Ferroli Optimax Fault codes & diagnostics

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

CombiSystem Natural Gas 25-38 kW 2008-2015 Discontinued
GC Numbers 41-267-2247-267-3447-267-43

Replaced by: Bluehelix

Download the Ferroli Optimax 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.
37 High 1 Medium 10 Low
Just want the most common Ferroli Optimax faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 48 documented codes

Tap any card for details

LED / flash-pattern codes
Display Flashing

Your boiler has stopped working because it cannot start the flame or the water pressure inside the system is too low.

High DIY-safe £0-220

What to check first

  1. Check the pressure gauge on the front of the boiler; it should be between 1.0 and 1.5 bar
  2. If the pressure is low, top up the system using the external filling loop
  3. Once pressure is restored, press the 'Reset' button to restart the boiler

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for Display Flashing
Digital codes
10

Your boiler has failed to ignite and cannot produce heat or hot water because it isn't receiving gas or cannot create a flame.

High DIY-safe £90-280

What to check first

  1. Check that your gas emergency control valve (at the meter) is in the 'on' position
  2. Ensure the gas isolation valve underneath the boiler is fully open
  3. Press and hold the RESET button for several seconds to clear the lockout

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 10
12

Your boiler has failed to ignite, which means it cannot heat your water or your home because it isn't getting a flame started.

High DIY-safe £120-350

What to check first

  1. Check that your gas emergency control valve and any boiler isolation valves are turned to the 'on' position
  2. Check if other gas appliances in your home, such as a hob, are working to confirm your gas supply is active
  3. Press the Reset button on the control panel to see if the boiler restarts

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame sensing electrode
  • PCB

Related codes

Full explanation for 12
13

Your boiler has failed to ignite, which means your heating and hot water will not work until the gas supply is confirmed and the system is restarted.

High DIY-safe £0-250

What to check first

  1. Check that your gas emergency control valve (at the meter) is in the 'on' position
  2. Ensure the gas isolation valve under the boiler is fully open
  3. Press and hold the RESET button for several seconds to restart the ignition sequence

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Electrode

Related codes

Full explanation for 13
14

Your boiler has tried to start but could not ignite the gas needed to generate heat.

High DIY-safe £100-£250

What to check first

  1. Check that your gas emergency control valve at the meter is in the 'on' position
  2. Ensure the gas isolation valve underneath the boiler is fully open
  3. Press and hold the 'RESET' button for a few seconds to try restarting the sequence

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Electrode

Related codes

Full explanation for 14
15

Your boiler has failed to ignite and cannot produce heating or hot water because it is not receiving or detecting a gas supply.

High DIY-safe £100-300

What to check first

  1. Check that your gas emergency control valve (at the meter) is in the 'on' position
  2. Ensure the gas isolation valve underneath the boiler is fully open
  3. Press and hold the 'RESET' button for one second to clear the fault

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • PCB (Control Board)

Related codes

Full explanation for 15
16

Your boiler is unable to start the flame despite trying, usually because gas isn't reaching the burner or the ignition spark is failing.

High DIY-safe £100-£280

What to check first

  1. Check that your gas meter isolation valve is in the 'on' position
  2. Ensure the gas supply lever underneath the boiler is fully open
  3. Press the Reset button (labeled 2) once to restart the ignition sequence

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Carbon Electrode

Related codes

Full explanation for 16
17

Your boiler has failed to ignite, meaning it is unable to burn gas to provide heating or hot water.

High DIY-safe £90-250

What to check first

  1. Check that your gas emergency control valve (at the meter) is in the 'on' position
  2. Ensure the gas isolation valve under the boiler is open
  3. Press the 'RESET' button on the boiler control panel for one second

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame sensing cable

Related codes

Full explanation for 17
18

Your boiler has failed to ignite, which means it cannot heat your water or your home.

