Overview
B7 257 on Worcester Bosch CDi Compact / Greenstar Si/I combi boilers indicates an internal fault flagged by the boiler control electronics. Most official references list B7 257 as an "internal error" with the likely cause being a control board (PCB/HCM) fault or related internal electronics problem. In short, the boiler has detected something within its control system that it cannot manage, so it has entered a fault/lockout state to protect the appliance and the property. This fault is generally more serious than routine user-level faults like low pressure or frozen condensate and usually requires an engineer to diagnose and repair. Homeowner actions are limited to basic, safe checks and a reset attempt. Because the suspected fault affects the boiler’s control electronics, DIY repair is not recommended. Work on control boards, gas valves, ignition circuits or internal wiring must be carried out by a competent, Gas Safe qualified engineer. If you see signs of burning, water ingress, a strong smell of gas, or smoke, isolate the boiler and call an emergency Gas Safe engineer and your gas supplier immediately.
Possible Cause: Possible control board problem.
Troubleshooting Steps
Safety precautions:
- If you smell gas, leave the property immediately and call your gas emergency number. Do not operate electrical switches or the boiler. Contact a Gas Safe engineer and your gas supplier.
- Before any visual checks, switch the boiler off at the isolator and, if safe, isolate the electrical supply at the fuse box. Do not remove covers or touch internal components unless you are a qualified engineer.
- If you see water leaking from the boiler, turn off the water and electricity to the boiler and call a professional.
Initial homeowner checks you can safely do:
1. Note the full fault code and any secondary/displayed cause code. Press the spanner/return button (where present) to view the cause code if the boiler shows one and write it down or photograph the display for the engineer.
2. Try a simple reset: on many Worcester models press and hold the reset button for ~3 seconds or use the temperature dial reset position for ~3 seconds. If the model requires, perform a full mains power cycle: switch the boiler off at the isolator, wait 30 seconds, then switch back on. Only attempt a reset once; repeated resets do not fix internal faults and can mask underlying problems.
3. Check the mains electrical supply: ensure other house circuits are powered and the boiler’s fused spur or isolator is ON. Check the consumer unit for a tripped MCB or RCD. Do not open the boiler to access internal fuses.
4. Look for obvious external signs: water ingress, corrosion, burn marks, loose wiring at external terminals, or external damage to the casing. Photograph any damage for the engineer.
5. Check that external controls and timers are calling for heat (room thermostat, programmer). A control error can sometimes appear if the boiler is not being commanded to run.
Specific diagnostic steps and information to gather for the engineer:
1. If reset fails and the B7 257 persists, record how and when the fault happened (e.g., during ignition, after power cut, after heavy rain or maintenance). Note any preceding symptoms: unusual noises, burner behaviour, loss of heating, flickering display or tripped breakers.
2. Use the boiler interface to access the block/cause code (press return/spanner as per manual). Record both the blocking fault code and the cause code; these help the engineer narrow the fault to sensors, fan, HCM or PCB issues.
3. Check for a visible blown connector or loose plug on the boiler casing only if you are competent and the electrical supply is isolated. Do not probe connectors or remove PCBs. If you see a loose wire on an external terminal, record its location and photograph it.
4. Note water pressure on the boiler gauge. Although B7 is not usually pressure-related, very low or very high pressure and repeated leaks can damage electronics; report pressure reading to the engineer.
5. If the boiler has experienced recent power interruptions, lightning, or water leaks, mention this. Brief power surges and water ingress are common causes of PCB faults.
When to call a professional and what to tell them:
- Call a Gas Safe registered engineer if the code remains after a single safe reset or if you see signs of water damage, burning, persistent fault codes, or loss of gas/ignition. Do not attempt to replace PCBs, ignition modules, gas valves, or internal wiring yourself.
- Provide the engineer with: boiler model, serial number, the exact fault code (B7 257) and any cause code or block code you recorded, description of events leading to the fault, results of your checks (power status, photographs, pressure reading), and whether a reset was attempted.
Why a professional is required:
- B7 257 commonly points to a control board or burner control unit issue. Diagnosis often needs specific test equipment, access to internal diagnostic menus, and safe handling of live circuits and gas components. A qualified engineer will test sensors, fan and ignition circuits, check the HCM/code plug, inspect PCB connectors and earthing, and replace the correct component if necessary. They will also perform post-repair safety and combustion checks.
Final note:
- Do not repeatedly reset the boiler to clear the fault; this can create unsafe conditions and delay correct repair. Treat B7 257 as an internal electronics failure that needs professional diagnosis. Arrange a Gas Safe engineer and give them the recorded fault details to speed up repair and ensure safe restoration of the boiler.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch CDi Compact / Greenstar 25/30 Si Combi / Greenstar 27/30 I system Boiler.