Ferroli F24b Fault codes & diagnostics

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

Combi Natural Gas 24 kW Discontinued
Download the Ferroli F24b 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 →

Just want the most common Ferroli F24b faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 4 documented codes

Tap any card for details

LED / flash-pattern codes
5 (Flashing Light)

Your boiler has stopped working because the water pressure inside your heating system has dropped too low to operate safely.

High DIY-safe £60-120

What to check first

  1. Locate the filling loop, which is usually a silver braided hose with one or two valves underneath your boiler.
  2. Open the valve(s) slowly and watch the pressure gauge on the front of the boiler until the needle reaches the green zone (typically between 1.0 and 1.5 bar).
  3. Close the valve(s) firmly and ensure the boiler resets itself or switch it off and back on at the power source.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Pressure sensor
Full explanation for 5 (Flashing Light)
LED 5 (Flashing light)

Your boiler has stopped working because the water pressure inside your heating system has dropped too low to operate safely.

High DIY-safe £80-120

What to check first

  1. Locate the external filling loop, which is a silver braided flexible hose connected to two valves under the boiler.
  2. Open both taps on the filling loop slowly to let water into the system until the pressure gauge on the front reads between 1 and 1.5 bar.
  3. Close both taps tightly and check for leaks, then reset the boiler.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Pressure sensor
Full explanation for LED 5 (Flashing light)
Digital codes
Boiler lock out

Your boiler has stopped working for safety reasons and needs a manual reset or a check of your gas and water drainage systems.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Condensate trap
  • Gas valve
  • Printed circuit board
Full explanation for Boiler lock out
Low water pressure

Your boiler has stopped working because the amount of water inside your heating system has dropped below the required level.

Medium DIY-safe £0-£100

What to check first

  1. Locate the filling loop, which is usually a flexible silver hose connected to two valves beneath the boiler.
  2. Open the valves slowly to allow water into the system until the pressure gauge on the front of the boiler reads between 1 bar and 1.5 bar.
  3. Close both valves tightly and ensure there is no dripping from the connection points.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Pressure gauge
Full explanation for Low water pressure
Status & diagnostic codes

5 operating states & engineer diagnostics

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

4 (Flashing Light) Status Your boiler is currently in standby mode and is simply waiting for a signal from your thermostat or hot water tap to start heating. 4 (Permanent Light) Status Your boiler is currently detecting that your heating system is active and running normally. 5 (Permanent Light) Status Your boiler is operating normally and is receiving mains electricity, so no action is required. 43 Diagnostic Your boiler is flagging that a small internal component which manages airflow is not correctly set for the length of the exhaust pipe installed in your home. 43 mm Diagnostic This is a technical configuration setting for your boiler's flue rather than a system fault, indicating the specific size of the air restrictor plate required for your flue pipe setup.