Ferroli Optimax 25 C Fault codes & diagnostics

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

Combi Natural Gas 25 kW Discontinued
GC Number 47-267-34
Download the Ferroli Optimax 25 C 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.
2 Emergency 35 High 2 Medium 10 Low
Just want the most common Ferroli Optimax 25 C faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 49 documented codes

Tap any card for details

LED / flash-pattern codes
Display Flashing

Your boiler is struggling to start because there is not enough water pressure in the system to operate safely.

High DIY-safe £80-150

What to check first

  1. Locate the filling loop (usually two silver flexible hoses with valves under the boiler).
  2. Open the valves slowly to allow water into the system until the pressure gauge reads between 1 and 1.5 bar.
  3. Close both valves tightly and restart the boiler.

Parts commonly replaced

  • Pressure relief valve
  • Pressure gauge
  • Expansion vessel

Related codes

Full explanation for Display Flashing
Digital codes
10

Your boiler is not starting because it is failing to ignite, meaning it cannot produce heat or hot water.

High DIY-safe £120-250

What to check first

  1. Check that your gas meter is turned on and other gas appliances like a hob are working
  2. Ensure the gas isolation valve located near the boiler is in the open position
  3. Press and release the reset button on the boiler control panel once

Parts commonly replaced

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

Related codes

Full explanation for 10
12

Your boiler is not firing up because it cannot detect a flame, which means you currently have no heating or hot water.

High DIY-safe £120-250

What to check first

  1. Check that your gas supply is turned on at the meter and the boiler isolation valve
  2. Ensure other gas appliances like your cooker are working to rule out a gas supply cut-off
  3. Press and hold the Reset button on the front of the boiler for a few seconds

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

Full explanation for 12
13

Your boiler is not starting because it is failing to ignite the flame needed to heat your water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Ignition transformer

Related codes

Full explanation for 13
14

Your boiler is currently unable to ignite the flame required to produce heat or hot water.

High Engineer only £120-£250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

Full explanation for 14
15

Your boiler is trying to start but cannot successfully ignite the flame to begin heating your home.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

Full explanation for 15
16

Your boiler is failing to ignite because it is not receiving enough gas or is struggling to start the flame.

High Engineer only £120-250

Parts commonly replaced

  • Fan
  • Gas valve
  • Ignition electrode

Related codes

Full explanation for 16
17

Your boiler is failing to ignite the flame needed to warm your water or heating system.

High DIY-safe £120-£250

What to check first

  1. Check that your gas supply is definitely on by verifying other gas appliances like a hob are working
  2. Ensure the gas isolation valve near the boiler is turned to the fully open position
  3. Press and release the reset button on the boiler control panel once

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

Full explanation for 17
18

Your boiler is trying to start but the flame is failing to ignite, meaning you currently have no heating or hot water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed circuit board

Related codes

Full explanation for 18
19

Your boiler is currently unable to ignite, meaning it cannot produce heating or hot water for your home.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

Full explanation for 19
Showing 11–20 of 49
20

Your boiler is not firing up because it cannot detect a flame, which means it currently cannot provide heating or hot water.

High DIY-safe £120-250

What to check first

  1. Check that your gas supply is turned on at the meter
  2. Ensure the gas isolation valve near the boiler is set to the open position
  3. Press the reset button once to try and restart the boiler

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

Full explanation for 20
21

Your boiler is currently unable to ignite, meaning it cannot produce heat or hot water because it is not receiving or burning gas safely.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

Full explanation for 21
23

Your boiler has stopped working because it cannot light the flame required to produce heat or hot water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

Full explanation for 23
24

Your boiler is failing to ignite the flame needed to produce heat and hot water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

Full explanation for 24
25

Your boiler is currently unable to start because it is not receiving or burning gas successfully.

High DIY-safe £120-250

What to check first

  1. Check that your gas supply is definitely on by testing another appliance like a gas hob
  2. Locate the gas isolation valve near the boiler and ensure the handle is in the 'on' position
  3. Press the reset button on your boiler's control panel and wait to see if it ignites

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board

Related codes

Full explanation for 25
26

Your boiler is failing to ignite the flame needed to heat your water and home, which is often caused by a blocked gas supply or an internal ignition issue.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

Full explanation for 26
30

Your boiler has detected a temporary inconsistency in its operating temperature and has paused itself to protect the system before it automatically restarts.

