Worcester Bosch B7 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch B7 fault code mean?
The B7 fault code — almost always displayed alongside cause code 257 on Worcester Bosch boilers — signals that the boiler's internal control board, commonly known as the PCB (Printed Circuit Board), has detected a fault or failed outright. The PCB acts as the central brain of your boiler: it receives signals from components such as sensors, the fan, and the air pressure switch, and sends instructions back to parts like the gas valve and pump. When the PCB is malfunctioning, that two-way communication breaks down, causing the boiler to lock out, misfire, or behave erratically. Note: a small number of sources associate B7 with a hot water sensor fault rather than a PCB failure; however, the dominant interpretation among heating specialists for B7 / cause code 257 is an internal control board fault, and that is what this guide addresses.
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
- Age-related PCB component wear Common
Over many years of use, the individual components soldered onto the PCB — capacitors, resistors, relays — deteriorate naturally. Older boilers (typically over 10–12 years) are far more susceptible to this type of gradual failure than newer, better-sealed units.
- Moisture ingress or internal water leak Common
Even a small drip from a pump seal, heat exchanger joint, or nearby pipe can spell disaster for a PCB. Direct water contact causes immediate short-circuits, while the surrounding humidity causes slow corrosion of board tracks and solder joints over time. This is one of the most frequent routes to a PCB failure.
- Burnt-out wiring or connector terminals Common
The wiring harness and plug-in connectors that link the PCB to other components can oxidise, overheat, or work loose over time. A poor connection generates localised heat that burns the terminal or the board track it feeds into, producing intermittent or permanent faults.
- Limescale-induced heat exchanger damage leading to leaks Sometimes
In hard-water areas, limescale builds up inside the heat exchanger and creates hot spots. Those hot spots eventually crack the heat exchanger, causing an internal leak that then migrates towards the PCB — so the root cause is limescale even though the presenting fault is a board failure.
- Vibration damage Sometimes
Unusual boiler vibration — particularly on ignition or shut-down — can shake solder joints loose or crack PCB tracks over time. If you have noticed your boiler rattling or vibrating more than usual, this may have contributed to the fault.
- Faulty upstream sensor or component giving bad signals Rare
Occasionally the PCB itself is healthy but a failed sensor, thermistor, or other component is feeding it corrupted data, causing the board to log an internal error. A thorough diagnostic by an engineer is needed to rule this out before condemning the PCB.
How to fix it
- Check your gas supply is active DIY safe
Before assuming a component fault, confirm that other gas appliances in your home (hob, fire) are working normally. If there is no gas supply across the property, contact your gas supplier rather than your heating engineer.
- Attempt a single boiler reset DIY safe
Press and hold the reset button on your Worcester Bosch boiler for roughly 3 seconds, then release. Wait several minutes to see if the boiler fires up and the B7 code clears. If it locks out again immediately, do not keep resetting — repeated resets on a PCB fault can drive components harder and risk causing further damage.
- Note down the full code displayed — including cause code 257 DIY safe
Take a photo or write down exactly what appears on the display (e.g. 'B7 257'). This information helps the engineer narrow down the diagnosis quickly and may indicate whether the fault is consistent with a PCB failure or a related component.
- Do not attempt to access or inspect the PCB yourself Gas Safe engineer
The PCB sits inside the boiler casing alongside live electrical components and gas-carrying parts. Removing panels or probing the board without the correct training and equipment is dangerous and will void any remaining warranty. This work must only be carried out by a qualified engineer.
- Call a Gas Safe registered engineer to carry out a full diagnostic Gas Safe engineer
A qualified engineer will test the PCB and its associated wiring, sensors, and connectors to establish whether the board itself is faulty or whether an upstream component is generating the error. They will check for signs of moisture ingress, burnt connectors, or corrosion before deciding on the appropriate repair. If the PCB is confirmed faulty, they will replace it with the correct part for your boiler model.
- Discuss repair vs replacement with the engineer, especially for older boilers DIY safe
PCB replacement on a boiler that is over 10–12 years old or that has a history of recurring faults may not represent good value. Ask the engineer for an honest assessment: if the repair cost approaches or exceeds a third of a new boiler's price, a full replacement may be the more economical long-term choice.
Parts you may need
- Replacement PCB (control board) · from £120
- Wiring harness / connector loom · from £45
- NTC thermistor / temperature sensor · from £20
- Pump seal kit (if leak caused PCB damage) · from £15
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 £180–£400, depending on the underlying cause.
Frequently asked questions
Can I reset my Worcester Bosch boiler to clear the B7 fault code?
You can try a single reset, but do not keep repeating it. If the boiler fires up and the B7 code returns within a short time, the PCB or a related component has a genuine fault that resetting will not cure. Repeated resets on a faulty control board can force already-stressed components to operate outside safe parameters, potentially causing additional damage and a larger repair bill.
How much does it cost to replace a PCB on a Worcester Bosch boiler in the UK?
Most homeowners pay in the region of £180–£400 all-in for a PCB replacement on a Worcester Bosch boiler, including parts and labour. The PCB part itself typically costs £80–£160 at trade prices, and engineers usually charge a call-out fee plus an hourly rate of around £50–£60. Costs at the higher end reflect boilers where the PCB is harder to access or where the specific board is a later, more sophisticated unit. If a heat exchanger replacement is also required (for example, where a leak caused the PCB failure), the overall bill can rise considerably — this is worth discussing with your engineer before authorising work.
Is the B7 fault code always a PCB failure, or could it be something else?
The B7 / cause code 257 combination almost always points to an internal control board issue on Worcester Bosch boilers, but it does not automatically mean the PCB needs to be replaced. Occasionally a faulty sensor, a burnt wiring connector, or corrupted signals from another component trigger the fault code even though the board itself is intact. A Gas Safe engineer will test the entire signal chain — sensors, wiring, and the board — before confirming which part is at fault. Replacing the PCB unnecessarily is expensive, so a proper diagnostic matters.
Should I repair or replace my boiler if it shows a B7 fault?
If your boiler is under 8–10 years old and otherwise in good condition, a PCB repair or replacement is usually worth doing. On boilers aged 10–12 years or more, especially those with a history of faults, the cost of a PCB replacement (£180–£400) starts to look less attractive when weighed against the reliability and efficiency gains of a new boiler. A new Worcester Bosch Greenstar combi typically costs £1,400–£2,550 supplied and installed. Ask your engineer for an honest view on the boiler's overall condition before committing to an expensive repair on an ageing unit.