Ideal Logic Code Combi ESP1 Fault codes & diagnostics

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

Combi Natural Gas 26-38 kW 2016 onwards
GC Numbers 47-349-3547-349-3647-349-37
Download the Ideal Logic Code Combi ESP1 manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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.
1 Emergency 48 High 5 Medium 3 Low
Versions & GC numbers

Which version do you have?

The GC number on your boiler's data badge identifies the exact appliance and production years, as recorded in the UK Product Characteristics Database.

GC number Appliance (register name) Produced
47-349-35 LOGIC CODE COMBI ESP1 26 2016–present
47-349-36 LOGIC CODE COMBI ESP1 33 2016–present
47-349-37 LOGIC CODE COMBI ESP1 38 2016–present
Just want the most common Ideal Logic Code Combi ESP1 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 57 documented codes

Tap any card for details

4.2

Your boiler has shut down because its internal temperature has become too high, often caused by a lack of water flow or a blockage.

High DIY-safe £100-350

What to check first

  1. Check that your boiler pressure is between 1 and 1.5 bar on the gauge
  2. Ensure all radiator valves and the isolation valves under the boiler are fully open
  3. Press the 'Restart' button on the front control panel

Parts commonly replaced

  • Circulation Pump
  • Overheat Thermostat
  • Main Heat Exchanger

Related codes

Full explanation for 4.2
4.3

Your boiler has failed to light after several attempts, meaning it has shut down as a safety precaution and will not provide heating or hot water.

High DIY-safe £100-£350

What to check first

  1. Check if other gas appliances in your home, such as a gas hob, are working to ensure gas is reaching the property.
  2. Ensure your pre-payment gas meter has credit if applicable.
  3. Press the 'Reset' button on the boiler control panel once to see if it restarts. Only attempt this three times maximum.
  4. If it is freezing outside, check that your white plastic condensate pipe leading outdoors is not frozen and blocked.

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame sensing probe
  • PCB (Printed Circuit Board)

Related codes

Full explanation for 4.3
4.4

Your boiler has detected a flame when it shouldn't be there, which usually means a internal component is stuck open or there is a fault with the electronics.

Emergency Engineer only £180-350

Parts commonly replaced

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

Related codes

Full explanation for 4.4
4.5

Your boiler has detected that the water pressure inside the system is too low to run safely and has switched itself off.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually blue-handled levers) underneath the boiler.
  2. Slowly open the valves until you hear water entering the system.
  3. Watch the pressure gauge and close the valves when the needle reaches 1.5 bar.
  4. Restart the boiler if it does not reset automatically.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure sensor

Related codes

Full explanation for 4.5
4.6

Your boiler is struggling to keep its flame lit, which means the system has safely shut down to prevent issues.

High DIY-safe £120-£350

What to check first

  1. Check that your gas credit hasn't run out if you have a prepayment meter
  2. Ensure other gas appliances like your hob are working to confirm your gas supply is active
  3. Check that the white plastic condensate pipe outside hasn't frozen in cold weather
  4. Press the 'Reset' button on the boiler display

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead
  • Gas valve
  • Spark generator

Related codes

Full explanation for 4.6
4.7

Your boiler has stopped working because the internal fan, which safely clears exhaust fumes, is not running correctly.

High Engineer only £280-450

Parts commonly replaced

  • Fan Assembly
  • Main PCB
  • Wiring Loom

Related codes

Full explanation for 4.7
4.8

Your boiler has detected an issue with the sensor that monitors the temperature of the water leaving the unit, preventing it from heating your home safely.

High Engineer only £90-160

Parts commonly replaced

  • Flow thermistor (NTC sensor)
  • Wiring harness / connector
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 4.8
4.9

Your boiler has detected an issue with the sensor that monitors the temperature of the water returning to the unit, likely causing it to shut down for safety.

High Engineer only £90-160

Parts commonly replaced

  • Return Thermistor (NTC Sensor)
  • Wiring harness
  • Main PCB

Related codes

Full explanation for 4.9
4.10

Your boiler has lost communication with the sensor that monitors the outdoor temperature, which means it cannot automatically adjust its settings based on the weather.

