Unical Alkon C 28 HE Fault codes & diagnostics

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

Combi Natural GasLPG 28 kW 2011-current
GC Number 41-010-15
Download the Unical Alkon C 28 HE manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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Severity at a glance

Each cell = one fault code. Hover to identify.
5 Emergency 33 High 2 Medium 3 Low
Just want the most common Unical Alkon C 28 HE faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 43 documented codes

Tap any card for details

6

Your boiler has switched itself off because it has detected that the water inside has become dangerously hot.

High DIY-safe £100-£350

What to check first

  1. Check that all radiator valves are fully open to allow water to flow.
  2. Ensure there is enough water pressure in the system by checking the gauge.
  3. Try resetting the boiler carefully once the unit has cooled down.

Parts commonly replaced

  • Circulation pump
  • Overheat thermostat
  • Temperature sensor (NTC)

Related codes

Full explanation for 6
08

Your boiler has detected that the water pressure is too low for it to operate safely and effectively.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver braided hose underneath the boiler).
  2. Slowly open the valve or valves until you hear water entering the system.
  3. Monitor the pressure gauge and close the valves once it reaches between 1.0 and 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure sensor

Related codes

Full explanation for 08
8

Your boiler has stopped working because the water pressure is too low for it to operate safely.

High DIY-safe £60-140

What to check first

  1. Locate the filling loop (usually a silver flexible hose) underneath or near the boiler
  2. Slowly open the small valves on the filling loop until you hear water flowing
  3. Watch the pressure gauge until it reaches 1.5 bar, then close the valves completely
  4. Press the 'Reset' button on the boiler to clear the fault

Parts commonly replaced

  • Filling loop
  • Water pressure switch
  • Expansion vessel recharge

Related codes

Full explanation for 8
09

Your boiler has lost connection with the sensor that measures the temperature outside, which helps it run more efficiently.

Low Engineer only £90-160

Parts commonly replaced

  • External temperature probe
  • Outdoor sensor wiring

Related codes

Full explanation for 09
10

Your boiler has reached an unsafe temperature and has shut itself down to prevent damage or overheating.

High DIY-safe £120-350

What to check first

  1. Check that all radiator valves are fully open to allow water to flow.
  2. Ensure there is enough water pressure in the system (typically 1.0 to 1.5 bar).
  3. Switch the boiler off and back on again to attempt a reset after it has cooled down.

Parts commonly replaced

  • Circulation pump
  • Overheat thermostat
  • Heat exchanger (flushing or replacement)

Related codes

Full explanation for 10
11

Your boiler has detected a flame inside when it shouldn't be there, so it has safely shut itself down to prevent a fire risk.

Emergency Engineer only £150-300

Parts commonly replaced

  • Ionisation electrode
  • Gas valve
  • Printed Circuit Board

Related codes

Full explanation for 11
12

Your boiler has stopped working because it is unable to accurately measure the temperature of the water used for your radiators.

High Engineer only £120-200

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring harness

Related codes

Full explanation for 12
13

Your boiler is struggling to detect the temperature of the water used for your taps and shower, which means you may have no hot water.

High Engineer only £120-220

Parts commonly replaced

  • DHW NTC sensor
  • Wiring harness

Related codes

Full explanation for 13
14

Your boiler has detected that a sensor monitoring the temperature of the water returning to the unit has stopped working or is disconnected.

High Engineer only £90-160

Parts commonly replaced

  • NTC Return Temperature Sensor
  • Sensor Wiring Harness

Related codes

Full explanation for 14
15

Your boiler is overheating because water isn't flowing through the system quickly enough to take the heat away.

High DIY-safe £120-£350

What to check first

  1. Check that your boiler pressure gauge is between 1.0 and 1.5 bar
  2. Ensure all radiator valves and the boiler isolation valves are fully open
  3. Bleed your radiators to remove any trapped air pockets

Parts commonly replaced

  • Circulation pump
  • Central heating filter (cleaning)
  • Main heat exchanger (flushing)

Related codes

Full explanation for 15
Showing 11–20 of 43
16

Your boiler has detected that the water inside its internal heat exchanger is at risk of freezing, so it has stopped operating to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • NTC sensor
  • Flow switch
  • PCB

Related codes

Full explanation for 16
17

Your boiler has detected an issue with the electrical power supply coming into your home, meaning the voltage or frequency is currently unstable for the unit to operate safely.

Medium DIY-safe £0-120

What to check first

  1. Turn the boiler off using the main power switch or at the fused spur wall switch.
  2. Wait for approximately 30 seconds.
  3. Turn the power back on and check if the error clears once the system reboots.

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Surge protector

Related codes

Full explanation for 17
18

Your boiler has detected that a component inside is either damp or touching metal where it shouldn't be, causing it to shut down for safety.

High Engineer only £120-220

Parts commonly replaced

  • Ignition electrode
  • Ionisation probe
  • Ignition lead

Related codes

Full explanation for 18
19

Your boiler is detecting 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

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

Related codes

Full explanation for 19
20

Your boiler's safety sensor is still detecting a flame even though the system has been told to turn off.

