Worcester Bosch 4 Long Flashes Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch 4 Long Flashes fault code mean?
On older Worcester Bosch boilers that use an LED flash sequence instead of an alphanumeric display, faults are communicated by a repeating pattern of quick flashes followed by a pause, then a series of longer flashes. Seeing 4 long flashes — in the repeating sequence of 4 quick flashes, a 5-second pause, then 4 long flashes — indicates a locking fault, most commonly an ignition or flame detection failure. This means the boiler's PCB has attempted to establish a flame multiple times (typically four attempts), failed to confirm ignition each time, and has shut the boiler down completely as a safety measure. The boiler will not restart on its own; a manual reset is required. This flash pattern is broadly equivalent to the EA fault code seen on Worcester Bosch models with alphanumeric displays, where the PCB locks out after repeated failed ignition attempts. Because this is a locking error rather than a temporary blocking fault, simply resetting without addressing the underlying cause will usually result in the fault returning.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Ignition electrode or lead failure Common
The ignition electrode generates the spark needed to light the burner. Over time, the electrode tip can wear, crack, or become coated with residue, preventing it from producing a reliable spark. A damaged or disconnected ignition lead will have the same effect. This is the most frequent root cause of repeated failed ignition attempts on Worcester Bosch boilers.
- Faulty or dirty flame sensor (ionisation electrode) Common
Once the burner lights, the boiler relies on an ionisation sensor to confirm that a flame is present. If the sensor is coated with oxidisation or debris, or has developed a fault, it may fail to detect the flame even when the burner has actually lit — causing the PCB to register a failed ignition attempt and eventually lock out.
- Frozen or blocked condensate pipe Common
In cold weather, the plastic condensate pipe that carries acidic water away from a condensing boiler can freeze solid, particularly where it runs externally. A blocked condensate pipe prevents the boiler from operating safely, triggering a lockout. This is one of the most common causes of winter boiler faults across all condensing boiler brands.
- Gas supply interruption Common
If the boiler is not receiving adequate gas — whether because the meter is turned off, a prepayment meter has run out of credit, or there is a wider supply interruption — it will attempt ignition repeatedly, fail each time, and lock out. Always confirm other gas appliances in the property are working before assuming the fault is internal to the boiler.
- Faulty gas valve Sometimes
The gas valve controls the flow of gas to the burner. A valve that is sticking, not opening fully, or failing to respond to PCB signals will starve the burner of fuel, resulting in failed ignition attempts and eventual lockout. Gas valve faults require a Gas Safe engineer to diagnose and replace.
- Fan failure or restricted flue Sometimes
The fan must reach the correct speed and prove adequate flue flow before ignition is permitted. A failing fan motor, a blocked flue terminal, or a significant flue restriction can prevent the boiler from ever reaching the pre-ignition stage, producing the same lockout behaviour. A fan fault may also trigger a separate C6 flash code alongside the 4 long flashes.
- PCB fault or wiring issue Rare
A fault on the printed circuit board itself — including failed components, moisture ingress, or loose connector contacts — can produce erratic ignition control or prevent the PCB from correctly interpreting signals from the flame sensor. Loose ignition leads connecting the PCB to the electrode are a simpler wiring cause worth checking by an engineer.
How to fix it
- Check that other gas appliances are working DIY safe
Turn on a gas hob ring or check another gas appliance in the property. If nothing gas-powered is working, the issue is with your gas supply rather than the boiler. Contact your gas supplier or, if you smell gas anywhere, call the National Gas Emergency number (0800 111 999) immediately.
- Check your prepayment meter credit DIY safe
If your property uses a prepayment gas meter, confirm there is sufficient credit. A meter that has run out will cut the gas supply and cause exactly this lockout pattern. Top up the meter, wait a few minutes, and then attempt a reset.
- Check the boiler's system pressure DIY safe
Look at the pressure gauge on the boiler — it should read between 1 and 1.5 bar when the system is cold. If it reads below 1 bar, top up the system using the filling loop (usually a braided hose or lever valve beneath the boiler) until the gauge reaches 1.2 bar, then close the loop. Do not overfill above 2 bar.