Low Engineer only £90-180

Parts commonly replaced

  • Outside weather compensation sensor
  • Sensor wiring harness

Related codes

Full explanation for 4.10
4.11

Your boiler has detected that the water pressure in the system is too low and needs to be topped up to work properly.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (the flexible silver hose) underneath your boiler.
  2. Slowly open the blue-handled valves until you hear water entering the system.
  3. Watch the pressure gauge and close the valves tightly once the needle reaches 1.5 bar.

Parts commonly replaced

  • Filling loop valves
  • Expansion vessel
  • Pressure sensor

Related codes

Full explanation for 4.11
Showing 11–20 of 57
4.12

Your boiler has stopped working because it cannot detect water moving through the internal pipes to keep the system safe.

High DIY-safe £100-£350

What to check first

  1. Check the pressure gauge on the front of the boiler is between 1 and 1.5 bar
  2. If the pressure is low, use the filling loop underneath the boiler to top it up
  3. Ensure all radiator valves and isolation valves under the boiler are fully open
  4. Press the Restart button to clear the fault

Parts commonly replaced

  • Pump
  • Water Pressure Sensor
  • Automatic Air Vent

Related codes

Full explanation for 4.12
4.13

Your boiler's internal computer parts are failing to talk to each other, meaning the system cannot operate or provide heat and hot water.

High Engineer only £180-450

Parts commonly replaced

  • Primary PCB
  • Wiring Loom
  • User Interface Board

Related codes

Full explanation for 4.13
4.15

Your boiler is heating your radiators normally, but it is failing to provide hot water when you turn on your taps.

Medium Engineer only £120-£250

Parts commonly replaced

  • Diverter Valve Actuator Motor
  • Diverter Valve Cartridge
  • DHW Flow Turbine

Related codes

Full explanation for 4.15
81

Your boiler is currently heating your tap water normally, but it is unable to send heat to your radiators.

Medium Engineer only £150-300

Parts commonly replaced

  • Diverter valve
  • Central heating sensor
  • Wiring harness

Related codes

Full explanation for 81
82

Your boiler is providing central heating as normal, but it is currently failing to heat your hot water taps.

Medium Engineer only £150-300

Parts commonly replaced

  • Diverter valve
  • Diverter valve motor
  • Flow turbine

Related codes

Full explanation for 82
83

Your boiler has lost all power to the control panel, meaning it is currently completely shut down and unable to provide heating or hot water.

High Engineer only £150-350

Parts commonly replaced

  • Printed circuit board (PCB)
  • Internal fuse
  • Wiring harness
Full explanation for 83
85

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

High Engineer only £180-350

Parts commonly replaced

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

Related codes

Full explanation for 85
86

Your boiler has stopped working because it is unable to accurately measure the temperature of the water inside it, likely due to a faulty sensor or loose wiring.

High Engineer only £120-£200

Parts commonly replaced

  • Flow Thermistor

Related codes

Full explanation for 86
89

Your boiler's hot water sensor has stopped working, meaning it cannot accurately tell the system the temperature of the water, so it has shut down to prevent damage.

High Engineer only £120-220

Parts commonly replaced

  • DHW NTC Thermistor
  • Wiring harness

Related codes

Full explanation for 89
91

Your boiler is currently unable to provide heating to your radiators, although it is still successfully heating your tap water.

Medium Engineer only £150-300

Parts commonly replaced

  • Diverter valve
  • Central heating sensor
  • PCB

Related codes

Full explanation for 91
Showing 21–30 of 57
93

Your boiler has lost all electrical power, meaning it cannot operate your heating or hot water.

High Engineer only £150-300

Parts commonly replaced

  • Main PCB
  • Wiring harness
  • Power supply fuse

Related codes

Full explanation for 93
C2

Your boiler's computer is missing its essential piece of software memory, which prevents it from knowing how to operate your heating and hot water.

High Engineer only £150-250

Parts commonly replaced

  • Boiler Chip Card (BCC)
  • Main Control PCB
Full explanation for C2
F1

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

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver flexible hose) underneath your boiler.
  2. Slowly open the one or two taps/handles on the filling loop until you hear water flowing.
  3. Watch the pressure gauge and close the taps when the needle reaches between 1 and 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Expansion vessel re-pressurisation
  • Pressure sensor

Related codes

Full explanation for F1
F2

Your boiler has started to light but the flame has gone out, which means the system has stopped providing heating and hot water 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 the property.
  2. Check that your gas pre-payment meter (if you have one) has credit.
  3. Restart the boiler by pressing the 'RESET' button firmly for two seconds.

