Heatline Sargon 24 Fault codes & diagnostics

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

Combi Natural Gas 24 kW Discontinued

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21 High 2 Low
Just want the most common Heatline Sargon 24 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 23 documented codes

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E01

Your boiler has tried to start up but cannot light the gas flame required to provide heating and hot water.

High DIY-safe £120-350

What to check first

  1. Check that your gas emergency control valve is in the 'on' position
  2. If you have a prepay meter, check that you have sufficient credit
  3. Test other gas appliances, like a hob, to see if the gas supply is active
  4. Press the Reset button on the boiler control panel

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Spark Leads
  • PCB

Related codes

Full explanation for E01
E02

Your boiler has switched itself off because the water inside it has become too hot, usually caused by a blockage or a pump failure.

High DIY-safe £150-£350

What to check first

  1. Check that all radiator valves are fully open to allow water to flow.
  2. Inspect the system pressure gauge and top it up to 1.5 bar if it is too low.
  3. Wait for the boiler to cool down and press the 'Reset' button.

Parts commonly replaced

  • Circulating Pump
  • Overheat Thermostat
  • Main Heat Exchanger

Related codes

Full explanation for E02
E03

Your boiler has shut down because it has detected that the exhaust gases are getting too hot or aren't clearing properly, which could indicate a blockage or a faulty sensor.

High Engineer only £100-£220

Parts commonly replaced

  • Flue gas sensor
  • Heat exchanger cleaning
  • Flue thermostat

Related codes

Full explanation for E03
E04

Your boiler has stopped working because the water pressure is too low and needs to be topped up.

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 valve or valves on the hose until you hear water moving.
  3. Monitor the pressure gauge until it reaches between 1.0 and 1.5 bar, then close the valves tightly.
  4. Restart the boiler to clear the fault code.

Parts commonly replaced

  • Filling loop
  • Pressure sensor
  • Expansion vessel

Related codes

Full explanation for E04
E05

Your boiler has detected a problem with the internal sensor that monitors the temperature of the water used to heat your radiators, preventing the system from running safely.

High Engineer only £90-£160

Parts commonly replaced

  • Central heating flow NTC thermistor
  • Wiring harness

Related codes

Full explanation for E05
E06

Your boiler is struggling to measure the temperature of your hot water, which usually means your taps will only run stone cold.

High Engineer only £90-160

Parts commonly replaced

  • Domestic Hot Water (DHW) NTC Sensor
  • Sensor Wiring Harness

Related codes

Full explanation for E06
E10

Your boiler has stopped working because it cannot correctly detect the water pressure, even if there is actually enough water in the system.

High Engineer only £120-190

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness

Related codes

Full explanation for E10
E11

Your boiler has stopped working because water is not flowing through the system correctly, which is causing it to overheat.

High DIY-safe £180-450

What to check first

  1. Check that the boiler pressure is between 1.0 and 1.5 bar on the gauge.
  2. Ensure all radiator valves and isolation valves under the boiler are fully open.
  3. Gently bleed air from your radiators using a radiator key.

Parts commonly replaced

  • Circulation pump
  • Main heat exchanger
  • Temperature sensors (NTC thermistors)

Related codes

Full explanation for E11
E16

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

High Engineer only £180-350

Parts commonly replaced

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

Related codes

Full explanation for E16
E22

Your boiler's internal computer has lost its settings or is incorrectly programmed, meaning it no longer knows how to operate the heating system safely.

High Engineer only £180-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • PCB Interface/Display

Related codes

Full explanation for E22
Showing 11–20 of 23
E25

Your boiler's main control brain has developed an internal electrical error and can no longer process commands to heat your home.

High Engineer only £280-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)

Related codes

Full explanation for E25
E35

Your boiler thinks it sees a flame even though the gas should be turned off, which is a safety conflict that prevents the system from starting.

High Engineer only £120-£280

Parts commonly replaced

  • Flame sensing electrode
  • Gas valve
  • Main PCB

Related codes

Full explanation for E35
F0

Your boiler has detected that the water pressure inside the system is either too low or too high to operate safely.

High DIY-safe £0-160

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 0.5 bar, use the filling loop valves underneath the boiler to top it up until it reaches 1.2 bar.
  3. If the pressure is above 2.5 bar, bleed a radiator using a radiator key to release water and lower the pressure.
  4. Reset the boiler once the correct pressure is achieved.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure sensor

Related codes

Full explanation for F0
F1

Your boiler has switched itself off for safety because it has become too hot internally.

High Engineer only £90-180

Parts commonly replaced

  • Overheat thermostat
  • Thermal fuse
  • Circulation pump

Related codes

Full explanation for F1
F2

Your boiler's hot water sensor has stopped working correctly, which means the system cannot accurately measure or provide hot water.

High Engineer only £90-160

Parts commonly replaced

  • DHW Temperature Sensor (NTC Thermistor)
  • Wiring Harness

Related codes

Full explanation for F2
F3

Your boiler's internal thermometer has stopped working correctly, so the system has shut down to prevent it from getting too hot.

High Engineer only £90-160

Parts commonly replaced

  • NTC Temperature Sensor
  • Sensor Wiring Harness

Related codes

Full explanation for F3
F4

Your boiler has stopped working because it cannot light the flame or the internal drainage pipe is backed up with water.

High DIY-safe £100-280

What to check first

  1. Check if your gas supply is active (e.g. check other gas appliances or prepay meter credit).
  2. Inspect the white plastic pipe exiting the building for ice or blockages; gently thaw with warm water if frozen.
  3. Press and hold the reset button for 5 seconds to try and restart the system.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Condensate Trap
  • Spark Generator

Related codes

Full explanation for F4
F5

Your boiler has detected that a safety sensor monitoring the temperature of exhaust gases is broken or malfunctioning.

High Engineer only £90-160

Parts commonly replaced

  • Flue Gas NTC Sensor
  • Sensor Wiring Harness

Related codes

Full explanation for F5
F6

Your boiler is having trouble readng the temperature outside, which means it cannot adjust its settings to keep your home at the right temperature efficiently.

Low Engineer only £90-160

Parts commonly replaced

  • External Temperature Sensor
  • Sensor Wiring

Related codes

Full explanation for F6
F7

Your boiler has detected that the fan, which safely removes waste gases, is spinning at the wrong speed and has stopped working for safety.

High Engineer only £180-350

Parts commonly replaced

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

Related codes

Full explanation for F7
Showing 21–23 of 23
F8

Your boiler has detected that the internal fan is spinning at the wrong time, which is preventing the system from starting up safely.

High Engineer only £180-£450

Parts commonly replaced

  • Main PCB
  • Fan Assembly
  • Wiring Loom

Related codes

Full explanation for F8
F9

Your boiler has switched itself off for safety because the water inside has become far too hot.

High DIY-safe £100-280

What to check first

  1. Check that all radiator valves are fully open to allow water to circulate.
  2. Ensure there is enough pressure in the system by checking the gauge is between 1 and 1.5 bar.
  3. Wait for the boiler to cool down and try pressing the reset button.

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Main Heat Exchanger (if blocked)

Related codes

Full explanation for F9
OC

Your boiler has detected an outdoor temperature sensor and is adjusting its settings to match the weather outside.

Low DIY-safe £0-60

What to check first

  1. Check that the outdoor sensor is not obstructed or damaged
  2. Monitor the boiler to ensure it continues to provide heating and hot water as normal
  3. If heating is too cold, adjust the heating curve settings via the control panel

Parts commonly replaced

  • External temperature sensor
  • Sensor wiring

Related codes

Full explanation for OC