High DIY-safe £100-280

What to check first

  1. Check that your gas emergency control valve (at the meter) is in the 'on' position
  2. Ensure the gas isolation valve under the boiler is fully open
  3. Press and hold the 'RESET' button for a few seconds to restart the ignition sequence

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • PCB

Related codes

Full explanation for 18
19

Your boiler has failed to ignite, which means it cannot heat your water or your home.

High DIY-safe £90-250

What to check first

  1. Check that your gas emergency control valve (at the meter) is in the 'on' position
  2. Ensure the gas isolation valve under the boiler is fully open
  3. Press the Reset button (marked '2' on the display panel) for one second

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Ionisation probe

Related codes

Full explanation for 19
Showing 11–20 of 48
20

Your boiler has failed to ignite, meaning it cannot create the flame needed to provide heating or hot water.

High DIY-safe £90-280

What to check first

  1. Check that the gas supply is turned on at the meter and the boiler isolation valve.
  2. Verify that other gas appliances in your home, such as a hob, are working.
  3. Press and hold the 'RESET' button for a few seconds to restart the ignition sequence.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • PCB (Printed Circuit Board)

Related codes

Full explanation for 20
21

Your boiler is not receiving gas or is unable to ignite a flame to provide heating and hot water.

High DIY-safe £90-280

What to check first

  1. Check that your gas meter has credit (if using a prepay meter).
  2. Ensure the gas isolation valves at the meter and the boiler are fully open.
  3. Press the Reset button on the control panel to restart the ignition sequence.

Parts commonly replaced

  • Gas valve
  • Ignition electrode
  • Flame sensing probe

Related codes

Full explanation for 21
23

Your boiler is not receiving gas or is unable to ignite a flame to provide heating and hot water.

High DIY-safe £90-180

What to check first

  1. Check that your gas meter has credit (if on a prepayment meter)
  2. Ensure your gas isolation valves are fully open at the meter and the boiler
  3. Press and hold the RESET button for one second to attempt a restart

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame sensing cable

Related codes

Full explanation for 23
24

Your boiler has failed to ignite, which means it cannot heat the water or your home.

High DIY-safe £100-350

What to check first

  1. Check that your gas emergency control valve (at the meter) is in the 'on' position
  2. Ensure the gas isolation valve underneath the boiler is open
  3. Press and hold the RESET button for several seconds to clear the fault

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame sensing cable
Full explanation for 24
25

Your boiler has failed to ignite, which means it isn't getting the gas it needs or the spark start is failing.

High DIY-safe £120-350

What to check first

  1. Check that your gas emergency control valve at the meter is in the 'on' position
  2. Ensure any gas isolation valves under the boiler are fully open
  3. Press and hold the 'RESET' button for a few seconds to restart the ignition sequence

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 25
26

Your boiler is not able to light the gas to start the heating process, which is often caused by a fuel supply issue or a faulty ignition component.

High DIY-safe £100-£280

What to check first

  1. Check that your gas emergency control valve (at the meter) is in the 'on' position
  2. Ensure the gas isolation valve under the boiler is fully open
  3. Press the Reset button on the control panel to attempt a restart

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Ignition Lead
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 26
30

Your boiler has temporarily paused operation because it has detected an unusual reading, such as high temperature, and will restart automatically once it cools down.

Low DIY-safe £0-120

What to check first

  1. Check that all radiator valves are open to ensure water can flow properly.
  2. Ensure the boiler pressure is between 1 and 1.5 bar on the pressure gauge.
  3. Wait for the boiler to cool down and see if it restarts automatically.

Parts commonly replaced

  • External Pump
  • Temperature Sensor (NTC)
  • Flow Switch

Related codes

Full explanation for 30
31

Your boiler has paused briefly because a sensor detected an unusual reading, and it will try to restart itself automatically once the reading returns to normal.