Parts commonly replaced

  • Flame Sensing Electrode
  • Gas Valve
  • Ignition Lead
  • Spark Generator

Related codes

Full explanation for F2
F2 Or Fn Or Ln

Your boiler is struggling to keep its internal flame lit while trying to heat your water or home.

High Engineer only £150-350

Parts commonly replaced

  • Ignition Electrode
  • Flame Rectification Probe
  • Gas Valve

Related codes

Full explanation for F2 Or Fn Or Ln
F3

Your boiler has detected an issue with the fan that pushes exhaust gases out of your home, causing the system to shut down for safety.

High Engineer only £280-£450

Parts commonly replaced

  • Fan assembly
  • Main PCB
  • Wiring harness

Related codes

Full explanation for F3
F4

Your boiler has detected an issue with the sensor that measures the temperature of the water leaving the unit, preventing it from heating your home safely.

High Engineer only £90-180

Parts commonly replaced

  • Flow thermistor (NTC sensor)
  • Wiring harness
  • Main PCB

Related codes

Full explanation for F4
F4 Or L4

Your boiler has stopped working because it cannot correctly read the temperature of the water flowing through the system.

High Engineer only £120-£200

Parts commonly replaced

  • Flow thermistor (NTC sensor)
  • Wiring harness

Related codes

Full explanation for F4 Or L4
F5

Your boiler has detected an issue with the sensor that monitors the temperature of the water returning from your radiators.

High Engineer only £100-£180

Parts commonly replaced

  • Return Thermistor (NTC)
  • Wiring Harness

Related codes

Full explanation for F5
F5 Or L5

Your boiler is struggling to accurately measure the temperature of the water coming back into the system, so it has safely shut down to prevent overheating.

High Engineer only £120-200

Parts commonly replaced

  • Return thermistor sensor
  • Wiring harness

Related codes

Full explanation for F5 Or L5
Showing 31–40 of 57
F6

Your boiler is having trouble communicating with the temperature sensor located on the outside of your house, which helps it decide how hard to work based on the weather.

Low DIY-safe £60-140

What to check first

  1. Locate the reset button on the front of the boiler display.
  2. Press and hold the reset button for 2 seconds.
  3. If the code persists, check that the external sensor on your outside wall hasn't been blocked or damaged.

Parts commonly replaced

  • Outside weather compensation sensor
  • Wiring harness

Related codes

Full explanation for F6
F7

Your boiler has stopped working because the electrical power coming into your home is at a lower voltage than it needs to operate safely.

High DIY-safe £0-120

What to check first

  1. Check if other appliances in your home are acting strangely or lights are flickering
  2. Contact your electricity network provider to report a potential power supply issue
  3. Once the external power issue is resolved, reset the boiler using the 'RE' dial or button

Parts commonly replaced

  • External mains supply (Power Grid)
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F7
F8

Your boiler's main internal computer has lost its settings or has developed an internal electrical fault, preventing it from starting.

High Engineer only £280-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Data Chip

Related codes

Full explanation for F8
F9 or L9 or F8 or L8

Your boiler has detected an internal communication or electrical control error, meaning it has safely shut itself down and stopped producing heating or hot water.

High Engineer only £250-450

Parts commonly replaced

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

Related codes

Full explanation for F9 or L9 or F8 or L8
FA

Your boiler has detected that the temperature sensors for the water going in and out are reporting incorrect or swapped readings, which has caused the system to shut down for safety.

High Engineer only £100-220

Parts commonly replaced

  • Flow Thermistor
  • Return Thermistor
  • Wiring Harness

Related codes

Full explanation for FA
Fan Fault

Your boiler has detected a problem with the fan that clears exhaust gases, preventing the system from starting up for safety reasons.

High Engineer only £220-450

Parts commonly replaced

  • Fan assembly
  • Main PCB
  • Wiring harness

Related codes

Full explanation for Fan Fault
Flame Loss

Your boiler has failed to keep its internal flame lit, so it has shut itself down as a safety precaution and is currently unable to provide heating or hot water.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Condensate trap assembly
  • Flame rectification probe

Related codes

Full explanation for Flame Loss
Flow Thermistor Fault

Your boiler has detected that a temperature sensor is not communicating correctly, which has caused the system to stop working to prevent overheating.

