Unical Alkon R 28 HE Fault codes & diagnostics

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

Regular Natural GasLPG 28 kW 2011-current
GC Number 41-010-18
Download the Unical Alkon R 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 2 Low
Just want the most common Unical Alkon R 28 HE faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 42 documented codes

Tap any card for details

6

Your boiler has shut down because it has detected it is getting too hot, likely due to a lack of water flow through the system.

High DIY-safe £120-350

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure the boiler pressure gauge is between 1.0 and 1.5 bar
  3. Press the reset button to see if the fault clears after the system cools down

Parts commonly replaced

  • Circulation Pump
  • Overheat Thermostat
  • NTC Temperature Sensor

Related codes

Full explanation for 6
08

Your boiler has detected that the water pressure is too low for it to operate safely and has shut down to avoid damage.

High DIY-safe £80-160

What to check first

  1. Locate the pressure gauge on the front of the boiler or underneath it.
  2. Find the filling loop, which is usually a flexible silver hose with one or two small black taps.
  3. Slowly open the taps until the pressure gauge needle sits between 1.0 and 1.5 bar.
  4. Securely close both taps and attempt to reset the boiler.

Parts commonly replaced

  • Filling loop valve
  • Pressure sensor
  • Expansion vessel

Related codes

Full explanation for 08
8

Your boiler has stopped working because the water pressure inside the system is too low.

High DIY-safe £60-140

What to check first

  1. Locate the filling loop (usually a silver braided hose with small valves underneath the boiler).
  2. Open the valves slowly to allow water into the system while watching the pressure gauge.
  3. Close the valves tightly once the pressure reaches between 1.0 and 1.5 bar.

Parts commonly replaced

  • Water pressure switch
  • Filling loop
  • Expansion vessel

Related codes

Full explanation for 8
09

Your boiler has lost connection with the sensor that monitors the temperature outside, which may cause your heating to behave unpredictably.

Low Engineer only £90-160

Parts commonly replaced

  • External temperature sensor (Outdoor probe)
  • Sensor wiring harness

Related codes

Full explanation for 09
9

Your boiler has shut down because the internal fan is spinning faster than the system allows, preventing the boiler from safely managing the gas flame.

High Engineer only £180-450

Parts commonly replaced

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

Related codes

Full explanation for 9
10

Your boiler has shut down because it has detected it is getting too hot, likely due to a lack of water flow through the system.

High DIY-safe £100-350

What to check first

  1. Check that your radiators are turned on and the valves are open
  2. Ensure your system water pressure is between 1.0 and 1.5 bar
  3. Press the 'Reset' button on the control panel to see if the fault clears

Parts commonly replaced

  • Circulation pump
  • Overheat thermostat
  • Main heat exchanger (flushing)

Related codes

Full explanation for 10
11

Your boiler is detecting a flame inside before it has actually been told to start, so it has shut down as a safety precaution.

Emergency Engineer only £150-300

Parts commonly replaced

  • Flame rectification electrode
  • Gas valve
  • PCB

Related codes

Full explanation for 11
12

Your boiler has stopped working because it cannot correctly detect the temperature of the water being used for your heating system.

High Engineer only £120-220

Parts commonly replaced

  • Central heating NTC temperature sensor
  • Wiring harness

Related codes

Full explanation for 12
13

Your boiler is struggling to detect the temperature of your hot water, which means it will likely stop producing hot water until the faulty sensor is fixed.

High Engineer only £120-220

Parts commonly replaced

  • DHW NTC temperature sensor

Related codes

Full explanation for 13
14

Your boiler has detected a communication failure with a temperature sensor that monitors water returning to the unit, causing it to shut down for safety.

High Engineer only £110-190

Parts commonly replaced

  • Return NTC Temperature Sensor (SRR)
  • Sensor wiring harness

Related codes

Full explanation for 14
Showing 11–20 of 42
15

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

High DIY-safe £150-£400

What to check first

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

Parts commonly replaced

  • Circulation pump
  • Main heat exchanger (de-scaling)
  • Flow temperature sensor

Related codes

Full explanation for 15
16

Your boiler's internal memory has been scrambled by electrical interference, meaning it has forgotten its essential operating instructions.

High Engineer only £100-350

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Display interface cable

Related codes

Full explanation for 16
17

Your boiler has detected an issue with the electrical power supply coming into your home, meaning the electrical frequency or voltage is outside of the safe operating range.

