Worcester Bosch 24si II Fault codes & diagnostics

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

Combi Natural GasLPG 24 kW 2002-2005 Discontinued
GC Numbers 47-311-6547-311-6647-311-6747-311-68

Replaced by: Greenstar 24i

Download the Worcester Bosch 24si II manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

This boiler uses LED flash patterns

Count the flashes in one repeating group before the pause. That number is your fault code. Do not reset until you have noted it. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
1 Emergency 11 High 1 Medium

Which version do you have?

The GC number on your boiler's data badge identifies the exact appliance and production years, as recorded in the UK Product Characteristics Database.

GC number Appliance (register name) Produced
47-311-65 24 SI II RSF 2002–2005
47-311-66 24 SI II RSF 2002–2005
47-311-67 28 SI II RSF 2002–2005
47-311-68 28 SI II RSF 2002–2005

All 13 documented codes

Tap any card for details

Flash 4 times per second

Your boiler has detected an internal communication or airflow issue that has caused it to shut down for safety.

High Engineer only £120-350

Parts commonly replaced

  • Fan
  • Air Pressure Switch
  • NTC Sensor
  • PCB
Flash once per second

Your boiler has stopped working because it has failed to light or has detected a dangerous build-up of heat.

High DIY-safe £120-450

What to check first

  1. Check that your gas supply is turned on (e.g., check other gas appliances)
  2. Check the pressure gauge is between 1.0 and 1.5 bar
  3. Press and hold the reset button for five seconds

Parts commonly replaced

  • Gas valve
  • Spark electrode
  • Circulation pump
  • Overheat thermostat
  • Printed Circuit Board (PCB)
LED 4 flash/second

Your boiler has detected an issue with the sensor that manages your hot water, meaning you will likely only have heating or very inconsistent water temperatures.

Medium Engineer only £90-160

Parts commonly replaced

  • DHW Temperature Sensor (Thermistor)
  • Wiring harness
Air flow fault

Your boiler is failing to clear combustion fumes properly and has automatically shut down to keep your home safe.

Emergency Engineer only £150-350

Parts commonly replaced

  • Air pressure switch
  • Flue fan
  • Venturi

Related codes

DHWsensor

Your boiler is struggling to detect the temperature of your hot water tap supply, meaning you likely have no hot water available.

High Engineer only £120-220

Parts commonly replaced

  • Domestic Hot Water (DHW) NTC Thermistor
  • Wiring harness
F1

Your boiler has stopped working because one of its internal electrical safety fuses has blown, preventing the system from powering up.

High Engineer only £120-220

Parts commonly replaced

  • 2A Glass fuse
  • Printed Circuit Board (PCB)
F2

Your boiler has lost electrical power to its internal control board due to a blown safety fuse, meaning it cannot operate until the circuit is restored.

High Engineer only £120-200

Parts commonly replaced

  • 1.25A Glass Fuse
  • Printed Circuit Board (PCB)

Related codes

Ignition lockout

Your boiler has stopped working because it failed to light the flame, which means your heating and hot water have turned off for safety.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Ignition lead
  • Printed Circuit Board (PCB)

Related codes

lock-out

Your boiler is failing to light the flame, which means it has stopped working to keep you safe.

High Engineer only £150-350

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board (PCB)
  • Flame rectification probe
LOCKOUT

Your boiler has stopped working because it failed to successfully light its internal flame.

High Engineer only £150-350

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board (PCB)
  • Limit thermostat
Overheat cut-off

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

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Pump
  • Diverter valve
  • Primary heat exchanger
Water temperature too cool

Your boiler is struggling to reach the temperature you have set because it is not receiving enough gas or is failing to detect the water flow correctly.

High Engineer only £150-350

Parts commonly replaced

  • Gas valve
  • Flow sensor
  • Main printed circuit board (PCB)
Water temperature too hot

Your boiler has sensed that the water inside it is getting dangerously hot and has shut itself down to prevent damage.

High Engineer only £120-220

Parts commonly replaced

  • NTC temperature sensor
  • Clip or securing bracket
  • Heat exchanger

1 operating states & engineer diagnostics

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