Common problems guide

The most common Worcester Bosch High Flow 400 BF problems

The faults most likely to send a Worcester Bosch High Flow 400 BF into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

This model is discontinued. Parts can be harder to source and expensive. If repair costs are mounting, a new A-rated boiler may be the smarter choice.
3 documented codes
3 most common
0 DIY-safe checks
3 engineer needed

The 3 most common faults

01
H8 High Engineer

Your boiler is sensing a flame even though the gas should be turned off, leading the system to shut down as a safety precaution.

Why it happens: This fault occurs when the boiler's internal sensors continue to detect a flame or electrical current even after the gas valve has been instructed to close. The most common reason is a faulty gas valve that is failing to shut off properly or a 'ghost' signal caused by a malfunctioning control board or dampness around the electrodes. Because the system thinks a fire is still burning when it shouldn't be, it shuts down immediately to ensure your safety.

Est. cost
£120-450
Parts
Gas valve, Control board (PCB), Ionisation electrode
Full guide for H8
02
A3 High Engineer

Your boiler has lost the signal from one of its internal heat sensors, meaning it cannot accurately monitor the temperature and has shut down to stay safe.

Est. cost
£150-250
Parts
NTC Temperature Sensor, Wiring harness
Full guide for A3
03
P? High Engineer

Your boiler is unable to safely drain away the water it creates during normal operation, causing it to block its own internal system and switch off.

Est. cost
£150-250
Parts
Condensate pump, Condensate trap, Float switch
Full guide for P?
Parts most often replaced

on the Worcester Bosch High Flow 400 BF

Gas valveControl board (PCB)Ionisation electrodeNTC Temperature SensorWiring harnessCondensate pump

Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.

Call a Gas Safe engineer if…

  • You can smell gas or see signs of a leak
  • The Worcester Bosch shows an Emergency or High severity code
  • The boiler keeps locking out after repeated resets
  • You've tried the DIY checks and the fault hasn't cleared
  • There's visible water leaking from the boiler
  • The flame is yellow or orange instead of blue
See the full 3-code list for the Worcester Bosch High Flow 400 BF

Related guides

Boiler losing pressure → No hot water → When to replace → Symptom checker →