Low Engineer only £100-200

Parts commonly replaced

  • NTC Temperature Sensor
  • System Filter

Related codes

Full explanation for 30
31

Your boiler is temporarily pausing its operation to protect itself because it has detected an unusual temperature reading, but it should restart on its own once it cools down.

Low Engineer only £120-250

Parts commonly replaced

  • NTC temperature sensor
  • Central heating pump
  • System pressure sensor

Related codes

Full explanation for 31
32

Your boiler has detected a temporary issue and is pausing its operation while it waits for conditions to return to normal, after which it should restart itself automatically.

Low Engineer only £100-200

Parts commonly replaced

  • Central heating flow sensor
  • PCB

Related codes

Full explanation for 32
34

Your boiler has detected a temporary issue and has paused itself to stay safe, but it should restart automatically once everything returns to normal.

Low Engineer only £90-180

Parts commonly replaced

  • System pressure sensor
  • Main printed circuit board
  • NTC thermistor

Related codes

Full explanation for 34
Showing 21–30 of 49
35

Your boiler has detected a temporary issue and stopped working as a safety precaution, but it should restart itself once the system returns to its normal operating temperature or pressure.

Low Engineer only £90-180

Parts commonly replaced

  • NTC sensor
  • Flow sensor
  • Main PCB

Related codes

Full explanation for 35
37

Your boiler has detected that the system pressure is briefly outside of the normal range and has paused itself while it waits for levels to recover.

Medium Engineer only £90-180

Parts commonly replaced

  • Pressure sensor
  • Expansion vessel
  • Pressure relief valve

Related codes

Full explanation for 37
40

Your boiler has detected a temporary inconsistency in its readings and has paused itself to stay safe, but it should restart on its own as soon as conditions return to normal.

Low Engineer only £100-200

Parts commonly replaced

  • NTC Temperature Sensor
  • System Pump
  • Pressure Sensor

Related codes

Full explanation for 40
42

Your boiler has temporarily paused itself because a sensor detected a reading outside of its normal operating limits, but it is currently trying to recover on its own.

Low Engineer only £100-250

Parts commonly replaced

  • Flow temperature sensor
  • Return temperature sensor
  • Wiring harness

Related codes

Full explanation for 42
43

Your boiler has temporarily paused itself because it detected that a system temperature is slightly outside of the normal range, but it should restart automatically without your help.

Low Engineer only

Parts commonly replaced

  • NTC Sensor
  • System Pump
  • Primary Heat Exchanger

Related codes

Full explanation for 43
45

Your boiler has detected a temporary fluctuation in its operating conditions and has paused itself to stay safe; it should restart on its own once it cools down or stabilizes.

Low Engineer only

Parts commonly replaced

  • NTC sensor
  • System pump
  • Main circuit board

Related codes

Full explanation for 45
46

Your boiler has detected a temporary temperature discrepancy and is pausing its operation to protect itself before automatically restarting.

Low Engineer only £80-150

Parts commonly replaced

  • NTC sensor
  • System bypass valve

Related codes

Full explanation for 46
F1

Your boiler is trying to start but cannot light the internal flame, so it has shut down for safety to prevent gas buildup.

High Engineer only £120-250

Parts commonly replaced

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

Related codes

Full explanation for F1
F2

Your boiler has detected internal activity that suggests a flame is present when it should be switched off, which is a safety lockout to prevent potential hazards.

Emergency Engineer only £150-300

Parts commonly replaced

  • Ionisation electrode
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F2
F02

Your boiler has detected a flame when it is not supposed to be lit, which acts as a safety lockout to prevent potential danger.

Emergency Engineer only £150-300

Parts commonly replaced

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

Related codes

Full explanation for F02
Showing 31–40 of 49
F3

Your boiler has detected that the water inside it has become dangerously hot and has shut itself down to prevent damage or safety risks.

High Engineer only £150-300

Parts commonly replaced

  • NTC sensor
  • Circulation pump
  • Heat exchanger

Related codes

Full explanation for F3
F5

Your boiler is struggling to clear exhaust gases safely, so it has turned itself off to prevent any potential danger.

High Engineer only £150-300

Parts commonly replaced

  • Fan assembly
  • Wiring harness
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F5
F8

Your boiler has attempted to light the internal flame several times but has failed to stay on, so it has turned itself off for safety.

