Worcester Boiler Blue Flashing Light
On many Worcester Bosch boilers a flashing blue light means the boiler has found a fault and shut down. On the Greenstar i, for example, Worcester's instructions say the reset button flashes once per second and the blue mains indicator flashes when there is a fault. One reset after checking the basics often brings it back. If the light starts flashing again, the boiler is telling you the fault has not gone, and that is the point to stop resetting and call a Gas Safe engineer.
The short answer
A flashing blue light is a fault signal, not a power-on light. A steady blue light normally just means the boiler has power. When it flashes, the boiler has stopped because something is wrong, and on boilers with a display a fault code shows alongside it. Check the pressure, the gas and the condensate pipe, reset once, and call an engineer if it comes back.
Blocking faults and locking faults
Worcester's instructions split faults into kinds, and knowing which you have tells you whether a reset can help:
| Kind | What the boiler does | What you do |
|---|---|---|
| Blocking fault | Shuts the heating down temporarily and restarts by itself when the fault clears | Fix the cause (for example low pressure); no reset needed |
| Locking fault | Shuts down and stays off until it is reset | Check the basics, then reset once |
Our guide to Worcester boiler fault codes lists the common codes and which kind each one is.
Check these before you reset
| Check | How | Why |
|---|---|---|
| Pressure | Read the gauge or display with the system cold | Below 1 bar stops many boilers. Top it up first. |
| Gas | Does the hob light? Is there credit on a prepayment meter? | No gas means no flame, and the boiler locks out. |
| Condensate pipe | In frost, find the plastic pipe outside, reachable from the ground | A frozen pipe backs up and stops the boiler. Thaw with warm, never boiling, water. |
| Fault code | Write down any code on the display | The engineer will want it, and it tells you whether a reset can help. |
How to reset a Worcester boiler
How you reset depends on the model, so check the user manual for yours (Worcester's MyBosch site holds the manuals). As examples from Worcester's own instructions:
| Model | Reset | Source |
|---|---|---|
| Greenstar i System | Press and hold the reset button for about five seconds. When it has reset, the button stops glowing and the blue light stops flashing. | User instructions 6 720 644 864a |
| Greenstar 4000 | Use the keys on the control panel until "Reset" shows on the display. | Installation instructions 6 720 891 162 |
Reset once. Worcester's Greenstar i instructions say that if the fault cannot be cleared with the reset button, contact Worcester for help. Pressing it again and again does not fix anything; it just repeats the failed start.
When the light keeps flashing
If the blue light flashes again straight after a reset, or every few days, the fault is real and needs diagnosing. The most common lockout on Worcester boilers is a failure to light, which Worcester's code table lists as 227, "no flame signal after ignition"; our fault 227 guide covers it. Anything inside the case, such as electrodes, the gas valve, the fan or the control board, is work for a Gas Safe registered engineer. Ours is registration 541116. If you smell gas at any point, open the windows, do not use switches, leave the property and call the National Gas Emergency Service on 0800 111 999.
Repair or replace?
A fault that clears with one reset or a top-up costs nothing. A worn electrode or a recharged expansion vessel on a boiler a few years old is a routine repair. The sums change when the boiler is twelve to fifteen years old, the fault is a gas valve, heat exchanger or control board, or the same lockout keeps coming back. As a Worcester Bosch Accredited Installer we fit the Greenstar range from £2,000 for the Greenstar 1000 to £2,850 for the Greenstar 8000 Life, fitted, with guarantees of up to 12 years on the 8000 range (subject to Worcester's conditions). Every model and price is on our boiler prices page, and finance is available. Ask for the repair figure and the replacement figure side by side and decide with both in front of you.
Common questions
What does a flashing blue light on a Worcester boiler mean?
That the boiler has found a fault and stopped. On the Greenstar i, Worcester's instructions say the reset button flashes once per second and the blue mains indicator flashes on a fault. A steady blue light normally just means the boiler has power.
How do I stop the blue light flashing on my Worcester boiler?
Check the pressure, the gas supply and, in frost, the condensate pipe, then reset the boiler once as your manual describes. On a Greenstar i you hold the reset button for about five seconds. If the light starts flashing again, call a Gas Safe engineer.
How many times can I reset a Worcester boiler?
Once, after checking the basics. If the fault comes straight back, more resets will not fix it. Worcester's own instructions say to get help if the fault cannot be cleared with the reset button.
Is a Worcester boiler with a flashing blue light dangerous?
The flashing light means the boiler has shut itself down to stay safe. It is not dangerous to leave it off. If you smell gas, though, treat it as an emergency and call 0800 111 999.
Why does my Worcester boiler flash blue on cold mornings?
Usually the condensate pipe freezing where it runs outside, which stops the boiler. Thaw it with warm water from the ground, then reset once. If it happens every cold spell, the pipe needs upsizing, lagging or re-routing.
Updated 18 October 2026