Emergency Engineer only £120-350

Parts commonly replaced

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

Related codes

Full explanation for 20
21

Your boiler is struggling to move water through the system, which causes it to shut down to prevent overheating.

High DIY-safe £120-£350

What to check first

  1. Check if the boiler pressure is between 1.0 and 1.5 bar
  2. Ensure all radiator valves and external isolation valves are fully open
  3. Bleed all radiators to remove trapped air pockets

Parts commonly replaced

  • Circulation Pump
  • Flow/Return Thermistor
  • System Filter

Related codes

Full explanation for 21
22

Your boiler has detected that its internal temperature sensors are reading heat in the wrong order, suggesting they may be swapped or fitted incorrectly.

High Engineer only £90-180

Parts commonly replaced

  • Flow NTC sensor
  • Return NTC sensor
  • Wiring loom

Related codes

Full explanation for 22
23

Your boiler has detected that a button on the control panel has been held down for too long, which usually means a button is stuck or the panel is faulty.

Low DIY-safe £0-180

What to check first

  1. Check all buttons on the front control panel to see if any are physically stuck in the 'in' position.
  2. Gently press and release each button several times to ensure they click and spring back out.
  3. Clean the control panel with a dry microfiber cloth to remove any sticky residue or grit hindering the buttons.
  4. Reset the boiler by turning the power off and back on at the fused spur switch.

Parts commonly replaced

  • Control Panel Overlay
  • User Interface PCB
Full explanation for 23
24

Your boiler’s internal fan is not spinning at the correct speed, which prevents the unit from starting up safely to heat your home.

High Engineer only £180-450

Parts commonly replaced

  • Combustion fan
  • Main PCB
  • Wiring harness

Related codes

Full explanation for 24
25

Your boiler's electronic control system has lost its tuning settings and cannot properly regulate the gas and air mixture for combustion.

High Engineer only £100-250

Parts commonly replaced

  • Ionisation probe
  • Gas valve
  • Main PCB

Related codes

Full explanation for 25
Showing 21–30 of 43
26

Your boiler has shut down because the internal fan is spinning faster than it is supposed to, which prevents the system from safely managing the gas flame.

High Engineer only £180-380

Parts commonly replaced

  • Combustion Fan
  • PCB (Main Circuit Board)
  • Wiring harness

Related codes

Full explanation for 26
27

Your boiler has detected that the water temperature is rising much faster than expected, which usually points to a restricted flow or a problem with the internal circulation pump.

High Engineer only £150-250

Parts commonly replaced

  • Circulation pump
  • Flow sensor
  • Primary heat exchanger

Related codes

Full explanation for 27
28

Your boiler has detected an internal communication failure between the control board and the gas supply system, meaning it cannot safely ignite.

High Engineer only £180-£450

Parts commonly replaced

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

Related codes

Full explanation for 28
30

Your boiler's internal computer settings have been disrupted by nearby electrical interference and need to be reset to work correctly.

High Engineer only £100-280

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Electronic Control Unit

Related codes

Full explanation for 30
32

Your boiler is struggling to ignite because the gas valve is taking too long to open, causing the system to shut down for safety.

High Engineer only £180-350

Parts commonly replaced

  • Gas Valve
  • Ignition Electrode
  • Control PCB

Related codes

Full explanation for 32
33

Your boiler is struggling to keep a flame lit even when trying to start up at full power, causing it to shut down for safety.

High Engineer only £120-350

Parts commonly replaced

  • Ionisation probe
  • Ignition electrode
  • Gas valve
  • PCB

Related codes

Full explanation for 33
38

Your boiler's computer has detected an internal error caused by electrical interference, causing it to lose its correct operating settings.

High Engineer only £120-250

Parts commonly replaced

  • Control PCB
  • Wiring harness

Related codes

Full explanation for 38
43

Your boiler has repeatedly failed to start the burner, so it has safely shut itself down to prevent a gas build-up.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Flame rectification probe
  • Gas valve

Related codes

Full explanation for 43
44

Your boiler has detected that the water pressure inside your heating system is currently set to an incorrect level.

High DIY-safe £80-150

What to check first

  1. Locate the filling loop (a flexible metal hose) underneath your boiler.
  2. Open the valves on the filling loop to allow cold mains water to enter the system.
  3. Watch the pressure gauge on the front of the boiler until it reaches 1.5 bar, then securely close both valves.

Parts commonly replaced

  • Pressure transducer
  • Pressure gauge
  • Filling loop

Related codes

Full explanation for 44
45

Your boiler has switched off because there is too much water pressure in the system, which can cause internal leaks if not corrected.