Low DIY-safe £0-120

What to check first

  1. Check if your heating system pressure is between 1 and 1.5 bar
  2. Ensure your external wireless thermostat has fresh batteries
  3. Wait for 15 minutes to see if the boiler clears the code and restarts on its own

Parts commonly replaced

  • External controller
  • Communication interface
  • Temperature sensor

Related codes

Full explanation for 31
32

Your boiler has temporarily paused because it detected an unusual temperature or pressure reading and it should restart itself once the levels return to normal.

Low DIY-safe £80-180

What to check first

  1. Check if the pressure gauge is between 1 and 1.5 bar
  2. Ensure all radiator valves are open to allow water flow
  3. Wait up to 30 minutes for the boiler to self-correct and restart

Parts commonly replaced

  • Temperature sensor (NTC thermistor)
  • Wiring harness

Related codes

Full explanation for 32
34

Your boiler has temporarily paused because the electrical voltage supplied to it is currently too low for it to operate safely.

Medium DIY-safe £0-120

What to check first

  1. Check if there is a local power cut or grid fluctuation in your area
  2. Switch off and unplug heavy-duty electrical appliances that may be overloading the circuit
  3. Monitor the boiler to see if it restarts automatically once the supply voltage stabilizes

Parts commonly replaced

  • Main PCB
  • External voltage stabilizer

Related codes

Full explanation for 34
Showing 21–30 of 48
35

Your boiler has detected an unusual signal or flame when it shouldn't have and is pausing for a short time to keep itself safe.

Low DIY-safe £90-180

What to check first

  1. Check if any nearby gas appliances were being used heavily at the same time
  2. Wait up to 10 minutes to see if the boiler resets itself automatically
  3. Locate the reset button and press it once if the code does not clear on its own

Parts commonly replaced

  • Flame sensing electrode
  • Ionisation cable
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 35
37

Your boiler has detected that the water pressure is too low or fluctuating, causing it to pause for safety until it returns to a normal level.

Low DIY-safe £0-120

What to check first

  1. Check the pressure gauge on the front of the boiler; it should be between 1 and 1.5 bar.
  2. If the pressure is below 1 bar, locate the filling loop (usually a silver flexible hose) under the boiler.
  3. Slowly open the valves on the filling loop to add water until the gauge reaches 1.2 bar, then close the valves tightly.

Parts commonly replaced

  • Water pressure switch
  • Filling loop valve
  • Expansion vessel

Related codes

Full explanation for 37
40

Your boiler has paused temporarily because it has detected an unusual reading, but it should restart itself automatically once the system settles down.

Low DIY-safe £0-120

What to check first

  1. Wait up to 15 minutes to see if the boiler clears the code and restarts itself
  2. Check that your radiators are bled and there is no trapped air in the system
  3. Ensure the central heating system pressure is between 1.0 and 1.5 bar

Parts commonly replaced

  • Temperature sensor (NTC thermistor)
  • Wiring harness connections

Related codes

Full explanation for 40
42

Your boiler has temporarily paused because it detected a measurement that was slightly outside its normal limits, and it will try to restart itself automatically once everything settles down.

Low DIY-safe £80-200

What to check first

  1. Wait for 10-15 minutes to see if the boiler resets and restarts itself
  2. Check that your radiators are bled and there is no air in the system
  3. Ensure your boiler pressure is topped up to between 1 and 1.5 bar

Parts commonly replaced

  • Temperature sensor (NTC thermistor)
  • Wiring harness
Full explanation for 42
43

Your boiler has paused itself because the water pressure in the system is falling outside of the safe operating range, but it should restart automatically once the pressure is corrected.

Low DIY-safe £0-120

What to check first

  1. Check the pressure gauge on the front of the boiler; it should ideally be between 1 and 1.5 bar.
  2. If the pressure is too low, use the filling loop to top up the system until the needle reaches the 1.5 bar mark.
  3. If the pressure is too high, bleed a small amount of water from a radiator to drop the pressure.
  4. Wait for the boiler to detect the correct pressure and clear the code automatically.

