Worcester Bosch 1 Long Flash Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch 1 Long Flash fault code mean?
The 1 Long Flash fault code is specific to Worcester Bosch Greenstar Ri boilers — models that use a flashing blue mains indicator light rather than an alphanumeric display to communicate faults. One flash repeated in a sequence with a pause between each group is a Group 1 locking fault, meaning the boiler has attempted to ignite the burner four times, failed to detect a flame on any of those attempts, and has shut itself down as a safety precaution. This is commonly called an ignition lockout. When this fault is active, you will also notice the reset button flashing roughly once per second alongside the blue indicator light. The boiler will not attempt to fire again until it has been reset. Before assuming a fault, it is worth confirming you are actually looking at a fault sequence: if the blue light pulses three to five times in a distinct pattern, the boiler may simply have been left in service mode by an engineer measuring flue gases — a quick check before anything else.
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
- Gas supply interrupted or insufficient Common
If the gas supply to the boiler has been disrupted — whether due to a prepayment meter running out, a closed isolation valve, or a wider supply problem — the burner cannot light and the boiler locks out. This is the first thing worth ruling out because it requires no tools and no engineer.
- Faulty or contaminated ignition or flame detection electrode Common
The Ri range uses a spark electrode to ignite the burner and a separate flame detection electrode to confirm combustion has started. If either electrode is cracked, coated in carbon deposits, or has shifted out of its correct position, the boiler will attempt to spark but either fail to light the gas or fail to register that it has. This is one of the most frequent hardware causes of ignition lockout on these boilers.
- Low system pressure Common
The Greenstar Ri boilers incorporate pressure sensing, and if the system pressure has dropped below roughly 0.5 bar the boiler will refuse to fire. The pressure gauge on the boiler front should normally sit between 1 and 1.5 bar when the system is cold. Pressure below this range can prevent successful ignition.
- Blocked or frozen condensate pipe Common
During cold weather the acidic condensate produced during normal operation can freeze in the external section of the condensate pipe. When this happens, the condensate backs up and the boiler shuts down on safety. This is a very seasonal cause but extremely common during cold snaps.
- Blocked or obstructed flue Sometimes
A blockage in the flue — caused by debris, bird nesting material, or ice in winter — prevents safe evacuation of combustion gases. The boiler's safety system detects the problem and prevents ignition. An obstructed flue also disrupts the correct air-to-gas ratio needed for reliable combustion.
- Gas valve fault or disconnected coil Sometimes
Worcester Bosch's own documentation lists a disconnected or faulty gas valve coil as a trigger for this lockout. If the valve is not opening correctly, gas cannot reach the burner regardless of whether the spark is functioning. This requires a Gas Safe engineer to diagnose and rectify.
- Boiler accidentally left in service mode Rare
After a service visit, an engineer must return a small switch on the boiler to its normal operating position. If it is left in service mode, the blue indicator light continues to flash in a pattern that can be mistaken for a fault code. This is an easy first check before assuming anything more serious.
How to fix it
- Check whether the boiler is in service mode DIY safe
Look at the service mode switch on the boiler — it is a small switch used by engineers when taking flue gas readings. If it was left in the service mode position after a recent service visit, the blue light will flash in a way that resembles a fault. Return the switch to its normal operating position. If you are unsure, contact the engineer who last serviced the boiler before changing anything.
- Confirm your gas supply is working DIY safe
Go to your gas hob or another gas appliance and try to light it. If it lights normally, your gas supply is fine. If it does not, call your gas supplier — in an emergency call the National Gas Emergency Service on 0800 111 999. Do not attempt to reset or operate the boiler until your gas supply is confirmed.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. For most Greenstar Ri models it is a round analogue dial. It should read between 1 and 1.5 bar when the heating system is cold. If it reads below around 0.8 bar, the system needs repressurising. Use the filling loop — usually a braided hose with one or two small valves — beneath the boiler to slowly add water until the gauge reads around 1.2 bar, then close the filling loop valves securely.
- Thaw the condensate pipe if it is frozen DIY safe
In cold weather, feel along the external section of the white or grey plastic condensate pipe (usually 21.5 mm in diameter, running from the boiler through an outside wall). If it is icy, pour warm — not boiling — water gently along the pipe to thaw it. You can also wrap the pipe in a warm damp cloth. Once thawed, the condensate can drain freely again.
- Reset the boiler DIY safe
On the Greenstar Ri range, reset is performed by turning the central heating temperature knob anticlockwise to the RESET position and holding it there for approximately five seconds, then returning the knob to your preferred temperature setting. The blue indicator light should stop flashing once the reset is accepted. If the boiler fires and runs normally, monitor it over the next few hours. Do not reset more than two or three times in total — repeated resets without resolving the underlying cause can cause additional damage.
- Call a Gas Safe registered engineer if the fault returns or persists Gas Safe engineer
If the boiler locks out again after resetting, or if none of the homeowner checks above have resolved the issue, you need a Gas Safe registered engineer. They will test the ignition and flame detection electrodes, check the gas valve and its coil connections, inspect the flue and condensate trap internally, and assess the PCB if an electrical fault is suspected. Do not open the boiler casing yourself — it is dangerous and will invalidate any remaining warranty.
Parts you may need
- Ignition electrode · from £25
- Flame detection electrode · from £20
- Gas valve · from £120
- Condensate trap · from £18
- PCB (printed circuit board) · from £180
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–£350, depending on the underlying cause.
Frequently asked questions
How do I tell if my Worcester Bosch Ri is showing 1 flash or is just in service mode?
Service mode and a genuine lockout fault can look similar because both cause the blue mains indicator light to flash. Service mode typically produces a steady pulsing pattern of three to five flashes, while a Group 1 locking fault is one flash repeated with a pause between each sequence, accompanied by the reset button also flashing about once per second. If an engineer has recently serviced the boiler, check the service mode switch first before assuming a fault.
Can I fix a 1 Long Flash fault myself without calling an engineer?
There are a handful of checks you can safely carry out yourself: confirming the gas supply is on, topping up low system pressure via the filling loop, thawing a frozen condensate pipe, and performing a boiler reset. If any of these resolve the issue and the boiler runs reliably afterwards, no engineer visit is needed. However, if the boiler relocks or none of those checks identify the problem, a Gas Safe registered engineer is required — work on electrodes, the gas valve, or the PCB must never be attempted by an unqualified person.
How much does it typically cost to fix this fault in the UK?
Most people pay between £150 and £350 all in, depending on what is causing the lockout. Replacing ignition or flame detection electrodes, which are a common cause, usually costs around £150–£250 including labour and parts. Gas valve replacement tends to fall in the £180–£220 range. If the fault turns out to be a PCB failure, costs can rise to £300–£400 or beyond, but this is less common. Worcester Bosch's own out-of-guarantee repair service starts from around £381 inc. VAT, so an independent Gas Safe engineer is often more cost-effective for older boilers.
My Greenstar Ri keeps locking out with 1 flash — is it worth repairing or should I replace the boiler?
If the boiler is under about 10 years old and the repair is straightforward — such as new electrodes or a gas valve — repair is almost always the better choice financially. If the boiler is over 12 years old and you are facing a larger repair bill, it is worth comparing the repair cost against the price of a new A-rated condensing boiler. As a general rule of thumb, if the repair would cost more than half the price of a replacement boiler, replacement usually makes more long-term financial sense, especially given the efficiency improvements a modern boiler would bring.