Ferroli Sigma Fault codes & diagnostics

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

Regular Natural Gas Discontinued
GC Numbers 41-267-1741-267-18
Download the Ferroli Sigma manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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Severity at a glance

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1 Emergency 7 High
Just want the most common Ferroli Sigma faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 8 documented codes

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37

Your boiler has stopped working because the amount of water inside your heating system has dropped too low.

High DIY-safe £80-150

What to check first

  1. Locate the filling loop, which is a silver braided hose usually found underneath the boiler.
  2. Open the valves at both ends of the hose to let mains water into the system.
  3. Watch the pressure gauge on the front of the boiler until the needle reaches the green zone (usually around 1.5 bar).
  4. Close both valves tightly and remove the filling loop if it is not a permanent fixture.

Parts commonly replaced

  • Pressure relief valve
  • Pressure sensor
  • Expansion vessel
Full explanation for 37
121

Your boiler has detected a temperature issue with the water flow, causing it to shut down for safety to prevent overheating.

High Engineer only £120-250

Parts commonly replaced

  • Flow temperature sensor
  • NTC sensor
  • Circulation pump

Related codes

Full explanation for 121
Boiler lockout

Your boiler has failed to light several times and has locked itself out for safety.

High DIY-safe £120-280

What to check first

  1. Check that your gas supply is active and other gas appliances are working
  2. Press the 'Reset' button on the boiler control panel
  3. Check for any obvious obstructions in the external flue pipe outside your home

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board (PCB)
Full explanation for Boiler lockout
Boiler sooting

Your boiler is not burning gas properly, which is causing a dangerous buildup of black soot and potentially harmful fumes.

Emergency Engineer only £120-300

Parts commonly replaced

  • Burner
  • Gas Valve
  • Air Intake Pipework
Full explanation for Boiler sooting
Burner fails to ignite

Your boiler is unable to light the flame needed to heat your water and radiators.

High Engineer only £120-350

Parts commonly replaced

  • Gas valve
  • Ignition electrode
  • Fan
  • Air pressure switch
Full explanation for Burner fails to ignite
Burner goes out

Your boiler is struggling to stay lit because it is either dirty or isn't getting a steady flow of gas.

High Engineer only £100-280

Parts commonly replaced

  • Flow temperature sensor
  • Gas valve
  • Burner injectors
Full explanation for Burner goes out
Condensate in boiler

Your boiler is producing excess moisture internally because it is running too cool or burning gas at the wrong rate.

High Engineer only £120-280

Parts commonly replaced

  • Gas valve
  • Main burner
  • Condensate trap
Full explanation for Condensate in boiler
Radiators cold in winter

Your central heating system is failing to warm up your radiators because water isn't being circulated correctly or the system isn't being told to turn on.

High DIY-safe £120-350

What to check first

  1. Check that your room thermostat is turned up high enough to call for heat
  2. Ensure your programmer or timer is set to the 'On' position for heating
  3. Replace the batteries in your wireless room thermostat if the screen is blank

Parts commonly replaced

  • Circulation Pump
  • Flow Temperature Sensor (NTC)
  • Room Thermostat
Full explanation for Radiators cold in winter