Daikin D2CND Fault codes & diagnostics

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

Combi Natural GasLPG 24-35 kW 2017-present
GC Numbers 47-439-0147-439-0247-439-03

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Severity at a glance

Each cell = one fault code. Hover to identify.
1 Emergency 35 High 3 Medium 7 Low
Just want the most common Daikin D2CND faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 46 documented codes

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1J-64

Your boiler has shut down because it has detected it is getting too hot, often due to low water pressure or poor circulation.

High DIY-safe £100-250

What to check first

  1. Check the pressure gauge and top up the water to between 1 and 1.5 bar if it is low
  2. Ensure at least two or three radiator valves are fully open to allow water to flow
  3. Restart the boiler using the reset button to see if the fault clears

Parts commonly replaced

  • Circulation Pump
  • Overheat Thermostat
  • Diverter Valve

Related codes

Full explanation for 1J-64
8A-46

Your boiler has detected that the water inside it is too cold and has switched itself off to prevent internal pipes from freezing and bursting.

High DIY-safe £0-120

What to check first

  1. Check that your home heating is turned on and the thermostat is set to a calling temperature.
  2. Gently warm the room where the boiler is located using an electric heater to raise the ambient temperature.
  3. Verify that your condensate pipe (the plastic pipe leading outside) isn't frozen, and thaw it with warm water if necessary.
  4. Reset the boiler once the area is warmer and flow has been restored.

Parts commonly replaced

  • Flow temperature sensor (NTC)
  • Trace heating cable (if installed)
Full explanation for 8A-46
8H-65

The water inside your boiler is heating up too quickly because it cannot flow through your radiators properly.

High DIY-safe £0-120

What to check first

  1. Ensure all your radiator valves (TRVs) are turned to the highest setting
  2. Check that any bypass valves or zone valves in your system are open
  3. Restart the boiler once you have ensured water can flow through the radiators

Parts commonly replaced

  • None (usually a settings issue)
  • Circulation pump
  • Bypass valve
Full explanation for 8H-65
10-64

Your boiler has detected a communication issue with its internal gas control system and has shut down as a safety precaution.

High DIY-safe £250-450

What to check first

  1. Locate the reset button or dial on the front control panel
  2. Press and hold the reset button for 5 seconds
  3. Wait for the boiler to complete its restart sequence to see if the flame ignites

Parts commonly replaced

  • Gas Valve
  • Main Printed Circuit Board (PCB)
  • Wiring Loom

Related codes

Full explanation for 10-64
10-65

Your boiler has detected an electrical issue with the gas control system and has shut down as a safety precaution.

High DIY-safe £180-350

What to check first

  1. Locate the reset button on the boiler control panel
  2. Press and hold the reset button for 2-3 seconds
  3. Wait for the boiler to attempt to restart
  4. If the code persists, contact a Gas Safe registered engineer

Parts commonly replaced

  • Gas valve
  • Main PCB (Printed Circuit Board)
  • Wiring harness

Related codes

Full explanation for 10-65
11-64

Your boiler is having trouble lighting the gas to start the heating process and may need to be restarted.

High DIY-safe £120-£350

What to check first

  1. Check that your gas emergency control valve is in the 'on' position
  2. Check if other gas appliances in your home are working
  3. Press the 'Reset' button on the boiler control panel up to three times

Parts commonly replaced

  • Gas Valve
  • Ignition Electrode
  • Spark Cable

Related codes

Full explanation for 11-64
11-65

Your boiler is struggling to keep the flame steady and will keep trying to restart itself until it succeeds.

High DIY-safe £120-350

What to check first

  1. Wait for the boiler to finish its automatic restart cycles
  2. Check that your gas meter has credit (if applicable)
  3. Ensure other gas appliances in your home are working correctly

Parts commonly replaced

  • Ignition electrode
  • Flame sensing probe
  • Gas valve

Related codes

Full explanation for 11-65
11-66

Your boiler is struggling to light or recognize that a flame has started, causing it to shut down for safety after several failed attempts.

High DIY-safe £120-350

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 meter is in credit if you use a prepay meter
  3. Locate the reset button on the control panel and press it once to attempt a restart

Parts commonly replaced

  • Ignition Electrode
  • Ionisation Probe
  • Gas Valve
  • Condensate Trap (if blocked)

Related codes

Full explanation for 11-66
11-67

Your boiler has temporarily lost its flame while running and is attempting to restart itself automatically.

Low DIY-safe £0-120

What to check first

  1. Wait for the boiler to complete its automatic restart process
  2. Check that your external gas supply is turned on (check other gas appliances)
  3. Reset the boiler if the error persists after several minutes

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead

Related codes

Full explanation for 11-67
12-64

Your boiler is having trouble managing the flame correctly and has locked itself for safety after failing to stabilise.

