Worcester Bosch 5 Long Flashes Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch 5 Long Flashes fault code mean?
On older Worcester Bosch models that use a flashing LED rather than a digital display — most notably the Greenstar 24i Junior and 28i Junior — five long blue flashes repeated in a looping sequence is the boiler's way of signalling one of two faults: low system water pressure (the most common cause) or a flame failure and ionisation fault (where the boiler attempts to ignite but cannot detect or sustain a flame). Because these boilers lack a text screen, counting the flashes carefully is the only way to identify which fault has triggered the lockout. In both cases the boiler stops producing heating and hot water until the underlying problem is resolved and the unit is reset.
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
- Low system water pressure Common
The pressure gauge reads below 1 bar (often near zero). This is the single most common trigger for the 5-flash code. Pressure drops gradually over time through micro-leaks, or more suddenly after radiators have been bled. The boiler's pressure sensor detects the low reading and locks the unit out as a safety measure.
- Blocked or frozen condensate pipe Common
The condensate pipe carries acidic waste water from the boiler to an external drain. During cold weather it can freeze solid, or at any time of year it can become partially blocked. A blockage causes condensate to back up, which can wet the ionisation probe and prevent flame detection — triggering a lockout that looks identical to a standard flame-failure fault.
- Dirty or damaged ignition electrode or ionisation probe Sometimes
Over time, carbon deposits and corrosion build up on the electrode tip and the ionisation sensing probe. A dirty probe cannot reliably detect the presence of a flame, so the boiler shuts down on safety grounds after a failed ignition attempt. This is particularly prevalent on older Junior models.
- Interrupted gas supply Sometimes
If gas pressure to the property drops, the meter tap is partially closed, or other gas appliances in the home have been turned off at the meter, the boiler may attempt ignition, fail to establish a flame, and lock out with 5 flashes.
- Faulty PCB (printed circuit board) Rare
Older Junior boilers are known to develop PCB faults with age — corrosion, damp ingress, or power surges can degrade the board. A failing PCB may misread sensor signals or fail to energise the ignition circuit correctly, producing intermittent or persistent 5-flash lockouts that do not resolve after re-pressurising or resetting.
- Faulty pressure sensor or gas valve Rare
If the pressure sensor itself has failed, it may report low pressure even when the system is correctly charged. Similarly, a gas valve that does not open fully will starve the burner of fuel. Both faults require a Gas Safe engineer to diagnose and replace the relevant component.
How to fix it
- Check the pressure gauge on the front of the boiler DIY safe
Look at the analogue or digital pressure gauge. For normal operation it should read between 1.0 and 1.5 bar when the heating is cold. If it reads below 1 bar — or is at zero — low pressure is almost certainly your culprit and you can address it yourself before calling anyone.
- Top up system pressure via the filling loop DIY safe
Locate the filling loop, which is usually a flexible braided hose with one or two valves connecting the mains cold supply to the central heating pipework beneath the boiler. Slowly open the valve(s) and watch the gauge climb. Stop when it reaches 1.2–1.5 bar, then close the valve(s) firmly. Never exceed 2 bar. If you cannot find the filling loop or are unsure, check your boiler's user guide or call an engineer.
- Check for visible leaks around radiators and pipework DIY safe
Walk around the property and inspect all radiators, towel rails, and any exposed pipework for drips, damp patches, or watermarks. Also check outside for water dripping from the pressure relief valve discharge pipe — a wet or dripping PRV pipe suggests the valve may be releasing, which requires an engineer to investigate.
- Check that your gas supply is working DIY safe
Try lighting a gas hob ring or another gas appliance. If it does not ignite or the flame is very weak, there may be a wider supply issue. Check that the gas meter isolation valve is fully open (handle aligned with the pipe). If you suspect a gas supply problem or smell gas, stop immediately and call the National Gas Emergency Service on 0800 111 999.
- Inspect and thaw a frozen condensate pipe if the weather is cold DIY safe
The condensate pipe is typically a white or grey plastic pipe (usually 21.5 mm or 32 mm diameter) that runs from the boiler through an external wall and terminates outside, often near a drain. In freezing temperatures it can ice up. You can carefully pour warm (not boiling) water over the exterior section of the pipe or apply a warm damp cloth to thaw it. Do not use a naked flame or pour boiling water, as this can crack the pipe.
- Reset the boiler DIY safe
Once you have addressed the pressure or condensate issue, press and hold the reset button (usually marked with a flame symbol or the word 'Reset') for around 3 seconds until the lock-out indicator clears. Allow the boiler up to 2 minutes to complete its start-up sequence. If it fires up successfully, monitor it over the next 24 hours to confirm the fault does not return. Do not reset more than 2–3 times in total — repeated resets without fixing the root cause can cause further damage.
- Call a Gas Safe registered engineer if the fault persists or recurs Gas Safe engineer
If the boiler locks out again after re-pressurising and resetting, or if the pressure gauge was already correct when you checked it (suggesting a flame-failure rather than a pressure fault), the problem is likely the ionisation probe, ignition electrode, gas valve, PCB, or another internal component. Only a Gas Safe registered engineer can safely inspect, diagnose, and repair these parts. Ask the engineer to check the electrode condition, ionisation probe, flue integrity, gas valve operation, and PCB — particularly on older Junior models where PCB failure is a known age-related issue.
Parts you may need
- Ignition electrode and ionisation probe assembly · from £35
- Condensate trap and pipe assembly · from £25
- System pressure sensor · from £40
- Filling loop assembly · from £20
- Gas valve · from £110
- PCB (printed circuit board) — Greenstar Junior · 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 £100–£350, depending on the underlying cause.
Frequently asked questions
How do I tell whether my 5 long flashes mean low pressure or a flame failure?
Check the pressure gauge first. If it reads below 1 bar (or is at zero), low pressure is almost certainly the cause and you can top up the system yourself via the filling loop. If the gauge already shows 1–1.5 bar and the boiler still locks out after a reset, the fault is more likely a flame-failure or ionisation issue — in which case you need a Gas Safe engineer to investigate the electrode, probe, gas supply, or internal components.
Can I fix the 5-flash fault myself, or do I need an engineer?
It depends on the cause. Topping up low system pressure through the filling loop is a straightforward homeowner task, and thawing a frozen condensate pipe in cold weather is also something you can do safely. However, if pressure is fine or the fault keeps coming back, the problem lies inside the boiler — with the ignition electrode, ionisation probe, gas valve, or PCB — and UK law requires a Gas Safe registered engineer to carry out that work. Attempting gas-side repairs yourself is both illegal and dangerous.
Why does my Worcester Bosch Junior keep losing pressure and showing 5 flashes?
A one-off pressure drop is normal and usually down to a small air pocket or radiator bleeding. But if you find yourself topping up the pressure repeatedly every few weeks, there is a leak somewhere in the system. Common culprits are weeping radiator valves, a faulty pressure relief valve that keeps releasing, or a pinhole leak in the pipework or heat exchanger. An engineer can pressure-test the system to find the source. Left unfixed, a persistent leak will keep triggering the 5-flash lockout.
My 5-flash code came back after I reset it — could it be the PCB?
It is possible, especially on older Greenstar Junior models. These boards can develop faults with age — corrosion or component degradation can cause the board to misread sensor signals or fail to fire the ignition circuit correctly. A PCB replacement typically costs £200–£400 including labour. However, a good engineer will always rule out cheaper causes first (dirty electrode, faulty pressure sensor, gas valve issue) before recommending a PCB swap, as it is one of the more expensive repairs on these boilers.