Worcester Bosch B7 257 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch B7 257 fault code mean?
The B7 257 fault code indicates an internal electrical error within the boiler. Specifically, the boiler's onboard diagnostics have detected a problem with the burner control unit (BCU) or the KIM (Kessel Identifikations Modul — the boiler identification module), both of which sit on or communicate directly with the main printed circuit board (PCB). Because this is a locking fault, the boiler shuts down completely and displays a flashing warning triangle alongside the code. It will not restart normally until the underlying fault is resolved. Worcester Bosch's own guidance for this code is to replace the control board, making this a job exclusively for a Gas Safe registered engineer. The sub-code 257 pinpoints the specific internal diagnostic path — a PCB-level communication or hardware failure — as opposed to other B7 sub-codes which may relate to different control-side issues.
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
- PCB or burner control unit hardware failure Common
The PCB is the electronic brain of the boiler, routing signals between every component — the gas valve, fan, pump, igniter, and sensors. When the BCU hardware itself degrades or fails outright, the boiler cannot coordinate these signals and shuts down. This is the most direct cause of a B7 257 and the one Worcester Bosch's own reset action addresses.
- Burnt or corroded wiring connections on the control board Common
Over time, the electrical connectors and soldered joints on the PCB can oxidise or become burnt. A loose or degraded connection can mimic a full board failure, triggering the B7 257 code even when the board itself is otherwise functional. Engineers routinely check and reseat connectors before condemning the whole PCB.
- Water or moisture ingress onto the PCB Common
Even a very small internal leak — from the pump seal, a heat exchanger joint, or a pressure relief valve — can allow water vapour to condense on the control board. Moisture causes short circuits and accelerated corrosion of the board's components, and is one of the most frequent real-world triggers for PCB faults.
- Age-related component degradation Sometimes
Older boilers — particularly those over ten years — are more susceptible to PCB failures. The sealing and build quality of earlier electronics means components become increasingly brittle and prone to failure over time, especially in a hot, humid boiler cabinet environment.
- Limescale-induced heat exchanger damage causing secondary PCB stress Sometimes
In hard-water areas, limescale deposits on the heat exchanger create hotspots that can eventually crack the heat exchanger, leading to minor internal leaks. Those leaks then expose the PCB to moisture (as above). The limescale itself does not directly cause the B7 257, but it can set off a chain of events that ends with a PCB fault.
- KIM (boiler identification module) fault Rare
The KIM stores the boiler's configuration data and communicates it to the BCU. In rare cases the KIM itself is the faulty component rather than the main PCB, though the symptom presented to the user is identical. An engineer can determine which module needs replacing through diagnostic testing.
How to fix it
- Check the boiler display and note the exact code shown DIY safe
Confirm the display is showing B7 257 (not a different B7 sub-code). Take a photograph of the display if possible — this helps the engineer prepare before arriving. Note whether the warning triangle is flashing, which confirms this is a locking fault.
- Attempt a single boiler reset DIY safe
Press and hold the reset button (typically marked with a flame and a line through it, or a rotating arrow symbol — check your user manual for your specific model) for around three seconds. If the boiler restarts, monitor it closely for the next 30–60 minutes. If the fault returns, do not reset again. Repeatedly resetting a PCB fault can mask the problem and potentially cause further damage to already-stressed components.
- Check your gas supply is working DIY safe
Confirm other gas appliances in your home (such as a gas hob) are working normally. If no gas appliances are functioning, contact your gas supplier. If other appliances work fine, the gas supply is not the issue here.
- Do not attempt to open the boiler casing or inspect internal components Gas Safe engineer
The B7 257 fault is an internal electronic fault. Removing the boiler casing and inspecting or touching the PCB, wiring loom, or any other internal part is not safe or legal for an unqualified person. No DIY intervention beyond the reset above is appropriate for this fault.
- Contact a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
A qualified engineer will test the PCB, BCU, KIM, and associated wiring. They will check connectors for corrosion or burning, look for evidence of internal leaks or moisture damage, and determine whether the board needs repair (reseating connections) or full replacement. Worcester Bosch's recommended action for this code is control board replacement. Always ask to see the engineer's Gas Safe ID card before work begins.
Parts you may need
- Main PCB / burner control unit (Worcester Bosch specific) · from £220
- KIM (boiler identification module) · from £85
- PCB wiring loom / connector set · from £35
- Pump seal kit (if moisture ingress is the root cause) · 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 £200–£420, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the boiler to get it working again?
You can try a single reset — the boiler may run for a short while afterwards. However, Worcester Bosch and most Gas Safe engineers advise against repeated resets for a B7 257 fault. A PCB failure is a hardware problem, not a transient software glitch. Running a boiler with a compromised control board can put other components under abnormal stress and potentially turn a £200–£400 PCB repair into a more expensive job. Reset once, and if the fault returns, call an engineer.
How much does it cost to replace a Worcester Bosch PCB in the UK?
For most Worcester Bosch combi and system boilers, PCB replacement typically costs between £200 and £420 all-in (parts and labour). The board itself usually accounts for £150–£280 of that, with the remainder covering the engineer's time. If the root cause turns out to be a corroded connector rather than a failed board, the bill could be lower. In rare cases where the heat exchanger has also been damaged by a related internal leak, that alone can cost £400–£600+ to replace — at that point, a new boiler may be worth considering, especially if the boiler is over ten years old.
What is the KIM module on a Worcester Bosch boiler and does it need replacing?
KIM stands for Kessel Identifikations Modul — it's a small module that holds the boiler's identity and configuration data, allowing the burner control unit to apply the correct operating parameters for that specific model. The B7 257 sub-code can relate to either the main PCB or the KIM. An engineer will test both before deciding which to replace, as the KIM is generally less expensive than the full PCB.
My boiler is over ten years old and showing B7 257 — should I repair or replace it?
If your boiler is approaching or past the ten-to-twelve year mark, it is worth getting a like-for-like repair quote alongside a replacement quote before committing to either. A PCB repair at £200–£420 is not unreasonable on a relatively young boiler, but if the boiler has had previous faults or is outside its manufacturer warranty period, the cost of repair can sometimes approach the deposit on a new, more efficient model. A new Worcester Bosch Greenstar 4000 typically starts from around £2,500 installed, but comes with a manufacturer guarantee of up to ten years — worth factoring into the comparison.