Worcester Boiler Flashing Blue Light: What It Means

A flashing blue light on a Worcester Bosch boiler usually means the boiler has detected a fault and shut itself down in a safety lockout. It will not fire again until the fault is cleared and the boiler is reset. Our guide on how to reset a Worcester boiler explains when a reset is safe and when it is the wrong move. On most Greenstar models with a display, a fault code appears at the same time, and that code is what tells you the cause. A steady blue light, by contrast, is normal: it simply shows the boiler has power or the burner is running.

Steady, Slow Flash or Fast Flash?

Worcester Bosch boilers use the blue light in slightly different ways across the Greenstar range, so check your user manual for your exact model. As a general guide:

• Steady blue light: normal operation. The boiler has power or is firing.
• Slow flashing blue light with a fault code on the display: the boiler has locked out on a fault. Read the code.
• Fast, continuous flashing with no code: on some models this indicates service or commissioning mode, which an engineer may have left the boiler in, rather than a fault.

If your boiler has no display, the blue light pattern and the reset button are all you have to go on, and the manual for your model is the best reference.

What the Common Codes Mean

The code shown alongside the flashing light narrows the cause down quickly. These are the Worcester Bosch codes we see most often in London homes, each with its own full guide:

• EA (often shown with 227): the boiler tried to light but did not detect a flame. Causes include a gas supply problem, a faulty ignition electrode or a flame sensor fault. See also fault 227.
• CE1: low system water pressure. This is the one most homeowners can fix themselves by topping up.
• D5: on some models, a condensate drainage fault, very often a frozen condensate pipe in cold weather.
• E9: an overheat lockout. Do not keep resetting a boiler that overheats.

Our full boiler fault codes reference lists every Worcester Bosch code we cover.

What You Can Safely Check Before Resetting

A reset clears the lockout, but if the underlying problem is still there, the light will start flashing again. Before you press reset, check the simple causes:

• Gas supply. Are other gas appliances such as the hob working? If you have a prepayment meter, does it have credit? Is the gas isolation valve under the boiler open, with its lever in line with the pipe?
• Pressure. The gauge on the front should read roughly 1 to 1.5 bar with the system cold. If it is lower, follow our guide to repressurising a boiler safely.
• Condensate pipe. In freezing weather, the plastic pipe running from the boiler to an outside drain can freeze solid. Our frozen condensate pipe guide explains how to thaw it with warm water.
• Power. A tripped fused spur or a recent power cut can leave the boiler in lockout. Check the switch next to the boiler.

If you smell gas at any point, stop. Do not operate switches, open windows, turn off the gas at the meter and call the National Gas Emergency Service on 0800 111 999.

How to Reset a Worcester Bosch Boiler

On most Greenstar models, the reset button is on the front control panel, often marked with a flame symbol or the word "reset". The usual process is to press and hold it for a few seconds, then wait for the boiler to run through its start-up sequence. Some models reset with a combination of buttons, so check your manual.

One reset is reasonable. If the blue light starts flashing again straight away, or within a day or two, leave it. Repeatedly resetting a boiler that keeps locking out does not fix anything, and on ignition and overheat faults it can make things worse. If you have already tried more than once, our article on what it means when a boiler won't reset explains what is going on inside.

Why a Worcester Boiler Keeps Locking Out

A boiler that locks out once after a power cut and then runs normally is not much of a concern. One that keeps flashing blue usually has one of these problems:

• Ignition or flame detection faults. A worn or badly positioned ignition electrode, a dirty flame sensor, or a gas valve that is not opening properly.
• Pressure loss. If you are topping up often, water is leaving the system through a leak, a dripping pressure relief valve or a failed expansion vessel.
• Condensate blockages that are not caused by freezing, such as debris in the trap or a badly routed pipe.
• Flue or air supply problems, which need investigating promptly.
• Fan or PCB faults in older boilers.

For a boiler that cuts out without a clear pattern, see why a Worcester Bosch boiler keeps turning off.

Getting It Fixed

Anything past gas supply, pressure, the condensate pipe and a single reset means opening the boiler, and that is legally work for a Gas Safe registered engineer. When you contact us, note the code on the display and roughly how often the light has been flashing. It helps us bring the likely parts on the first visit. You can book a diagnostic visit or read about our boiler repair service across London.