Worcester Bosch 2 Long Flashes Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch 2 Long Flashes fault code mean?
On older Worcester Bosch boilers fitted with a control box (such as the Danfoss-style unit), the diagnostic light communicates faults through a series of slow or fast flashes. Two long (slow) flashes specifically indicate that the boiler's internal service mode switch has been left at the minimum burner output position. This is not a mechanical breakdown — it is a mode indicator telling you the boiler is sitting in a service or test state rather than normal operation. The pace of the flash matters: slow pulses (2–5) relate to the service mode switch position (minimum at 2, maximum at 5), while rapid or fast flashing points to a separate issue such as a blocked code plug or an internal control board fault. If your boiler is showing exactly 2 slow, rhythmic pulses, the fix is usually straightforward and can often be done without an engineer.
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
- Service mode switch left at minimum position Common
The most frequent reason for 2 long flashes. The boiler's service or diagnostic switch — located inside the control box — has been set to the minimum burner output position. This can happen accidentally during a power cycle, after an engineer has been servicing the boiler, or if someone has interacted with the controls without realising the effect. The boiler is technically operational but stuck in a test state.
- Engineer left boiler in service/test mode after maintenance Sometimes
After carrying out a service or repair, an engineer may have used the service mode switch to test the boiler at different output levels and then not returned it to the standard running position before leaving. The boiler continues to signal this via the 2-flash pattern.
- Loose or intermittent wiring connections to the control box Rare
In a small number of cases, the flashing pattern can be triggered by a wiring or connector issue affecting the control box or heat control module, rather than the switch position itself. If pressing the service button and resetting the boiler has no effect, this becomes more likely.
How to fix it
- Check the flash pattern carefully — confirm it is slow, not rapid DIY safe
Stand in front of the boiler and count the pulses. Two distinct, slow (long) flashes that repeat in a steady rhythm confirm this is the service mode indicator. Rapid or continuous flashing is a different fault entirely and may require an engineer straight away. Getting this right before you do anything else saves unnecessary work.
- Press the service button to exit service mode DIY safe
On the boiler's front panel or control box, locate the service button (sometimes labelled 'S' or 'Service'). Press it firmly once. On some older Worcester Bosch control boxes, turning the control knob anticlockwise will achieve the same result. The boiler should return to normal operating mode and the 2-flash pattern should stop. This is the primary fix and works in the majority of cases.
- Reset the boiler and observe whether the fault clears DIY safe
If pressing the service button alone does not clear the display, perform a standard boiler reset: press and hold the reset button (usually marked with a flame symbol) for a few seconds until the boiler attempts to relight. Do this no more than 2–3 times. If the 2-flash pattern returns immediately after each reset, do not keep resetting — move to the next step instead.
- Power cycle the boiler at the mains DIY safe
Switch the boiler off at the mains isolator switch (or unplug it if it is plug-in type). Wait at least 30 seconds to allow the electronics to fully discharge, then restore power. This clears any temporary software state that might be holding the boiler in service mode. Check whether the 2-flash pattern reappears once the boiler restarts.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If the 2 long flashes continue after following all the steps above, an engineer needs to open the boiler casing and inspect the service mode switch setting directly, check wiring and connector integrity to the PCB or heat control module, and rule out any underlying issue with the control board itself. Do not attempt to open the boiler casing or adjust internal components yourself — this is unsafe and will invalidate your warranty.
Parts you may need
- Worcester Bosch control box / heat control module · from £180
- PCB (printed circuit board) · from £280
- Control box wiring loom · from £45
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 £0–£150, depending on the underlying cause.
Frequently asked questions
Is 2 long flashes on a Worcester Bosch boiler serious?
In most cases, no. Two long (slow) flashes simply mean the boiler's service mode switch is at the minimum position — it is a mode indicator rather than a sign of a broken component. Pressing the service button or resetting the boiler usually resolves it within minutes at no cost. It only becomes a more involved problem if the flashing continues after a reset, which may point to a wiring or control board issue.
What is the difference between slow flashes and fast flashing on a Worcester Bosch boiler?
The speed of the flash means very different things on Worcester Bosch control box models. Slow (long) flashes — between 2 and 5 pulses — indicate the service mode switch position: 2 slow flashes means minimum output, 5 slow flashes means maximum output. Rapid or fast flashing is a separate fault category, typically indicating a blocked code plug or an internal control board fault that usually requires an engineer and potentially a PCB replacement.
Can I fix 2 long flashes myself without an engineer?
Yes, in most cases. Start by pressing the service button on the control panel to exit service mode, or turn the control knob anticlockwise. If that does not work, try a full boiler reset and then a mains power cycle. These are all homeowner-safe actions. You only need a Gas Safe engineer if the fault persists after these steps, since any further investigation involves opening the boiler casing and inspecting internal components.
How much will it cost to fix 2 long flashes if I need an engineer?
Because this fault is usually a service mode switch issue rather than a broken part, many people fix it for free by pressing the service button themselves. If you do need an engineer, a standard call-out and reset typically costs £50–£100, and a full diagnostic visit around £100–£150. Parts are rarely needed for this specific fault, but if a thorough inspection reveals a faulty control board, a PCB replacement can cost £250–£400 including labour — though this is uncommon for 2 long flashes on its own.