell you honestly if a new boiler makes more sense.

A yearly boiler service helps stop many of these faults. The engineer checks the boiler is safe, cleans key parts and spots wear before it causes a breakdown.

Other boiler guides

• Boiler losing pressure: what the gauge should read and how to top it up.
• Frozen condensate pipe: how to thaw it safely.
• Heating works but no hot water: checks to try, and what a diverter valve does.
• Cold radiators: what cold at the top or bottom means.
• Leaking boiler: what to do first, and where the water is coming from.
• Hot water but no heating: thermostat, timer and valve checks.
• All common boiler problems.

Boiler still won't turn on? Book a boiler repair and we'll find the cause.