Quick answer: A flashing blue light on a Worcester Bosch boiler means the appliance has locked out on a fault. A steady blue light is normal. A slow flash, roughly one every two to three seconds, is a fault lockout and is by far the most common pattern. A fast continuous flash usually means the boiler has been left in service mode. No blue light at all means no power. The pattern tells you which of those four situations you’re in, so count the flashes before you do anything else.
Written by the engineering team at Edinburgh Boiler Company, Scotland’s number one boiler installer. “My blue light is flashing” is one of the most common calls we take, and a good share of them get resolved on the phone.
Why is the blue light on my Worcester boiler flashing?
The blue light is Worcester Bosch’s status indicator, and flashing is how it reports that the boiler has stopped itself. It isn’t a fault in its own right. It’s the boiler telling you that one of its internal safety checks has failed and it has shut down rather than continue running.
What the blue light won’t tell you on its own is why. That’s what the flash rate and any Worcester Bosch error code on the display are for. Two boilers can both show a flashing blue light while having entirely different problems, one a frozen pipe and the other a failed fan, so the first job is always to narrow it down.
Before you touch anything, watch the light for thirty seconds and count. Then photograph the display, including any code, because that photo is the single most useful thing you can give an engineer.
| Blue light behaviour | What it means | How urgent |
|---|---|---|
| Steady blue, no flashing | Normal operation | Nothing to do |
| Slow flash, every 2–3 seconds | Fault lockout | Work through the checks below |
| Fast continuous flash | Usually service mode | Often clears with a power cycle |
| No blue light at all | No power reaching the boiler | Check the electrical supply |
The blue light behaves the same way across the Greenstar range, including the i, Si, CDi and the 2000, 4000 and 8000 series. The display alongside it varies between models, but the light itself is consistent.
What does a steady blue light mean?
A steady, unflashing blue light means the boiler is powered up and operating normally. That’s the state it should be in almost all the time.
Worth saying explicitly, because we get calls from people worried about a blue light that is doing exactly what it’s meant to. If the light is solid and your heating and hot water are working, nothing is wrong. Some models illuminate the blue light constantly whenever there’s power, whether or not the burner is currently firing, so a steady light with the boiler sitting quiet between cycles is also normal.
What does a slow flashing blue light mean?
A slow flashing blue light, pulsing roughly once every two to three seconds, means the boiler has locked out on a fault. This is the pattern behind the large majority of blue light call-outs we attend.
When the blue light flashes slowly, the boiler has attempted to run, hit a problem, and shut itself down. Almost always there’ll be a code on the display alongside it. The slow flash is telling you a lockout has occurred, and the code is telling you what caused it.
Here’s what we actually find behind a slow flashing blue light, ranked by frequency:
| Cause of the slow flashing blue light | How common | Code usually shown |
|---|---|---|
| No flame detected at ignition | Very common | EA or EA 227 |
| Frozen or blocked condensate pipe | Very common in winter | Usually EA |
| Low system water pressure | Common | CE 207 or similar |
| Fan or airflow fault | Occasional | C6 or C7 |
| Sensor faults | Occasional | Varies by model |
| Gas valve or PCB faults | Less common | Varies by model |
The top two account for most Scottish winter call-outs, and both have safe homeowner checks. If your blue light started flashing slowly during a cold snap, read the condensate section below before you do anything else.
What does a fast flashing blue light mean?
A fast, continuous flashing blue light usually means the boiler is in service mode rather than faulted. On some models it can indicate a different type of lockout, but service mode is the far more likely explanation, particularly if an engineer has been out recently.
Service mode forces the boiler to fire at maximum output so an engineer can take combustion readings. It’s meant to be switched off before they leave, and occasionally it isn’t. The tell-tale signs of a fast flashing blue light caused by service mode are that the boiler is still producing heat, often more aggressively than usual, and that there’s no fault code on the display.
If your blue light is flashing fast and the boiler is otherwise working, try a full power cycle. Switch the boiler off at the fused spur, wait 30 to 60 seconds, then switch it back on. That clears service mode on most Greenstar models.
If the fast flashing blue light comes with a code, or the boiler isn’t producing heat, treat it as a lockout and work through the checks below instead.
Why is my blue light flashing with no error code?
A flashing blue light with a blank display or no code most often points at service mode, a power supply issue, or a communication problem between the control board and the display.
Work through it in this order. First, if the flash is fast, try the power cycle described above, because service mode is the most likely answer. Second, check whether the flashing started after a power cut or an electrical fault in the property, since a supply glitch can leave the boiler in an odd state that a power cycle resolves. Third, if the blue light keeps flashing after a full power cycle and still shows nothing on the display, you’re into board or communication territory, and that needs an engineer.
