Worcester Bosch B7 fault code: b7 257 explained

What does the Worcester Bosch B7 fault code mean?

The Worcester Bosch B7 fault code means the boiler's control box or its HCM is defective. That is Worcester's own wording, not ours. The error code table in the Greenstar i ErP installation, commissioning and servicing instructions, document 6 720 806 944 (2016/07), pairs fault code b7 with cause code 257 under the display text 'Internal error' and describes it as 'Control box or the HCM is defective.' The same table tells the engineer to check that the 'HCM is inserted properly' and the 'Connections of the control box', then to 'Contact technical helpline'. The identical row appears in the Greenstar i System ErP instructions, document 6 720 806 945.

The HCM is the small code plug that identifies your appliance to the control board. Worcester spells the acronym out in the parts-replacement section of those same manuals as 'HCM (Heating Control Module)', and its Greenstar 8000 Life fault list calls the same part the code plug. If the board cannot read a valid code plug, or cannot trust its own electronics, it stops rather than run on a configuration it cannot verify.

B7 is a locking fault rather than a temporary blocking one. Worcester's Greenstar Si Compact ErP instructions, document 6 720 813 278 (2015/07), define a locking error as one where 'The boiler display will flash a warning triangle and fault code. At the same time a (static) cause code will also be displayed', and where manual intervention is required after the fault is cleared: 'Press the reset button on the boiler fascia'. The same manual explains that the cause code is a 'Three digit number' which 'flashes on the display' during a locking error. In practice the boiler shuts down and stays locked out, with no heating and no hot water, until the cause is dealt with.

The code turns up written several ways. Worcester's manuals print a lowercase b, so it reads b7, while the display and most third party pages show B7. The Si Compact ErP manual lists the pair the other way round, as '257 b7'. The Greenstar 8000 Life range does not show a letter code at all: its user instructions say 'the cause of the fault is coded (e.g. fault code 232)', so those owners see 257 on its own. B7, b7 257 and a bare 257 on that range are the same fault.

Key facts

The short version, before the detail.

• Meaning: 'Control box or the HCM is defective.' That is the verbatim description in Worcester's Greenstar i ErP instructions, document 6 720 806 944.
• Also written as: B7, b7, b7 257, 257 b7, or on the Greenstar 8000 Life range simply 257 on its own.
• Fault type: locking. The boiler shuts down and needs the reset button pressed once the cause has been cleared.
• What the HCM is: the code plug that identifies your appliance to the control board. Worcester writes it out as 'HCM (Heating Control Module)'.
• DIY fixable? No. A single reset is the limit. Everything else sits behind the casing next to 230V electronics.
• Listed in the manuals for: Greenstar i ErP (25i and 30i), Greenstar i System ErP, Greenstar Si Compact ErP (25Si and 30Si Compact), Greenstar CDi Compact ErP (28CDi, 32CDi and 36CDi Compact) and, as bare cause code 257, the Greenstar 8000 Life. If your model is not one of those, check your own installer manual rather than a third party list.
• Where it is documented: the installer and servicing manuals, not the user instructions. If a homeowner cannot find B7 in the booklet that came with the boiler, that is why. Ask your engineer to confirm the code.
• Not a gas emergency: B7 is an electronics fault. It is safe to switch the boiler off and leave it off until an engineer attends. Call the National Gas Emergency Service on 0800 111 999 only if you smell gas or a carbon monoxide alarm sounds.
• Smart Plan boiler and central heating cover: parts and labour up to £500 per 12-month period if the boiler is under 7 years old, or up to £200 if it is over 7. Those are annual caps shared across every fault in the period, not a promise the whole repair is paid for, and you pay anything above the limit. A new plan covers future breakdowns, not a fault your boiler is already showing, so it is not the answer to tonight's b7 257.
• Cover still applies from day one, and a £95 call-out fee, paid in advance, applies in your first 30 days. The same £95 fee applies for faults the plan does not cover, for no-access visits and for early annual-service requests.
• Cover-plan call-outs run Monday to Friday, 08:00 to 18:00. Using a module starts a 12-month agreement period the first time you use a service. There is a 14-day cooling-off period, but it ends the moment a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance.
• Booking a one-off repair instead: those appointments run Monday to Friday, 09:00 to 17:00, carry a £95 call-out fee paid in advance, and are not covered by a workmanship guarantee.

Why does the B7 257 code appear?

B7 is an electronics fault, not a gas, flame or pressure fault. Worcester's Greenstar i ErP instructions give the engineer two checks against this code, in this order: that the 'HCM is inserted properly', and the 'Connections of the control box'. Only after that does the manual send the installer to the technical helpline.

The later Greenstar 8000 Life combi instructions, document 6 720 883 865 (2021/11), are more explicit for the same cause code. They describe 257 as a 'System fault boiler electronics / basic controller' and give a five step remedy: '1. Restart the appliance to see if the problem re-occurs. 2. Check the electrical connections on the board (check for loose wires etc.). 3. Check the code plug is not damaged. 4. Replace the code plug if required. 5. Replace the control unit (PCB).' A new board is the last step on that list, not the first.

That order matters, because Worcester's Si Compact ErP manual puts the same code far more bluntly. Its table reads '257 b7 Internal error - replace control board', with no mention of the cheaper checks first. Several third party fault code lists have taken that shorter wording and run with it, telling homeowners the board has failed. Worcester's more detailed manuals do not start there, and neither should you.

