Common problems guide

The most common Worcester Bosch Highflow 400 CDi problems

The faults most likely to send a Worcester Bosch Highflow 400 CDi 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.
4 documented codes
4 most common
1 DIY-safe checks
3 engineer needed

The 4 most common faults

01
overheat lock-out state High DIY-safe

Your boiler has switched itself off because it detected that the water inside it has become too hot, likely as a safety precaution.

What to try first

Locate the reset button on the bottom right-hand side of the boiler control panel.Press and hold the button firmly for a few seconds.Wait to see if the boiler restarts and the fault light clears.

Est. cost
£120-250
Parts
NTC Sensor, Primary Heat Exchanger, Pump
Full guide for overheat lock-out state
02
Heatbank Overheat Stat High Engineer

Your boiler has detected that the water storage tank has become too hot and has automatically shut down to stay safe.

Est. cost
£150-300
Parts
High limit overheat thermostat, NTC sensor, Diverter valve
Full guide for Heatbank Overheat Stat
03
Safety limit stat High Engineer

Your boiler has detected that it is getting too hot and has automatically shut itself down to prevent damage.

Est. cost
£150-300
Parts
Overheat thermostat, Pump, Diverter valve
Full guide for Safety limit stat
04
Hot water high limit stat High Engineer

Your boiler has detected that it is getting dangerously hot and has shut itself down to prevent damage or fire risk.

Est. cost
£150-300
Parts
High limit thermostat, NTC sensor, Diverter valve
Full guide for Hot water high limit stat
Parts most often replaced

on the Worcester Bosch Highflow 400 CDi

NTC SensorPrimary Heat ExchangerPumpHigh limit overheat thermostatNTC sensorDiverter valve

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 4-code list for the Worcester Bosch Highflow 400 CDi

Related guides

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