Ideal Logic Combi ESP1 Fault codes & diagnostics

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

Combi Natural Gas 24-35 kW 2015-2022 Discontinued

Replaced by: Logic2 Combi

Download the Ideal Logic 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.
26 High 1 Low
Just want the most common Ideal Logic Combi ESP1 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 27 documented codes

Tap any card for details

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 the boiler.
  2. Slowly open the blue or black taps on the filling loop until you hear water entering.
  3. Watch the pressure gauge and close the taps firmly when the needle reaches between 1 and 1.5 bar.

Parts commonly replaced

  • Filling loop valve
  • Expansion vessel
  • Pressure sensor

Related codes

Full explanation for F1
F2

Your boiler has switched itself off because it started to light but the flame went out or could not be detected.

High DIY-safe £100-£350

What to check first

  1. Check if your other gas appliances (like a hob) are working to ensure gas is reaching the property
  2. If you have a prepay meter, check that you have credit and the valve is open
  3. In freezing weather, check if the white plastic condensate pipe outside has frozen and gently thaw it with warm water
  4. Reset the boiler by turning the control knob to 'Reset' and then back to your desired setting

Parts commonly replaced

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

Related codes

Full explanation for F2
F2 or Fn or Ln

Your boiler is failing to stay lit, which means it has safely shut itself down because it cannot consistently produce or hold a flame.

High Engineer only £150-350

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Rectification Probe

Related codes

Full explanation for F2 or Fn or Ln
F3

Your boiler has stopped working because the internal fan, which safely exhausts gases produced during heating, is not running correctly.

High Engineer only £280-£450

Parts commonly replaced

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

Related codes

Full explanation for F3
F4

Your boiler has likely stopped heating because a sensor that measures the temperature of the water leaving the unit has failed or become disconnected.

High Engineer only £90-£160

Parts commonly replaced

  • Flow Thermistor
  • Connecting wiring harness

Related codes

Full explanation for F4
F4 or L4

Your boiler's temperature sensor is providing incorrect readings or has disconnected, so the system has shut down to prevent overheating.

High Engineer only £120-180

Parts commonly replaced

  • Flow Thermistor
  • Wiring harness

Related codes

Full explanation for F4 or L4
F5

Your boiler's internal sensor that monitors the temperature of the water coming back into the unit has developed a fault, preventing the system from operating correctly.

High Engineer only £90-180

Parts commonly replaced

  • Return Thermistor (NTC Sensor)
  • Wiring harness
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F5
F5 or L5

Your boiler is unable to 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 sensor
  • Wiring harness

Related codes

Full explanation for F5 or L5
F6

Your boiler is having trouble communicating with the optional sensor that measures the temperature outside, which may cause your heating to behave unpredictably.

Low DIY-safe £90-160

What to check first

  1. Check if any external wires leading to the sensor outside have been damaged or cut.
  2. Restart the boiler by turning the control knob to 'Reset' and then back to the desired setting.
  3. If the fault persists, contact a professional; the boiler will usually continue to provide heat using a default setting in the meantime.

Parts commonly replaced

  • Outdoor weather compensation sensor
  • Wiring harness

Related codes

Full explanation for F6
F7

Your boiler is not receiving enough electrical power from the grid to operate safely and has shut down as a precaution.

High Engineer only £0-120

Parts commonly replaced

  • Printed Circuit Board (PCB)

Related codes

Full explanation for F7
Showing 11–20 of 27
F8

Your boiler's internal computer brain either hasn't been set up correctly or has developed a hardware failure, preventing the unit from starting.

High Engineer only £250-£450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Transformer

Related codes

Full explanation for F8
F9

Your boiler's main control board hasn't been set up correctly to tell it which model it is, so it doesn't know how to operate yet.

High Engineer only £100-180

Parts commonly replaced

  • Main PCB
  • Programming Chip (if separate)

Related codes

Full explanation for F9
F9 or L9 or F8 or L8

Your boiler's computer system has lost its settings or is failing to communicate, meaning the boiler cannot safely operate. Because the central control unit is malfunctioning, you will currently have no heating or hot water.

High Engineer only £250-450

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for F9 or L9 or F8 or L8
FA

Your boiler has detected that the temperature sensors are reading back-to-front or are failing, causing it to shut down for safety.

High Engineer only £120-220

Parts commonly replaced

  • Flow thermistor
  • Return thermistor
  • Wire harness

Related codes

Full explanation for FA
Fn

Your boiler has failed to maintain a flame, which means it has stopped providing heating and hot water to protect itself.

High DIY-safe £100-350

What to check first

  1. Check if other gas appliances in your home, like a gas hob, are working correctly.
  2. Verify that your gas prepaid meter (if applicable) has enough credit.
  3. Press the Reset button on the boiler control panel to attempt a restart.

