Worcester Bosch Highflow 400 CDi Fault codes & diagnostics

3 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

Combi Natural Gas 40 kW 1997-2002 Discontinued

Replaced by: Greenstar Highflow 440 CDi

Download the Worcester Bosch Highflow 400 CDi manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

The fault code is already on your display

Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →

Just want the most common Worcester Bosch Highflow 400 CDi faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 3 documented codes

Tap any card for details

Heatbank Overheat Stat

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

High Engineer only £150-300

Parts commonly replaced

  • High limit overheat thermostat
  • NTC sensor
  • Diverter valve
Full explanation for Heatbank Overheat Stat
Hot water high limit stat

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

High Engineer only £150-300

Parts commonly replaced

  • High limit thermostat
  • NTC sensor
  • Diverter valve
  • Primary heat exchanger
Full explanation for Hot water high limit stat
Safety limit stat

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

High Engineer only £150-300

Parts commonly replaced

  • Overheat thermostat
  • Pump
  • Diverter valve
  • Heat exchanger
Full explanation for Safety limit stat
Status & diagnostic codes

3 operating states & engineer diagnostics

Not faults — these are normal operating states. Tap to expand.

72 Diagnostic Your boiler has detected that the internal exhaust vent settings do not match the length of the flue pipe installed, which is usually due to a recent installation error or a change in the exhaust configuration. 75 Diagnostic Your boiler has detected that the internal airflow setup does not match the flue length installed, which may be preventing it from firing up correctly. 77 Diagnostic Your boiler has detected that the internal airflow restrictor has been set to the wrong size for your specific flue length, preventing it from operating safely.