High Engineer only £100-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Surge Protector
  • External Voltage Regulator

Related codes

Full explanation for 17
18

Your boiler is failing to ignite because the internal sensor that detects the flame is likely touching a metal surface or has become damp.

High Engineer only £120-220

Parts commonly replaced

  • Ignition electrode
  • Ignition lead
  • Ionisation probe

Related codes

Full explanation for 18
19

Your boiler has detected a flame when there shouldn't be one, which means it has shut down for safety reasons.

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 detects a flame still burning even after it has been told to turn off.

Emergency Engineer only £150-350

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 around the system correctly, which is preventing it from heating up properly.

High DIY-safe £120-£350

What to check first

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

Parts commonly replaced

  • Circulation pump
  • Flow and return thermistors
  • Central heating filter (cleaning)

Related codes

Full explanation for 21
22

Your boiler has detected that the temperature sensors monitoring incoming and outgoing water have been swapped or are reading incorrectly, causing the system to stop for safety.

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 pressed or stuck for too long, causing the system to stop for safety.

Medium DIY-safe £90-180

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 to ensure they click and move freely.
  3. Wipe the control panel with a dry cloth to remove any sticky residue that might be holding a button down.
  4. Restart the boiler once all buttons appear to be released.

Parts commonly replaced

  • Control Interface PCB
  • Button Rubber Membrane
  • Display Housing

Related codes

Full explanation for 23
24

Your boiler has shut down because its internal fan is not spinning at the correct speed to safely clear exhaust fumes.

High Engineer only £180-450

Parts commonly replaced

  • Extractor Fan
  • PCB (Main Control Board)
  • Wiring Harness

Related codes

Full explanation for 24
Showing 21–30 of 42
25

Your boiler has lost its fuel-to-air combustion settings and cannot run safely until it is re-calibrated.

High Engineer only £90-180

Parts commonly replaced

  • Gas Valve
  • Control PCB
  • Ionisation Electrode

Related codes

Full explanation for 25
26

Your boiler has shut down because the internal fan is spinning faster than the system's safety settings allow.

High Engineer only £180-450

Parts commonly replaced

  • Extraction Fan
  • Main PCB
  • Fan Wiring Harness

Related codes

Full explanation for 26
27

Your boiler is struggling because the temperature of the water flowing out is rising too quickly, suggesting a potential blockage or restricted water flow within the system.

High Engineer only £150-300

Parts commonly replaced

  • Central heating pump
  • Flow/Return sensors
  • Heat exchanger

Related codes

Full explanation for 27
28

Your boiler has detected an internal electrical communication error with the gas valve, preventing it from heating your water Safely.

High Engineer only £220-380

Parts commonly replaced

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

Related codes

Full explanation for 28
30

Your boiler's electronic settings have been disrupted by outside electrical interference, causing the system to stop working properly.

High DIY-safe £100-280

What to check first

  1. Locate the reset button on the control panel.
  2. Press and hold the reset button for 5 seconds.
  3. Wait 2 minutes to see if the boiler resumes normal operation; if the code persists, a professional must recalibrate the settings.

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • External EMC Filter

Related codes

Full explanation for 30
32

Your boiler is struggling to ignite because the gas valve is taking too long to open or supply fuel correctly.

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 after trying to adjust the gas flow, causing the system 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 internal computer settings have become scrambled, likely due to a sudden power surge or nearby electrical interference.

High Engineer only £120-250

Parts commonly replaced

  • Main PCB
  • Control Panel

Related codes

Full explanation for 38
43

Your boiler is failing to stay lit because it cannot successfully establish a steady flame during the start-up process.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Flame rectification probe
  • Gas valve
  • Control board

Related codes

Full explanation for 43
44

Your boiler has detected that the water pressure inside the system is currently too high.

Medium DIY-safe £80-150

What to check first

  1. Locate the external filling loop, usually a small metal braided hose under the boiler.
  2. Ensure both valves on the filling loop are fully closed to prevent extra water from entering.
  3. Drain a small amount of water from a nearby radiator bleed valve until the pressure gauge on the front of the boiler returns to the green zone, typically between 1.0 and 1.5 bar.

