Error 5P3: the flame goes out while the boiler is running

What it means

Unlike 501, 5P1 and 5P2, which are ignition failures, 5P3 means the boiler did light: the flame has lifted away from the burner while it was running.

The boiler detects this, shuts off the gas for safety and shows 5P3. Most likely, the flue isn't venting properly or the gas supply isn't strong enough with the burner running.

Most likely causes

1. The flue is blocked or has a defect. This is one of the most likely causes.
2. The gas supply falls short with the burner running. There's gas at rest, but once the boiler is running the supply is too weak: a half-open valve, a nearly empty gas bottle or the gas installation.
3. An internal fault in the boiler. If the flue is clear and the gas supply is fine, that's a job for an engineer.

What you can check yourself

Without tools and without opening the unit.

1. 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.
2. Check the boiler's gas tap, 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, check that it isn't nearly empty.

If 5P3 comes back, don't keep trying: book an engineer. The flue and the combustion need to be checked by an engineer. And if you smell gas, switch the boiler off and follow the instructions below.