High Engineer only £90-180

Parts commonly replaced

  • Flow Thermistor (NTC Sensor)
  • Wiring Harness
  • Main Printed Circuit Board (PCB)

Related codes

Full explanation for Flow Thermistor Fault
Fn

Your boiler has lost its flame while trying to run, which means it cannot heat your water or your radiators.

High DIY-safe £120-350

What to check first

  1. Check that your gas supply is turned on at the meter
  2. If you have a prepay meter, check that you have enough credit
  3. Press the Reset button on the boiler display
  4. Ensure your external condensate pipe hasn't frozen in very cold weather

Parts commonly replaced

  • Gas Valve
  • Ignition Electrode
  • Flame Sensing Probe
  • Spark Lead

Related codes

Full explanation for Fn
FU

Your boiler has detected that heat is building up too quickly and cannot circulate around your home, likely due to a blockage or a pump failure.

High DIY-safe £150-£350

What to check first

  1. Check that the main isolation valves underneath the boiler are fully open
  2. Ensure at least two or three radiators have their valves turned to the maximum setting
  3. Check that the boiler pressure is between 1.0 and 1.5 bar on the gauge

Parts commonly replaced

  • Central heating pump
  • Main heat exchanger
  • Return thermistor

Related codes

Full explanation for FU
Showing 41–50 of 57
Ignition Lockout

Your boiler has failed to start the flame after several attempts and has shut itself down as a safety precaution, meaning you currently have no heating or hot water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Spark generator

Related codes

Full explanation for Ignition Lockout
L1

Your boiler has switched itself off because the internal water temperature has become too hot, usually caused by a lack of water flow through the system.

High DIY-safe £120-£350

What to check first

  1. Check that your radiator valves are fully open
  2. Ensure your boiler pressure is between 1 and 1.5 bar on the gauge
  3. Reset the boiler by turning the control knob to 'RE' or pressing the reset button

Parts commonly replaced

  • Circulating pump
  • Thermistor (NTC sensor)
  • Main heat exchanger

Related codes

Full explanation for L1
L2

Your boiler has tried to light several times but cannot establish a flame, meaning it has shut down for safety to prevent gas build-up.

High DIY-safe £100-280

What to check first

  1. Check if your gas supply is active by testing another gas appliance like a hob.
  2. If you have a prepay meter, ensure you have sufficient credit.
  3. In freezing temperatures, check if the external condensate pipe has frozen.
  4. Reset the boiler by turning the mode knob to 'RE' and back to your desired setting.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Ignition Lead
  • Ionisation Probe

Related codes

Full explanation for L2
L4

Your boiler has sensed a problem with the internal thermometer that monitors the water temperature leaving the unit, causing it to shut down for safety.

High Engineer only £100-£180

Parts commonly replaced

  • Flow thermistor (NTC sensor)
  • Wiring harness
  • Main PCB

Related codes

Full explanation for L4
L4 or F4

Your boiler's temperature-sensing component is malfunctioning, which prevents it from safely controlling the heat, so it has turned itself off to prevent overheating.

High Engineer only £150-250

Parts commonly replaced

  • Flow NTC Thermistor
  • Wiring harness

Related codes

Full explanation for L4 or F4
L5

Your boiler has stopped working because a sensor that monitors the temperature of the water coming back into the unit has detected a problem or failed.

High Engineer only £100-£190

Parts commonly replaced

  • Return thermistor (NTC sensor)
  • Wiring harness
  • Main PCB

Related codes

Full explanation for L5
L5 or F5

Your boiler is struggling because it cannot accurately read the temperature of the water returning to it, so it has safely shut down to prevent overheating.

High Engineer only £120-200

Parts commonly replaced

  • Return thermistor
  • Wiring harness

Related codes

Full explanation for L5 or F5
L6

Your boiler thinks it sees a flame even though it hasn't tried to light yet, causing it to shut down as a safety precaution.

High Engineer only £120-£250

Parts commonly replaced

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

Related codes

Full explanation for L6
L8

Your boiler's main internal computer has lost its settings or has developed an internal electrical fault, preventing it from starting.

High Engineer only £280-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Data Chip

Related codes

Full explanation for L8
L9

Your boiler's main internal computer has lost its settings or has developed an internal electrical fault, preventing it from starting.