Parts commonly replaced

  • Pressure sensor
  • Expansion vessel
  • Filling loop valve

Related codes

Full explanation for 43
45

Your boiler has temporarily paused operation because it has detected an unusual reading, and it will restart itself once conditions return to normal.

Low DIY-safe £0-120

What to check first

  1. Check if your radiators are bled and the system pressure is between 1 and 1.5 bar
  2. Wait for 10-15 minutes to see if the boiler clears the code and restarts automatically
  3. Ensure any external vents or flues are not obstructed by debris

Parts commonly replaced

  • Temperature sensor (NTC thermistor)
  • Wiring harness

Related codes

Full explanation for 45
46

Your boiler has paused briefly because a sensor detected an unusual reading, but it should restart itself automatically once the temperature or pressure stabilizes.

Low DIY-safe £0-120

What to check first

  1. Check if the system pressure is between 1 and 1.5 bar on the pressure gauge
  2. Ensure all radiator valves are open to allow proper water flow
  3. Wait up to 15 minutes to see if the boiler clears the code and restarts on its own

Parts commonly replaced

  • NTC Temperature Sensor
  • Circulation Pump

Related codes

Full explanation for 46
F1

Your boiler is not lighting, which means it cannot provide heat or hot water because the burner is failing to ignite.

High DIY-safe £120-350

What to check first

  1. Check that your gas stopcock is open and other gas appliances are working.
  2. Inspect the condensate pipe (the plastic pipe outside) for blockages or freezing.
  3. Press the 'Reset' button on the control panel to see if the boiler restarts.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • PCB

Related codes

Full explanation for F1
F2

Your boiler thinks a flame is present even though it hasn't been told to fire up, which is causing it to shut down for safety.

High Engineer only £120-350

Parts commonly replaced

  • Ionisation electrode
  • Ignition lead
  • Main printed circuit board (PCB)

Related codes

Full explanation for F2
F3

Your boiler has overheated because the water is not moving through the system correctly, causing a safety sensor to shut it down.

High DIY-safe £120-350

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure the boiler pressure is between 1.0 and 1.5 bar
  3. Switch the boiler off and on again to attempt a reset

Parts commonly replaced

  • Circulation pump
  • Overheat thermostat
  • Flow sensor

Related codes

Full explanation for F3
Showing 31–40 of 48
F5

Your boiler has detected an issue with the fan that is responsible for clearing exhaust gases, preventing the unit from starting safely.

High Engineer only £250-£450

Parts commonly replaced

  • Fan Assembly
  • PCB (Printed Circuit Board)
  • Wiring Harness

Related codes

Full explanation for F5
F8

Your boiler is failing to stay alight after it tries to start up, meaning it cannot provide heat or hot water.

High Engineer only £180-450

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Ignition Electrode
  • Flame Sensing Electrode

Related codes

Full explanation for F8
F9

Your boiler has lost its connection with the internal gas control system, preventing it from lighting safely.

High Engineer only £180-350

Parts commonly replaced

  • Gas Valve
  • Wiring Harness
  • PCB (Printed Circuit Board)

Related codes

Full explanation for F9
F10

Your boiler's computer has encountered an internal error and cannot process its normal operations, causing it to shut down for safety.

High Engineer only £200-400

Parts commonly replaced

  • PCB (Printed Circuit Board)

Related codes

Full explanation for F10
F22

Your boiler's internal computer is struggling to communicate with its sensors, which has caused the system to shut down for safety.

High Engineer only £150-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for F22
F25

Your boiler's internal computer brain has encountered a glitch or software error that is preventing it from running properly.

High DIY-safe £180-450

What to check first

  1. Locate the 'Reset' button on the front control panel.
  2. Press and hold the button for 2 to 3 seconds.
  3. Wait for the boiler to complete its restart sequence.