Emergency DIY-safe £90-180

What to check first

  1. Locate a radiator bleed valve and use a key to release water into a container until the boiler pressure gauge drops below 1.5 bar.
  2. Ensure the external filling loop valves are tightly closed to prevent more water from entering.
  3. Check if the pressure rises again on its own, which may indicate a faulty filling loop or internal valve.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure relief valve (PRV)

Related codes

Full explanation for 45
Showing 31–40 of 43
53

Your boiler is struggling to push out waste gases because the exhaust pipe or ventilation path is blocked or obstructed.

Emergency Engineer only £150-300

Parts commonly replaced

  • Flue assembly
  • Fan
  • Air pressure switch

Related codes

Full explanation for 53
55

Your boiler has lost its internal settings for how to burn fuel efficiently and needs a professional to recalibrate the system.

High Engineer only £120-220

Parts commonly replaced

  • Gas valve
  • PCB

Related codes

Full explanation for 55
75

Your boiler is having trouble communicating with the outdoor temperature sensor, which helps it run efficiently.

Low Engineer only £90-180

Parts commonly replaced

  • External temperature sensor
  • Sensor wiring

Related codes

Full explanation for 75
80

Your boiler's internal control panel (the Regolafacile) has detected a fault with its own built-in temperature sensor, meaning it cannot accurately manage the heating settings.

Medium Engineer only £120-250

Parts commonly replaced

  • Regolafacile remote controller unit
  • Internal sensor NTC
  • Wiring harness

Related codes

Full explanation for 80
81

Your boiler's internal memory chip has developed a software communication fault, meaning the control system cannot remember its settings or operate correctly.

High Engineer only £250-450

Parts commonly replaced

  • Main Control PCB
  • Regolafacile Module
  • EPROM Chip

Related codes

Full explanation for 81
AF

Your boiler has detected that waste gases are not escaping correctly through the flue, so it has shut down to prevent them from entering your home.

Emergency Engineer only £120-280

Parts commonly replaced

  • Flue gas thermostat
  • Flue terminal
  • Heat exchanger gaskets

Related codes

Full explanation for AF
BLOCK

Your boiler has encountered an unspecified error and has shut itself down for safety, requiring a system reset.

High DIY-safe £100-350

What to check first

  1. Locate the 'R' or 'Reset' button on the boiler control panel.
  2. Press and hold the button for 2 to 5 seconds.
  3. Wait to see if the burner ignites and the display returns to a standard temperature reading.

Parts commonly replaced

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

Related codes

Full explanation for BLOCK
Fd

Your boiler is failing to light the gas flame required to heat your water and home.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Detection electrode
  • Ignition lead
  • Printed Circuit Board (PCB)

Related codes

Full explanation for Fd
FL

Your boiler has detected a problem with the fan that safely pushes waste gases out of your home, causing the system to shut down for safety.

High Engineer only £220-450

Parts commonly replaced

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

Related codes

Full explanation for FL
FR

Your boiler has stopped working because it has detected that the main part responsible for heating your water is failing to transfer heat correctly.

High Engineer only £400-£1,200

Parts commonly replaced

  • Heat Exchanger
  • Temperature Sensors
  • Primary Circuit Seals

Related codes

Full explanation for FR
Showing 41–43 of 43
HL

Your boiler has shut down because it has detected an unsafe build-up of heat inside the unit.

High DIY-safe £120-250

What to check first

  1. Check that all radiator valves are fully open to allow water flow.
  2. Ensure the boiler pressure is between 1.0 and 1.5 bar on the gauge.
  3. Wait for the boiler to cool down and press the 'Reset' button.

Parts commonly replaced

  • Safety thermostat (High Limit Stat)
  • Circulation pump
  • Heat exchanger

Related codes

Full explanation for HL
Lp

Your boiler has detected that the water pressure is too low to operate safely, causing it to shut down to prevent damage.

High DIY-safe £0-180

What to check first

  1. Locate the filling loop (usually a silver flexible hose) underneath your boiler.
  2. Slowly open the small valve(s) to allow water into the system.
  3. Watch the pressure gauge and close the valve(s) once it reaches 1.5 bar.
  4. The fault code should disappear, allowing the boiler to restart.

Parts commonly replaced

  • Water pressure switch
  • Expansion vessel repressurisation
  • Filling loop

Related codes

Full explanation for Lp
tf

Your boiler has shut down because the exhaust gases have reached a temperature that is too high, triggering a safety sensor to prevent damage.

High Engineer only £120-£280

Parts commonly replaced

  • Flue gas thermostat
  • Heat exchanger cleaning service
  • Main heat exchanger

Related codes

Full explanation for tf
Status & diagnostic codes

4 operating states & engineer diagnostics

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

db Diagnostic Your boiler is having trouble measuring the temperature of your hot water, which usually means it cannot heat water for your taps and showers until it is fixed. Hb Diagnostic Your boiler has stopped working because it cannot accurately measure the temperature of the water used to heat your radiators. Ht Diagnostic Your boiler has stopped working because the circulation pump, which moves hot water around your radiators, has detected a fault or is unable to spin. rb Diagnostic Your boiler has detected an issue with the sensor that monitors water returning from your radiators, which may cause the system to stop running to prevent damage.
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