Parts commonly replaced

  • Gas Valve
  • Spark Generator
  • Flame Sensing Electrode
  • Condensate Trap or Pipe

Related codes

Full explanation for Fn
FU

Your boiler has detected that water is getting too hot too quickly because it isn't circulating through your radiators or pipes correctly.

High DIY-safe £150-£350

What to check first

  1. Check that all radiator valves (TRVs) are turned to a high setting
  2. Ensure the blue and white isolation valves underneath the boiler are in the open position
  3. Gently bleed your radiators to remove any trapped air pockets

Parts commonly replaced

  • Central Heating Pump
  • Main Heat Exchanger
  • Flow/Return Thermistors

Related codes

Full explanation for FU
L1

Your boiler has shut down because the water inside it has become too hot, usually due to a lack of water flow or a circulation problem.

High DIY-safe £100-£350

What to check first

  1. Check that your boiler pressure is between 1.0 and 1.5 bar on the front gauge.
  2. Ensure all radiator valves are fully open and not turned off.
  3. Reset the boiler by turning the mode dial to 'reset' for two seconds and then back to your desired setting.

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.

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 if your gas meter credit has run out
  3. If it is freezing outside, check if the white plastic condensate pipe leading outdoors is frozen
  4. Press the Reset button on the front of the boiler

Parts commonly replaced

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

Related codes

Full explanation for L2
L4

Your boiler has detected a problem with the internal sensor that monitors the temperature of the water as it leaves the unit, causing the system to shut down as a safety precaution.

High Engineer only £90-160

Parts commonly replaced

  • Flow Thermistor
  • Wiring Harness

Related codes

Full explanation for L4
L4 or F4

Your boiler has lost the ability to accurately measure the temperature of the water flowing through it, so it has shut down to prevent damage.

High Engineer only £120-200

Parts commonly replaced

  • Flow thermistor (NTC sensor)
  • Wiring harness

Related codes

Full explanation for L4 or F4
Showing 21–27 of 27
L5

Your boiler has detected a problem with the sensor that monitors the temperature of the water returning to the unit, 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 L5
L5 or F5

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

High Engineer only £120-220

Parts commonly replaced

  • Return thermistor
  • Wiring harness

Related codes

Full explanation for L5 or F5
L6

Your boiler thinks it sees a flame when there shouldn't be one, so it has shut down as a safety precaution.

High Engineer only £120-350

Parts commonly replaced

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

Related codes

Full explanation for L6
L8

Your boiler's internal computer brain either isn't set up correctly or has developed a technical hardware fault, preventing the unit from starting.

High Engineer only £250-£450

Parts commonly replaced

  • Main PCB (Control Board)
  • External Programming Key

Related codes

Full explanation for L8
L9

Your boiler’s main computer chip has forgotten its settings or has developed an internal electrical failure, meaning the unit cannot function.

High Engineer only £280-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Configuration Chip

Related codes

Full explanation for L9
LC

Your boiler is repeatedly failing to start and has locked itself out for safety after five unsuccessful attempts in a short period.

High Engineer only £100-£350

Parts commonly replaced

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

Related codes

Full explanation for LC
Ln

Your boiler is struggling to keep its flame lit, which means it cannot heat your water or radiators.

High DIY-safe £100-£350

What to check first

  1. Check if other gas appliances in your home, like a hob, are working to confirm gas supply.
  2. If your gas is on a pre-pay meter, ensure you have sufficient credit.
  3. Reset the boiler by turning the mode knob to 'RE' and back to the 'On' position.

Parts commonly replaced

  • Ignition Electrode
  • Gas Control Valve
  • Flame Sensing Electrode

Related codes

Full explanation for Ln
Status & diagnostic codes

7 operating states & engineer diagnostics

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

00 Status Your boiler is working perfectly and is simply waiting for you to turn on the heating or a hot tap. DU Diagnostic Your boiler is currently in a manual service mode used for internal checks and is not indicating a broken part. oF Diagnostic Your boiler is currently stuck in a technical programming mode used by engineers and is not currently in its normal operating state. SH Diagnostic Your boiler is currently in service mode, which is an intentional setting used by engineers to test the system and not a mechanical breakdown. SL Diagnostic Your boiler is not actually broken; it is currently in a manual service mode used by engineers to test the system or displays a history of recent issues. UF Diagnostic Your boiler is currently in a manual testing or service mode used by engineers and is not actually reporting a breakage or fault. uP Diagnostic Your boiler is currently stuck in its internal computer programming mode and hasn't been set up to operate as a heater yet.