Mention the absence of a code when you book. It genuinely changes what the engineer looks at first.
Why has my blue light gone out completely?
No blue light at all means the boiler isn’t getting power, which is a different problem from a flashing blue light and usually a simpler one.
Check the fused spur or switch that feeds the boiler, often on the wall nearby. Check the fuse in it, which is typically 3 amp. Then check your consumer unit for a tripped breaker or RCD. If other things in the house have lost power at the same time, the boiler isn’t the issue.
Where a dead blue light does become an engineer’s job is if the supply tests fine but the boiler still shows nothing. At that point you’re looking at internal wiring or the board.
Why does my blue light flash every winter?
Because in Scotland a flashing blue light in cold weather is very often a frozen condensate pipe, and that’s a seasonal, weather-driven fault rather than anything failing in the boiler.
Every condensing boiler produces acidic wastewater that has to drain away. Ideally that pipe runs internally to a soil stack. In practice, especially in tenement flats, conversions and older retrofits across Edinburgh, Glasgow and beyond, it exits through the wall to a gully or drain outside. When temperatures sit below freezing for a day or two, the water in that pipe turns to ice, condensate backs up into the boiler, and the boiler refuses to fire. Slow flashing blue light, EA on the display.
You can be fairly confident that’s what your flashing blue light means if:
- It started overnight during a cold spell.
- You hear gurgling or bubbling from inside the boiler when it tries to fire.
- The white or grey plastic pipe running outside your wall feels solid and ice-cold.
- Neighbours with similar setups are having the same problem at the same time.
This is a fault you can very often clear yourself in twenty minutes, and it’s the reason we tell people not to panic at a flashing blue light in January.
Why did my blue light start flashing right after a service?
Two reasons, and they’re both common enough to check before you call anyone back out.
The most likely is service mode left switched on, which shows as a fast flashing blue light with the boiler still running. A power cycle clears it.
The other is a connection disturbed during the work. Sensors and connectors get unplugged and replugged during a service, and one that hasn’t seated properly will produce a lockout and a slow flashing blue light within days. That one does need the engineer back, and if they serviced it, it’s their job to put right.
What can I safely do when the blue light is flashing?
Work through these in order and stop as soon as the light goes steady.
1. Count the flash rate and photograph the display. Ten seconds, and it determines everything else.
2. Test another gas appliance. Try the hob or a gas fire. If nothing gas-fired works, the boiler isn’t at fault. Check your prepayment meter has credit, because an empty meter behind a flashing blue light is one of the most common wasted call-outs we get booked for. Check the gas isolation valve under the boiler is open, meaning the handle runs parallel with the pipe rather than across it.
3. Check the pressure gauge with the system cold. Target 1.0 to 1.5 bar. Top up slowly via the filling loop and close both filling loop valves fully afterwards. If the pressure drops again over the next few days, you have a leak.
4. Thaw the condensate pipe if it’s frozen. Pour warm water along the exposed section, concentrating on any bend and the open end. Warm, not boiling. Boiling water can crack cold plastic and split the pipe, which turns a free fix into a repair. A hot water bottle or wheat bag against the pipe works too and is gentler.
5. Clear visible debris from the condensate outlet, or from the flue terminal if you can reach it safely from standing height.
6. Reset once. Hold the reset button for three to five seconds and give it a minute to run its sequence.
7. Power cycle if the flash is fast with no code. Off at the fused spur, 30 to 60 seconds, back on.
If you smell gas at any point: leave the property, then call the National Gas Emergency Service on 0800 111 999 from outside. If you’re on LPG, which covers a lot of properties off the mains grid in the Highlands, Borders and Argyll, call your supplier’s emergency line instead, which is on your tank or your last delivery note.
What should I never do when the blue light is flashing?
- Don’t remove the boiler casing. Everything past that point is gas and mains electrical work, and it’s illegal without Gas Safe registration.
- Don’t keep resetting a flashing blue light. One or two attempts is diagnosis. Beyond that you’re repeatedly asking a boiler that has flagged a fault to try again.
- Don’t use boiling water on the condensate pipe.
- Don’t climb to reach a flue. Ground-level checks only.
- Don’t ignore a flashing blue light that clears and returns. An intermittent lockout is a component on its way out, and it’s cheaper to deal with in September than in December.
When should I call a Gas Safe engineer about a flashing blue light?
Call when the blue light returns after you’ve confirmed the gas supply, checked pressure and dealt with any frozen condensate. At that point the remaining causes sit inside the casing, and they need proper test equipment rather than guesswork.