Parts commonly replaced

  • Primary Circuit Board (PCB)
  • Control Panel Interface

Related codes

Full explanation for F25
F26

Your boiler has tried to light itself five times but hasn't detected a flame, so it has shut down for safety.

High DIY-safe £100-350

What to check first

  1. Check if other gas appliances, like a hob, are working to ensure gas is reaching your home.
  2. If it is freezing outside, check if your white plastic condensate pipe is frozen and thaw it with warm water.
  3. Press the Reset button on the control panel once to see if the boiler restarts.

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame sensing electrode
  • Ignition cable

Related codes

Full explanation for F26
F30

Your boiler's internal thermometer that measures the temperature of the water being sent to your taps and radiators has developed a fault, preventing the system from heating up correctly.

High Engineer only £90-160

Parts commonly replaced

  • Central Heating (Flow) NTC Sensor
  • Sensor wiring harness
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F30
F31

Your boiler has lost signal from the temperature sensor that monitors the hot water leaving the unit, meaning it cannot safely regulate how much it heats your home.

High Engineer only £90-160

Parts commonly replaced

  • Flow temperature sensor (NTC thermistor)
  • Sensor wiring harness

Related codes

Full explanation for F31
F34

Your boiler has detected an issue with the electrical power supply coming into your home, meaning the voltage is either too low or too high for the unit to operate safely.

High Engineer only £100-250

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Surge Protector

Related codes

Full explanation for F34
Showing 41–48 of 48
F35

Your boiler has detected an issue with the quality of your home's electrical supply, meaning the power entering the unit is inconsistent.

High Engineer only £100-£350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • External Power Supply Filter

Related codes

Full explanation for F35
F36

Your boiler’s internal computer has detected a fault within its own circuitry, meaning it can no longer communicate with the boiler's components correctly.

High Engineer only £220-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • CVBC Control Module

Related codes

Full explanation for F36
F37

Your boiler has switched itself off because the water pressure inside the system has dropped too low to operate safely.

High DIY-safe £0-160

What to check first

  1. Locate the filling loop (usually a silver flexible hose) under or near the boiler.
  2. Slowly open the valve(s) to allow fresh water into the system until the pressure gauge reads between 1.0 and 1.5 bar.
  3. Close the valve(s) tightly and restart the boiler.

Parts commonly replaced

  • Water pressure sensor
  • Expansion vessel
  • Filling loop valve

Related codes

Full explanation for F37
F43

Your boiler has detected an issue with the sensor that measures the temperature of the water coming back into the unit, which has caused it to stop working as a safety precaution.

High Engineer only £90-160

Parts commonly replaced

  • Return temperature sensor (NTC thermistor)
  • Sensor wiring harness

Related codes

Full explanation for F43
F44

Your boiler's internal thermometer that monitors the water returning from your radiators has developed a fault, preventing the system from heating up correctly.

High Engineer only £100-180

Parts commonly replaced

  • Return NTC thermistor
  • Wiring harness
  • Control PCB

Related codes

Full explanation for F44
F45

Your boiler has detected an issue with the sensor that monitors the temperature of the exhaust gases, causing the system to shut down for safety.

High Engineer only £120-220

Parts commonly replaced

  • Flue gas NTC sensor
  • Wiring harness
  • Main PCB

Related codes

Full explanation for F45
F46

Your boiler has detected a problem with the sensor that monitors the exhaust gases, and it has shut down as a safety precaution.

High Engineer only £120-190

Parts commonly replaced

  • Flue gas NTC sensor
  • Connecting wiring harness

Related codes

Full explanation for F46
No burner ignition

Your boiler is not lighting, which means you have no heating or hot water because the system cannot produce a flame.

High DIY-safe £100-350

What to check first

  1. Check that your gas emergency control valve and meter valve are in the 'open' position
  2. Press and hold the RESET button for one second to clear the fault
  3. Ensure you have credit on your gas meter if you use a pre-payment system

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB (Control board)

Related codes

Full explanation for No burner ignition