High Engineer only £120-£250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

Full explanation for F8
F9

Your boiler has lost the electronic connection needed to control the gas supply, meaning it cannot ignite or provide heating and hot water.

High Engineer only £150-300

Parts commonly replaced

  • Wiring harness
  • Gas valve
  • Control board (CVBC)

Related codes

Full explanation for F9
F10

Your boiler's computer is experiencing a software glitch or communication error and needs to be reset to try and clear the fault.

High DIY-safe £150-300

What to check first

  1. Locate the Reset button on your boiler control panel.
  2. Press and hold the Reset button for 3 to 5 seconds.
  3. Wait a few minutes to see if the boiler restarts and the code clears.

Parts commonly replaced

  • Main Control PCB
  • Wiring harness

Related codes

Full explanation for F10
F22

Your boiler has detected an issue with its internal computer system, which is preventing it from operating correctly.

High Engineer only £150-350

Parts commonly replaced

  • Main printed circuit board (PCB)
  • Wiring harness

Related codes

Full explanation for F22
F25

Your boiler's computer system has detected an internal error, which has caused it to stop working for safety reasons.

High Engineer only £150-300

Parts commonly replaced

  • Main PCB

Related codes

Full explanation for F25
F26

Your boiler is failing to light the internal flame, which means it cannot produce any heat or hot water for your home.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board

Related codes

Full explanation for F26
F30

Your boiler is struggling to detect the flow of water necessary to provide you with heating and hot water.

High Engineer only £120-220

Parts commonly replaced

  • Flow sensor
  • Wiring loom

Related codes

Full explanation for F30
F31

Your boiler is struggling to detect the flow of water through the system, which means it cannot safely start up to provide you with heating or hot water.

High Engineer only £150-250

Parts commonly replaced

  • Flow sensor/turbine
  • Wiring harness

Related codes

Full explanation for F31
Showing 41–49 of 49
F34

Your boiler is detecting an unstable power supply coming from your home's electrical system, which is preventing it from operating safely.

High Engineer only £120-250

Parts commonly replaced

  • PCB (Printed Circuit Board)
  • Wiring harness

Related codes

Full explanation for F34
F35

Your boiler is struggling because the electricity supply feeding it is unstable or fluctuating outside of the normal range.

Medium Engineer only £120-220

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for F35
F36

Your boiler’s internal computer is experiencing a communication error and can no longer safely control the heating or hot water system.

High Engineer only £250-450

Parts commonly replaced

  • Main printed circuit board (PCB)
  • Wiring harness

Related codes

Full explanation for F36
F37

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

High DIY-safe £80-150

What to check first

  1. Locate the external filling loop (a flexible silver hose connecting two pipes near the boiler).
  2. Open the valves on both ends of the hose to allow water into the system until the gauge reads between 1.0 and 1.5 bar.
  3. Close both valves tightly and ensure no water is leaking from the hose connections.

Parts commonly replaced

  • Pressure relief valve
  • Filling loop
  • Pressure sensor

Related codes

Full explanation for F37
F43

Your boiler is currently unable to read the water temperature accurately, which has caused it to stop working for your own safety.

High Engineer only £120-200

Parts commonly replaced

  • Return temperature sensor
  • Wiring harness

Related codes

Full explanation for F43
F44

Your boiler has lost the ability to accurately measure the temperature of the water circulating back into it, so it has shut down as a precaution to prevent overheating.

High Engineer only £120-220

Parts commonly replaced

  • Return NTC thermistor
  • Wiring harness

Related codes

Full explanation for F44
F45

Your boiler has detected an issue with the sensor that monitors the exhaust gases, so it has safely shut down to prevent any unsafe operation.

High Engineer only £150-250

Parts commonly replaced

  • Flue sensor
  • Wiring loom
  • PCB

Related codes

Full explanation for F45
F46

Your boiler has detected an issue with the sensor that monitors the temperature and safety of the exhaust gases leaving your system, so it has shut down to keep you safe.

High Engineer only £120-220

Parts commonly replaced

  • Flue gas thermostat
  • Wiring harness

Related codes

Full explanation for F46
No burner ignition

Your boiler is failing to light the internal flame needed to produce heat or hot water.

High Engineer only £120-250

Parts commonly replaced

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

Related codes

Full explanation for No burner ignition