Ideal RD2 Fault codes & diagnostics

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

Regular Natural Gas 11-23 kW 2001 onwards Discontinued

Replaced by: Classic

Choose your boiler type

The RD2 range comes in 3 types

Pick the one you have. The fault codes, GC numbers and manual differ by type.

RD2 4100 8 codes · 1 GC number RD2 4125 8 codes · 1 GC number RD109 8 codes · 1 GC number
Each model has its own manual A family spans several manuals, one per model or generation. Pick your model above and its page has the manual its fault codes were read from.

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.
4 High 1 Medium 3 Low
Just want the most common Ideal RD2 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 8 documented codes

Tap any card for details

Application of power

Your boiler is simply indicating that electrical power has just been applied to it, which is a normal startup status rather than an actual fault.

Low DIY-safe Shared across the British / Scottish range

What to check first

  1. Wait a moment to see if the boiler transitions to normal operation automatically.
  2. If the boiler remains stuck on this startup indicator, try turning the main power switch off and on again to reset it.
Full explanation for Application of power
Control 'stat open circuit or waiting for demand

Your boiler is currently paused because your home thermostat is either not sending a signal to turn on or is set below the current room temperature.

Medium DIY-safe £80-150 Shared across the British / Scottish range

What to check first

  1. Check your main room thermostat to ensure the target temperature is set higher than the current room temperature.
  2. Replace the batteries in your wireless wall thermostat if it has a digital display.
  3. Turn the boiler off at the mains power switch, wait 30 seconds, and turn it back on to reset the system.

Parts commonly replaced

  • Room thermostat
  • Wireless receiver

Related codes

Full explanation for Control 'stat open circuit or waiting for demand
Fan start up

Your boiler is failing to start up because the internal fan is not operating correctly.

High Engineer only £150-300 Shared across the British / Scottish range

Parts commonly replaced

  • Fan assembly
  • PCB

Related codes

Full explanation for Fan start up
Lock-out

Your boiler has safely shut itself down to prevent a potential hazard and needs to be manually reset to get working again.

High DIY-safe Shared across the British / Scottish range

What to check first

  1. Locate the reset button or turn the mode dial on the front of your boiler to the reset position.
  2. Hold the reset button down for about 3 to 5 seconds, then release it.
  3. Wait to see if the boiler fires up normally; if the fault immediately returns, do not keep resetting it.

Parts commonly replaced

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

Related codes

Full explanation for Lock-out
Reverse polarity

Your boiler is failing to start because the electrical supply wires have been connected the wrong way around.

High Engineer only £90-150 Shared across the British / Scottish range

Parts commonly replaced

  • External electrical supply wiring
  • Fused spur
Full explanation for Reverse polarity
Run

Your boiler is simply switched off or in standby mode and is not currently operating.

Low DIY-safe Shared across the British / Scottish range

What to check first

  1. Check that the boiler power supply is switched on at the fused spur.
  2. Check your central heating programmer, timer, or thermostat to see if a call for heat is currently scheduled.
Full explanation for Run
Sensor Faults

Your boiler has detected an issue with one of its internal heat sensors and has stopped working to ensure safety.

High Engineer only £120-220 Shared across the British / Scottish range

Parts commonly replaced

  • NTC Sensor
  • Wiring harness
Full explanation for Sensor Faults
Status & diagnostic codes

2 operating states & engineer diagnostics

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

LED 3 Diagnostic Your boiler is currently experiencing a sensor error or electrical fault that is preventing it from operating correctly. LED 4 Diagnostic Your boiler is signalling an electrical wiring issue or a faulty thermostat connection, which is preventing it from communicating correctly to start your heating or hot water.