The table below follows the order Worcester's own instructions work through. Every row is engineer work, because every part named sits behind the boiler casing.

B7 257 causes and what needs to happen

What you can safely check yourself

There is no homeowner fix for B7. Every part the code points at sits behind the casing next to 230V electronics, so the safe list is short. Work through it in this order, then stop.

1. Read the pressure gauge and note the reading for the engineer. Low pressure is not a cause of b7 257, so do not try to repressurise the boiler for this fault. The reading is only useful as background when you describe the problem.

2. Look for any sign of water leaking from or beneath the boiler, and check the consumer unit for a tripped fuse or RCD. Look only. Do not remove the boiler case.

3. Press reset once. Worcester's Greenstar i ErP instructions describe it plainly: 'To reset the appliance, press the reset button, the tick symbol will be displayed briefly for a successful reset.' If the tick does not appear, or the code comes straight back, the fault is still there.

If b7 257 comes back, stop and book an engineer. Repeated resetting will not seat a loose code plug or repair a connection, and it tells you nothing new.

While you wait, leaving the boiler switched off is safe. It has already locked itself out, so nothing is running unsafely. You will have no heating and no hot water in the meantime, so in freezing weather keep one room warm by another means and keep an eye on any exposed pipes.

When to call a Gas Safe registered engineer

Call one as soon as a single reset fails to clear the code. Everything left is engineer-only work: taking the front panel off, checking the HCM code plug is properly seated and undamaged, testing the control box connections and harness, replacing the code plug or the control unit, and speaking to Worcester's technical helpline. This is a gas appliance, so that work must be done by someone Gas Safe registered. Smart Plan works with a national network of thousands of Gas Safe registered engineers across the UK.

Be wary of anyone who prices the job before seeing it. Worcester's own manuals show this fault can end at anything from re-seating a code plug to a new control unit, so ask for the cause to be confirmed and the job quoted before any work goes ahead.

For the b7 257 in front of you now, the thing to book is a one-off repair. Those appointments run Monday to Friday, 09:00 to 17:00, carry a £95 call-out fee paid in advance, and are not covered by a workmanship guarantee. The engineer attends, diagnoses the fault and tells you what is needed before any work goes ahead. You can book online or call 0333 772 6247.

A cover plan is a different thing, and it is worth being straight about what it does. A new plan covers future breakdowns, not a fault your boiler is already showing, so it will not pay for tonight's b7 257. What it is for is the next fault. A Smart Plan boiler and central heating module covers parts and labour up to £500 per 12-month period if your boiler is under 7 years old, or up to £200 if it is over 7. Those are annual caps shared across every fault in the period, not a promise the whole repair is paid for, and anything above your cover limit is paid by you. On a control unit replacement for an older boiler, that gap can be real. Smart Plan is a service plan, not insurance: it arranges and funds covered repairs up to your limit rather than paying you a sum of money.

The small print worth reading before you sign up. Cover applies from day one, and a £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan does not cover, for no-access visits and for early annual-service requests. Cover-plan call-outs run Monday to Friday, 08:00 to 18:00, and outside those hours you are only seen sooner for a genuine emergency breakdown, so a Saturday fault may wait until Monday. Within those hours we aim to get an engineer out to you within two working days, which is a target for attendance rather than a promise about how long a repair takes. Using a module starts a 12-month agreement period the first time you use a service. There is a 14-day cooling-off period, but it ends the moment a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance. Smart Plan comes from UK Boiler Company Ltd, which has traded since 2014.

One safety note. B7 is an electronics fault rather than a gas one, and it is not a gas emergency. But if you ever smell gas or suspect carbon monoxide, do not wait for an engineer. Call the National Gas Emergency Service on 0800 111 999 straight away.

Worcester Bosch B7 fault code FAQs

Stuck on b7 257? Book an engineer.

For the fault on your display now, book a one-off repair and a Gas Safe registered engineer will attend and diagnose it. One-off repair appointments run Monday to Friday, 09:00 to 17:00, carry a £95 call-out fee paid in advance, and are not covered by a workmanship guarantee. Or set up a Smart Plan boiler and central heating module so the next boiler breakdown is already covered: parts and labour up to £500 per 12-month period for a boiler under 7 years old, or up to £200 if it is older. Those are annual caps shared across every fault in the period, not a promise the whole repair is paid for, and you pay anything above the limit. A new plan covers future breakdowns, not a fault your boiler is already showing. Smart Plan is a service plan, not insurance. Cover applies from day one, and a £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan does not cover, for no-access visits and for early annual-service requests. Cover-plan call-outs run Monday to Friday, 08:00 to 18:00, and within those hours we aim to get an engineer out to you within two working days, which is a target for attendance rather than a promise about how long a repair takes. Using a module starts a 12-month agreement period the first time you use a service, there is a 14-day cooling-off period which ends the moment a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance. The wizard shows your price before you commit to anything. Call 0333 772 6247, Monday to Friday, 08:00 to 18:00.

Helpful links

• Build your boiler cover and see your price
• All Worcester Bosch fault codes explained
• C6 fault code, the fan speed lockout
• C7 fault code explained
• EA 227 fault code explained
• Worcester Bosch boiler reviews
• Book a boiler repair
• Boiler & central heating cover
• Worcester Bosch installer literature and manuals
• Check an engineer on the Gas Safe Register