Worcester Boiler Flashing Blue Light: What It Means and What to Do
On most Worcester Bosch Greenstar boilers a steady blue light means the burner is lit and the boiler is working, and a flashing blue light means it has detected a fault and shut itself down. The display will normally show a code alongside it. The commonest causes are a failed ignition, often because the gas supply or a frozen condensate pipe has stopped the boiler lighting, low system pressure, and overheating. Note the code, make the simple checks below, and try one reset. If the light comes back, the boiler is telling you the cause is still there, and it needs a Gas Safe registered engineer rather than more resets.
Steady blue or flashing blue?
The difference is the whole story. On Greenstar models with a blue indicator, a steady light is the burner on symbol: the boiler has lit and is heating. It goes out when the boiler reaches temperature and comes back when it fires again, which is normal and is not a fault.
A flashing light, usually with a code on the display and the boiler silent, means a lockout. The boiler has tried to do something, found a condition it does not accept, and stopped. Heating and hot water both stop with it, which is why most people notice the light at the same moment they notice cold taps.
Older Worcester models and some newer ones with a text display behave slightly differently, so the manual for your exact model is the final word. What follows applies to the Greenstar boilers we see most across London.
Read the code before you do anything else
The code on the display is the boiler telling you which of its checks failed. Write it down or photograph it before you reset, because a reset clears it and an engineer can do far more with the code than with a description of a light.
Worcester codes are usually a pair of letters or numbers followed by a three digit number. Codes beginning EA are the family most people meet, and they relate to the boiler failing to detect a flame after it has tried to light. Codes relating to pressure, overheating and sensors each have their own families. The manual for your model lists what each one means and which ones a householder can clear.
The common causes, and which ones you can check
Most flashing blue lights come down to one of a handful of causes. Some are outside the boiler and cost nothing to check. Others are inside it and are engineer work.
- No gas reaching the boiler. Light a hob to test. A prepayment meter that has run out of credit, or a meter valve knocked off, will stop the boiler lighting and show as an ignition lockout.
- A frozen condensate pipe. In a cold snap the plastic pipe that drains the boiler outside can freeze, the water backs up and the boiler locks out. Thaw it with warm water, not boiling, then reset.
- Low pressure. If the gauge reads below about 1 bar cold, the boiler may refuse to run. Top up through the filling loop as your manual describes.
- Overheating. Radiators all turned off, a pump that has stopped, or sludge restricting the flow can all make the boiler overheat and cut out.
- An ignition or flame sensing fault. A worn electrode, a failed ignition lead or a gas valve problem stops the boiler lighting even when gas is present. This is inside the case and is Gas Safe work.
- A fan, flue or sensor fault. These have their own codes and need testing with instruments rather than guessing.
How to reset it safely
On most Greenstar boilers the reset is a button on the front panel, sometimes marked with a flame or a circular arrow, which you press and hold for a few seconds. The boiler goes through its start sequence and, if the cause has cleared, lights and runs normally.
Two attempts is the sensible limit. A lockout is a safety response. If the boiler locks out again straight away or later the same day, resetting it a third or fourth time does not fix anything, and on an overheat or combustion fault it can make a modest repair into a bigger one. The pattern is also useful information: a boiler that locks out every morning in cold weather points somewhere very different from one that locks out at random.
If you can smell gas, do not reset the boiler and do not operate electrical switches. Turn the gas off at the meter, open windows and call the gas emergency line on 0800 111 999.
When the flashing light comes back
A lockout that returns means the condition is still there. At that point the useful thing is an engineer who arrives with a gas analyser, a manometer and a multimeter rather than a van full of parts. The job is to prove which component failed: measure the gas pressure at the boiler while it tries to fire, confirm the spark, confirm the flame signal, check the fan and flue, and read the sensors against their expected values.
Most of the time that ends in a single part and the boiler running again. Occasionally the diagnosis shows a boiler at the end of its useful life, with a heat exchanger or gas valve fault on an old unit where parts are costly or hard to get. We tell you which situation you are in and what each option involves, and the choice is yours. Our boiler diagnosis page explains how that visit works.
Anything inside the boiler case is gas work. By law it must be done by a Gas Safe registered engineer, and you can check the registration of whoever attends in a couple of minutes on the public register.
If you rent
A boiler that is locking out is your landlord or agent to fix, not yours, and a boiler that has left you without heating and hot water is treated as urgent. Report it in writing with the code and a photo of the display, and keep a note of when you reported it. You can still make the outside checks above, because they are quick and often solve it, but do not take the boiler apart or pay for repairs you expect to be reimbursed for without agreeing it first.
Getting an engineer out
If the light keeps coming back, or you have no heating and no hot water and the checks have not helped, call us or send an enquiry with the boiler model and the code on the display. We have 20 engineers across London and can usually attend the same day. Our rate is set out on the pricing page and agreed before anyone arrives.
Questions we get asked
- Is a flashing blue light on a Worcester boiler dangerous?
- Not in itself. It means the boiler has detected a problem and shut down, which is the safe response. The exception is if you can also smell gas, in which case turn the gas off at the meter, ventilate and call 0800 111 999 before anything else.
- Why does my Worcester boiler blue light flash in cold weather?
- The most likely cause is a frozen condensate pipe. When the pipe that drains the boiler outside freezes, the boiler cannot get rid of its condensate and locks out. Thawing the pipe with warm water and resetting usually gets it running. Lagging the outside section helps stop it happening again.
- Should the blue light on my Worcester boiler be on all the time?
- A steady blue light while the boiler is heating is normal on models where it shows the burner is lit. It turns off when the boiler reaches temperature. It is only a fault when it flashes, usually with a code on the display.
- How many times can I reset my Worcester boiler?
- Try once or twice. If it locks out again, stop and get it looked at. Repeated resets do not fix the cause and on some faults they can make the damage worse.
- Can low pressure make the blue light flash?
- Yes. Many Greenstar boilers will not run below about 1 bar and will show a pressure related fault. Topping up through the filling loop usually clears it, but if the pressure falls again within days, there is a leak on the system that needs finding.
- Will the boiler still give me hot water with the light flashing?
- No. A lockout stops the boiler entirely, so on a combi you lose hot water and heating together. If you have a cylinder with an immersion heater, you can switch the immersion on to get hot water while you wait for a repair.