Worcester Bosch 24/28CDi II RSF Fault codes & diagnostics

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

Combi Natural Gas 24-28 kW 2003-2007 Discontinued

Replaced by: Greenstar 25/30i

Download the Worcester Bosch 24/28CDi II RSF 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 →

Severity at a glance

Each cell = one fault code. Hover to identify.
11 High
Just want the most common Worcester Bosch 24/28CDi II RSF faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 11 documented codes

Tap any card for details

FAIL POINT A

Your boiler appears to have lost all electrical power, meaning it cannot start up or provide any heating or hot water.

High DIY-safe £120-350

What to check first

  1. Check if other appliances in your home have power
  2. Ensure the boiler's electrical isolation switch is turned on
  3. Check your home's main fuse box (consumer unit) for any tripped circuit breakers

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Internal Glass Fuses
  • Transformer
Full explanation for FAIL POINT A
FAIL POINT B

Your boiler has lost power or has a serious internal electronic failure, meaning it cannot start the ignition process to provide heating or hot water.

High Engineer only £180-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Internal Fuse
  • Transformer

Related codes

Full explanation for FAIL POINT B
FAIL POINT C

Your boiler has lost power or has a major electrical fault, meaning it cannot provide any heating or hot water despite the fuse board appearing normal.

High Engineer only £120-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Internal Fuses
  • Transformer
Full explanation for FAIL POINT C
FAIL POINT D

Your boiler is not reacting when you turn on the heating because it isn't receiving the correct signal to start that specific mode.

High DIY-safe £120-£280

What to check first

  1. Check that the room thermostat is turned up higher than the current room temperature.
  2. Ensure the mechanical clock or digital programmer is set to 'ON' or 'AUTO'.
  3. Replace the batteries in your wireless room thermostat if it has them.

Parts commonly replaced

  • External Room Thermostat
  • Internal Diverter Valve Microswitch
  • Control Board (PCB)

Related codes

Full explanation for FAIL POINT D
FAIL POINT E

Your boiler is not detecting that you have turned on a hot water tap, so it isn't starting the burner to heat the water.

High Engineer only £120-£220

Parts commonly replaced

  • DHW flow switch
  • Diaphragm assembly
  • Pressure microswitch
Full explanation for FAIL POINT E
FAIL POINT F

Your boiler is trying to run the heating even though you have turned the control knob to the off position, which usually indicates an internal electronic fault or a stuck component.

High Engineer only £180-£450

Parts commonly replaced

  • Main PCB (Control Board)
  • Diverter Valve
  • Flow Sensor

Related codes

Full explanation for FAIL POINT F
FAIL POINT G

Your boiler has shut down because it has detected its internal temperature is too hot and is now in a safety lockout.

High DIY-safe £120-350

What to check first

  1. Check that all radiator valves are fully open to allow water to flow.
  2. Wait for the boiler to cool down naturally.
  3. Press and hold the reset button for five seconds.

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Thermistor
Full explanation for FAIL POINT G
FAIL POINT H

Your boiler has failed to light the flame or detects that the flame has gone out, causing the system to shut down for safety.

High DIY-safe £100-350

What to check first

  1. Check if other gas appliances, like a cooker, are working to ensure gas is reaching the property
  2. Check that your prepaid gas meter has sufficient credit
  3. Press and hold the reset button for five seconds to see if the boiler restarts

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Probe
  • Gas Valve

Related codes

Full explanation for FAIL POINT H
FAIL POINT I

Your boiler has detected a problem with one of its internal temperature sensors and has stopped working as a safety precaution.

High Engineer only £120-£220

Parts commonly replaced

  • Central Heating Temperature Sensor (NTC Thermistor)
  • Hot Water Temperature Sensor
  • Wiring Harness

Related codes

Full explanation for FAIL POINT I
FAIL POINT J

Your boiler has failed to ignite properly, meaning it cannot create the flame needed to provide you with heat or hot water.

High DIY-safe £120-350

What to check first

  1. Check that your gas pre-pay meter has credit (if applicable)
  2. Check that other gas appliances in your home, like a hob, are working correctly
  3. Press and hold the Reset button for five seconds

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Printed Circuit Board (PCB)
Full explanation for FAIL POINT J
FAIL POINT K

Your boiler has encountered a general electrical failure, likely caused by moisture or damage to the internal brain of the unit.

High Engineer only £250-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Control Board Housing
Full explanation for FAIL POINT K
Status & diagnostic codes

12 operating states & engineer diagnostics

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

E00-711 Diagnostic Your boiler is struggling to transfer heat to your hot water because a key internal component is likely blocked or damaged. E00-716 Diagnostic Your boiler has detected a serious problem with the fan that safely removes exhaust gases, meaning the system has shut down for your safety. E00-720 Diagnostic Your boiler has detected a fault with the burner unit, meaning it cannot safely ignite or maintain a flame to provide heat or hot water. E00-721 Diagnostic Your boiler has detected a serious internal problem with the gas burner system, causing it to shut down for safety. E00-722 Diagnostic Your boiler's main control computer has developed an internal electrical fault and can no longer safely operate the system. E00-730 Diagnostic Your boiler has detected a problem with how it moves air and exhaust gases, which has caused it to shut down for safety. E00-737 Diagnostic Your boiler's burner has encountered a serious ignition or combustion failure, meaning it cannot safely light to provide heat or hot water. E00-738 Diagnostic Your boiler has detected a critical internal fault with the burner system that regulates the fuel supply, meaning the appliance cannot ignite safely. E00-741 Diagnostic Your boiler has detected an issue with the internal burner assembly components, likely related to the incorrect gas injection setup for your fuel type. E01-840 Diagnostic Your boiler has lost water pressure, and the internal component used to refill it is likely disconnected or faulty. E80-450 Diagnostic Your boiler has detected a failure with the internal gas control valve, meaning it cannot safely ignite to provide heating or hot water. E80-451 Diagnostic Your boiler has detected a specific internal electronic fault with the valve that controls the gas supply, meaning the appliance cannot ignite safely and will remain shut down.