Call immediately, without trying anything, if the flashing blue light comes with any of these:
- A smell of gas.
- Scorching or black staining around the boiler or flue.
- A bang when the boiler attempts to light.
- Anyone in the house feeling unwell or getting headaches when the heating runs.
Every home with a gas appliance should have a working carbon monoxide alarm, regardless of what the blue light is doing.
What will the engineer do about it?
- Read the fault history stored in the boiler, which shows the pattern of lockouts rather than just the current one.
- Gas tightness test and working gas pressure checks at the meter and the appliance.
- Inspect the ignition and flame-sensing electrodes, checking the gap, tip condition and ceramic insulator for cracks.
- Strip and clean the condensate trap.
- Inspect the flue and air intake along their full length.
- Test the flame sensing circuit and the gas valve.
- Check the fan where fan codes are involved.
- Run a full combustion analysis after any repair and record the readings on your paperwork.
That last step isn’t optional on a gas-side repair. If an engineer swaps a part and leaves without analysing the flue gases, get someone else in.
How much does it cost to stop the blue light flashing?
Anywhere from nothing to about £650, depending entirely on what’s behind the light. Guide prices for Scotland in 2026, parts fitted.
| Fix | Guide cost | Time on site |
|---|---|---|
| Power cycling out of service mode yourself | Free | 2 mins |
| Thawing a frozen condensate pipe yourself | Free | 20 mins |
| Engineer call-out to thaw and lag condensate | £80–£160 | 30–60 mins |
| Diagnostic visit | £70–£130 | 45–90 mins |
| Electrode clean or replacement | £120–£230 | 1–1.5 hours |
| Condensate trap clean or replacement | £110–£190 | 1 hour |
| Flue clearing or terminal replacement | £120–£350 | 1–2 hours, more with access equipment |
| Fan replacement | £280–£480 | 1.5–2.5 hours |
| Gas valve replacement | £320–£550 | 2–3 hours |
| PCB replacement | £350–£650 | 2–3 hours |
Flue access is the biggest variable in Scotland. A terminal three floors up a tenement gable is a different job from one on a bungalow wall, and any quote should say clearly whether access equipment is included.
Will my Worcester guarantee cover it?
If the boiler is within its guarantee period and your annual servicing is current, the boiler components that cause a flashing blue light, electrodes, fans, gas valves and PCBs, should be covered. Worcester Bosch guarantees run up to 10 or 12 years on Greenstar models when installed by an accredited installer and registered correctly.
What generally isn’t covered is anything classed as maintenance or system-related: thawing a frozen condensate pipe, clearing a blocked flue terminal, or topping up pressure. A missed annual service invalidates most manufacturer guarantees, so find your service records before you call.
How do I stop the blue light flashing again?
Lag the condensate pipe. For a Scottish property this is the highest-value item on the list by a wide margin, because frozen condensate is the single biggest cause of a flashing blue light here. Use waterproof insulation rated for external use, since standard foam sleeving degrades fast in our weather.
Get a badly routed condensate pipe re-routed. A long external run, a sag holding standing water, or an exposed termination will keep freezing however well you lag it. Rerouting internally to a soil stack, or increasing the external section to 32mm, solves it permanently.
Keep the flue terminal clear of leaves, moss and ivy. Check after autumn leaf fall and after any storm.
Keep pressure between 1.0 and 1.5 bar. Check the gauge every couple of months with the system cold.
Book the annual service. Electrodes get cleaned, the condensate trap gets checked, gas pressures get verified, and your guarantee stays valid.
Ask your engineer to confirm the boiler is out of service mode before they leave. It sounds trivial, and it prevents a call-out.
The bottom line
A flashing blue light tells you the boiler has locked out, not what’s wrong with it. The flash rate narrows it down and the code on the display finishes the job. In a Scottish winter, a slow flashing blue light with an EA code is most often a frozen condensate pipe, and warm water clears it in twenty minutes. A fast flashing blue light with no code is usually service mode, and a power cycle clears that. Anything still flashing after the safe checks needs a Gas Safe registered engineer.
Still flashing? Edinburgh Boiler Company covers Edinburgh, the Lothians and central Scotland with Gas Safe registered engineers, full combustion testing on every gas repair, and written pricing before any work begins.
This guide is general advice. Refer to your specific Worcester Bosch model’s user manual for reset instructions, and never remove the boiler casing. Gas work must be carried out by a Gas Safe registered engineer. If you smell gas, leave the property and call 0800 111 999, or your LPG supplier’s emergency line if you’re off the mains gas grid.