- Inspect the condensate pipe for freezing (cold weather only) DIY safe
If the outside temperature is at or below freezing, locate the condensate pipe — typically a white plastic pipe (22 mm or 32 mm diameter) exiting through an external wall low down. If it feels solid or has visible ice, thaw it by pouring warm (not boiling) water slowly along its length. Do not use a naked flame. Once thawed, attempt a boiler reset.
- Perform a single boiler reset DIY safe
On most older Worcester Bosch models, reset is performed by turning the control knob anti-clockwise to the reset position and holding it for approximately 3 seconds, then returning it to your desired setting. Alternatively, some models have a dedicated reset button. Attempt the reset once. If the boiler fires up and runs normally, monitor it for the next 24–48 hours. If the fault returns promptly or the boiler fails to light at all, do not keep resetting — move on to calling an engineer.
- Do not attempt further resets or open the boiler casing Gas Safe engineer
Repeatedly resetting a locking fault without addressing the cause can damage internal components further. Working inside the boiler casing — checking the electrode, flame sensor, gas valve, fan, or PCB — is not safe or legal for an unregistered person. These checks must be carried out by a Gas Safe registered engineer.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
A qualified engineer will carry out a systematic inspection: testing the ignition electrode and lead for continuity and condition, cleaning or replacing the flame sensor, checking gas pressure and valve operation, inspecting the fan and flue, and reading any stored fault history from the PCB. They will identify the root cause and carry out any required parts replacement safely and legally. Verify your engineer's Gas Safe registration at gassaferegister.co.uk before booking.
Parts you may need
- Ignition electrode · from £25
- Ignition lead · from £18
- Flame sensor (ionisation electrode) · from £22
- Gas valve (Worcester Bosch compatible) · from £95
- Boiler fan motor · from £85
- Condensate trap · from £20
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£380, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting my Worcester Bosch boiler to clear the 4 long flashes?
You should only reset the boiler once or twice at most. If the fault clears and does not return, it may have been a one-off event such as a brief gas pressure dip. However, if the lockout comes back after resetting, there is an underlying fault that resetting alone will not fix. Repeatedly forcing the boiler through ignition attempts when there is a genuine fault can stress components further and may cause additional damage — particularly to the gas valve or PCB. At that point you need a Gas Safe engineer, not another reset.
Is the 4 long flashes code the same as the EA fault code on Worcester Bosch boilers?
Yes, broadly speaking. Newer Worcester Bosch models with alphanumeric displays show the EA code (and sometimes a sub-code such as EA 227) for ignition and flame detection failures. Older models without that display communicate the same class of fault using the LED flash sequence — 4 quick flashes, 5-second pause, 4 long flashes. Both indicate the PCB has locked out after failing to confirm a flame within the permitted number of ignition attempts. If you have a model that shows alphanumeric codes rather than flash sequences, search for the EA fault code page instead.
My condensate pipe has thawed and I have reset the boiler, but it keeps locking out again — why?
If the condensate pipe was the original cause, the boiler should run normally once it is thawed and reset. If lockouts continue, the condensate pipe may be re-freezing quickly, or there is a separate fault — such as a worn ignition electrode or a faulty flame sensor — that coincidentally appeared at the same time. Persistent lockouts after a successful thaw need an engineer visit to properly diagnose the remaining fault.
How much does it typically cost to repair a Worcester Bosch boiler showing 4 long flashes?
Most repairs that cause this fault — replacing the ignition electrode, ignition lead, or flame sensor — fall in the range of £150 to £250 including parts and labour. A gas valve replacement typically costs £180 to £220 all-in. If the fan needs replacing, expect to pay roughly £200 to £380 depending on the model and labour rates in your area. PCB replacement is a higher-cost repair that can exceed £400 and is worth weighing against the age of the boiler. Worcester Bosch's own out-of-guarantee repair service starts from around £381 inc. VAT and includes a 91% first-time fix rate if you prefer the manufacturer's network.