Errors 5P1, 5P2 and 5P3: the boiler won't light or loses its flame

What it means

When it starts up, the boiler turns on the gas, creates a spark and checks that a flame has formed. If the first attempt fails, it shows 5P1 and tries again by itself; if the second fails, 5P2. If it still doesn't light, it locks out on ignition failure with error 501.

5P3 is a flame lift-off: the boiler did light, but the flame has lifted away from the burner while it was running. It's most likely down to the flue, or to the gas supply falling short with the burner running.

Most likely causes

1. Not enough gas. The shut-off valve is closed or only half open, the supply has been cut off or, with propane or butane, the gas bottle or tank is nearly empty. It's the first thing to rule out with failed ignition attempts.
2. Air in the gas pipe after changing the gas bottle or after building work: the first attempts may fail until the gas reaches the burner.
3. Ignition or ionisation electrodes that are dirty or worn: the spark doesn't jump properly or the boiler fails to detect the flame.
4. The flue is blocked or has a defect: this is one of the most likely causes of 5P3.
5. The gas supply falls short with the burner running. There's pressure at rest, but it drops once the boiler is running: a half-open valve, a nearly empty gas bottle or the regulator.
6. Gas valve or circuit board.

What you can check yourself

Without tools and without opening the unit.

1. Check whether there's gas in the rest of the house. If you have a gas hob, light a ring. If there's no gas there either, the problem is most likely in the supply, not the boiler.
2. Check the boiler's gas valve, which is usually underneath. With the handle in line with the pipe, it's open; across the pipe, it's closed. If it's half open, open it fully: half open, it may let enough gas through at rest but fall short once the boiler is running.
3. If you use a gas bottle or tank, check there's gas left. Right after changing the gas bottle, it's normal for the first ignition attempt to fail.
4. Check the flue terminal on the outside wall or the roof, if you can see it from your home or from the street without climbing on anything: make sure it isn't blocked by a nest, a plant, an awning or something leaning against it.
5. If it appears after the boiler hasn't been used for a few days or after a gas supply interruption, it will normally try again by itself. If it stays locked out, reset it with the gas turned on.

If the warnings come up often, or if the boiler ends up locked out with 501 after two or three resets, don't keep trying: book an engineer. The same applies if 5P3 keeps coming back: the flue needs to be checked by an engineer. And if you smell gas, don't reset it: follow the instructions below.