High Engineer only £280-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Data Chip

Related codes

Full explanation for L9
Showing 51–57 of 57
LC

Your boiler has attempted to restart itself too many times in a short period because it is detecting a recurring problem.

High Engineer only £100-350

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Electrode
  • Gas Control Valve
  • PCB

Related codes

Full explanation for LC
Ln

Your boiler has lost its flame while running, meaning it is not getting enough gas or the spark is failing to keep the burner lit.

High DIY-safe £100-350

What to check first

  1. Check if your gas meter has credit (if on a prepayment meter)
  2. Check if other gas appliances, like a hob, are working to confirm gas supply
  3. Reset the boiler by turning the mode knob to 'RE' and then back to the desired setting
  4. Ensure the external condensate pipe has not frozen if it is winter

Parts commonly replaced

  • Flame Sensing Electrode
  • Gas Valve
  • Ignition Lead
  • Spark Generator

Related codes

Full explanation for Ln
LOW MAINS VOLTAGE

Your boiler is receiving less electrical power from your home's main supply than it needs to operate reliably.

Medium Engineer only £100-250

Parts commonly replaced

  • PCB
  • Wiring harness

Related codes

Full explanation for LOW MAINS VOLTAGE
Low Water Pressure

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

High DIY-safe

What to check first

  1. Locate the two small levers or the silver filling loop underneath your boiler.
  2. Open the valves slowly to allow water into the system while watching the pressure gauge on the front.
  3. Close the valves firmly once the needle sits between 1.0 and 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Pressure sensor

Related codes

Full explanation for Low Water Pressure
Outside Sensor Fault

Your boiler is struggling to understand the outdoor temperature, which might make your heating feel less efficient or slightly inconsistent.

Low Engineer only £90-180

Parts commonly replaced

  • Outdoor weather compensation sensor
  • Sensor wiring harness

Related codes

Full explanation for Outside Sensor Fault
Overheat Lockout

Your boiler has detected that it is running too hot and has automatically shut down to prevent damage to its internal parts.

High Engineer only £120-250

Parts commonly replaced

  • Pump
  • Diverter Valve
  • Heat Exchanger
  • NTC Sensor

Related codes

Full explanation for Overheat Lockout
Return Thermistor Fault

Your boiler is struggling to monitor the temperature of the water returning from your radiators, which prevents it from heating your home or water efficiently.

High Engineer only £110-180

Parts commonly replaced

  • Return Thermistor (NTC Sensor)
  • Wiring Harness
  • Main PCB

Related codes

Full explanation for Return Thermistor Fault
Status & diagnostic codes

14 operating states & engineer diagnostics

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

00 Status Your boiler is perfectly healthy and is simply resting because it is not currently being asked to provide heating or hot water. dU Diagnostic Your boiler is currently set to a specific maintenance mode used by engineers to replace internal parts, rather than being in a fault state. F9 Diagnostic Your boiler’s internal control board has lost its configuration or is brand new and hasn't been programmed yet to tell it which model it is. FL Status Your boiler is accidentally trying to use a feature it doesn't have, which is usually caused by a minor electronic glitch or a recent settings change. FP Status Your boiler has detected that the temperature is very low and has automatically turned itself on to prevent the internal pipes from freezing. oF Status Your boiler is currently updating its internal settings and is showing this code to indicate it has been reset during its configuration process. P1 Diagnostic Your boiler is displaying a service setting notification to run the pump at maximum speed, which usually indicates an installation or maintenance mode rather than a dangerous mechanical failure. P7 Diagnostic Your boiler is displaying a configuration message regarding how fast its internal pump should run to move water through your radiators. SE Diagnostic Your boiler is simply reminding you that its annual service is due soon; it is a notification rather than a breakdown. SH Diagnostic Your boiler is currently in a manual testing mode where it runs at full power, usually activated by a technician during a service. SL Diagnostic Your boiler is currently in a manual service mode used for testing and is not experiencing an actual fault. UF Diagnostic Your boiler is currently stuck in a professional service mode used by engineers when they are replacing an internal valve. uP Diagnostic Your boiler has detected that it was turned on while its internal computer chip was being replaced or updated. VF Diagnostic Your boiler is currently running a self-cleaning cycle to remove air bubbles from the pipes and will return to normal operation shortly.
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