Parts commonly replaced

  • Pressure transducer
  • Pressure relief valve
  • Filling loop valves

Related codes

Full explanation for 44
Showing 31–40 of 42
45

Your boiler has switched off because the water pressure inside the system is too high.

Emergency DIY-safe £80-160

What to check first

  1. Ensure the blue filling loop valves under the boiler are fully closed.
  2. Bleed water from a radiator using a radiator key to release pressure.
  3. Monitor the pressure gauge until it drops between 1.0 and 1.5 bar.

Parts commonly replaced

  • Filling loop valve
  • Expansion vessel
  • Pressure transducer

Related codes

Full explanation for 45
53

Your boiler is detecting a blockage in the exhaust or air intake system, preventing it from venting safely.

Emergency Engineer only £120-250

Parts commonly replaced

  • Flue system components
  • Condensate trap
  • Fan assembly

Related codes

Full explanation for 53
55

Your boiler has lost the internal settings required to balance its gas and air mixture, meaning it cannot safely start up or heat your home.

High Engineer only £120-250

Parts commonly replaced

  • Gas valve
  • PCB (Printed Circuit Board)

Related codes

Full explanation for 55
75

Your boiler is struggling to work out how much to heat your home because the sensor that monitors the temperature outside has lost its connection or stopped working.

Low Engineer only £90-180

Parts commonly replaced

  • External weather compensation sensor
  • Sensor wiring

Related codes

Full explanation for 75
80

Your boiler's remote control unit has a faulty internal temperature sensor, preventing the system from accurately managing your home's heat.

High Engineer only £120-250

Parts commonly replaced

  • Regolafacile Remote Controller
  • Internal Room Sensor
  • Communication Cable

Related codes

Full explanation for 80
81

Your boiler's main internal computer card has suffered a memory failure and can no longer process instructions correctly.

High Engineer only £250-450

Parts commonly replaced

  • Regolafacile Control Board
  • EPROM chip

Related codes

Full explanation for 81
AF

Your boiler has detected that dangerous exhaust fumes are not escaping through the flue correctly and has shut down to prevent them from entering your home.

Emergency Engineer only £120-250

Parts commonly replaced

  • Flue gas thermostat
  • Flue seals
  • Heat exchanger cleaning kit
  • Fan assembly

Related codes

Full explanation for AF
BLOCK

Your boiler has encountered an unknown internal error and has shut down as a safety precaution to prevent damage.

High DIY-safe £90-350

What to check first

  1. Press the Reset button on the control panel to see if the error clears
  2. Check that your gas supply is turned on and other gas appliances are working
  3. Turn the power supply to the boiler off and back on again after 30 seconds

Parts commonly replaced

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

Related codes

Full explanation for BLOCK
Fd

Your boiler is struggling to light or detect its own flame, which prevents it from firing up to provide heating and hot water.

High Engineer only £120-£250

Parts commonly replaced

  • Ignition electrode
  • Detection electrode
  • Ignition lead
  • PCB

Related codes

Full explanation for Fd
HL

Your boiler has overheated, causing a safety sensor to automatically shut the system down to prevent damage.

High Engineer only £120-280

Parts commonly replaced

  • Safety thermostat
  • Overheat limit sensor
  • Circulation pump

Related codes

Full explanation for HL
Showing 41–42 of 42
Lp

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

High DIY-safe £0-160

What to check first

  1. Locate the filling loop (a flexible silver hose) underneath or near the boiler.
  2. Slowly open the valve(s) to allow water into the system while watching the pressure gauge.
  3. Close the valve(s) tightly once the pressure reaches between 1.0 and 1.5 bar.

Parts commonly replaced

  • Water pressure switch
  • Filling loop valve
  • Expansion vessel

Related codes

Full explanation for Lp
tf

Your boiler has shut down because a safety sensor has detected that the exhaust gases are becoming too hot, which could indicate a blockage or a heat exchange problem.

High Engineer only £120-£280

Parts commonly replaced

  • Flue gas thermostat
  • Heat exchanger cleaning kit
  • Secondary 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 your bath and shower water won't heat up properly. Hb Diagnostic Your boiler has detected an issue with the sensor that monitors central heating temperature, meaning it can no longer safely regulate the heat going to your radiators. Ht Diagnostic Your boiler's internal pump has stopped circulating water, which prevents the system from providing heating and hot water. rb Diagnostic Your boiler has detected a problem 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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