High DIY-safe £120-£350

What to check first

  1. Locate the reset button on your boiler's control panel
  2. Press and hold the reset button for at least 5 seconds
  3. Wait for the boiler to attempt to restart; if the code returns immediately, call a Gas Safe engineer

Parts commonly replaced

  • Ionisation electrode
  • Ignition lead
  • Gas valve
  • PCB
Full explanation for 12-64
Showing 11–20 of 46
12-65

Your boiler is having trouble starting the burner safely and needs a simple restart to clear a temporary electronic glitch.

High DIY-safe £100-£250

What to check first

  1. Locate the Reset button on the boiler control panel
  2. Press and hold the Reset button for 5 seconds
  3. Wait for the boiler to complete its startup sequence

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • PCB (Control Board)

Related codes

Full explanation for 12-65
12-66

Your boiler is having trouble detecting the steady flame it needs to run safely, but it may just be a temporary sensing error.

Low DIY-safe £100-220

What to check first

  1. Locate the reset button (usually a circular arrow or 'Reset' text) on the control panel.
  2. Press and hold the reset button for five seconds.
  3. Wait for the boiler to restart and monitor if the code clears.

Parts commonly replaced

  • Ionisation electrode
  • Ignition lead
  • Burner gasket

Related codes

Full explanation for 12-66
12-67

Your boiler's flame sensor is detecting an unusual reading that is outside of its normal operating range, which has caused the system to stop as a safety precaution.

High DIY-safe £120-280

What to check first

  1. Locate the reset button on the boiler control panel
  2. Press and hold the reset button for 2-3 seconds
  3. Wait for the boiler to complete its restart sequence and check if the code clears

Parts commonly replaced

  • Ionisation electrode
  • Ignition lead
  • Control PCB

Related codes

Full explanation for 12-67
12-68

Your boiler has failed to correctly detect its own flame during start-up, likely due to a minor sensor reading inconsistency.

High DIY-safe £90-220

What to check first

  1. Locate the Power/Reset button on the front control panel
  2. Press and hold the Reset button for 2-3 seconds
  3. Wait up to 5 minutes for the boiler to complete its restart sequence

Parts commonly replaced

  • Ionisation electrode
  • Ignition cable
  • Burner gasket

Related codes

Full explanation for 12-68
12-69

Your boiler is struggling to automatically adjust its internal settings to ensure the gas burns efficiently, which may lead to it shutting down for safety.

High Engineer only £120-350

Parts commonly replaced

  • Ionisation electrode
  • Gas valve
  • Burner seal

Related codes

Full explanation for 12-69
13-64 / 13-65

Your boiler's fan is not spinning at the correct speed to safely clear waste gases, causing the system to shut down for safety.

High DIY-safe £250-£450

What to check first

  1. Press the Reset button on the boiler control panel once.
  2. Wait for the boiler to complete its startup sequence.
  3. Ensure there are no external blockages to the flue pipe outside your home.

Parts commonly replaced

  • Fan assembly
  • Main PCB
  • Wiring harness

Related codes

Full explanation for 13-64 / 13-65
16-64

Your boiler has shut down because the exhaust gases are getting too hot, indicating a potential blockage or safety issue in the flue pipe.

Emergency Engineer only £120-350

Parts commonly replaced

  • Flue thermistor
  • Heat exchanger (cleaning/descale)
  • Burner seals
Full explanation for 16-64
80-01

Your boiler has detected a problem with the sensor that monitors the temperature of the water returning to the unit, causing it to stop for safety.

High DIY-safe £120-£190

What to check first

  1. Press the Reset button on the boiler control panel once
  2. Wait for the boiler to complete its restart sequence
  3. If the code persists, contact a Gas Safe registered engineer

Parts commonly replaced

  • Return temperature sensor (NTC thermistor)
  • Connecting wiring harness

Related codes

Full explanation for 80-01
81-01

Your boiler has detected an issue with the sensor that measures the temperature of the water leaving the unit, which usually results in the system shutting down for safety.

High DIY-safe £100-180

What to check first

  1. Locate the power button or reset switch on the boiler control panel
  2. Press and hold the reset button for five seconds
  3. Wait for the boiler to restart and check if the error code clears

Parts commonly replaced

  • Flow temperature NTC sensor
  • Wiring harness

Related codes

Full explanation for 81-01
81-65

Your boiler is still providing heating and hot water, but it has detected a problem with the temperature sensor for your solar thermal system.

Low Engineer only £90-180

Parts commonly replaced

  • Solar temperature sensor (NTC)
  • Sensor wiring harness
Full explanation for 81-65
Showing 21–30 of 46
E1-64

Your boiler has sensed a flame inside the system before it was even supposed to start up, causing it to shut down for safety.

High DIY-safe £120-350

What to check first

  1. Locate the reset button on your boiler's control panel
  2. Press and hold the reset button for five seconds
  3. Wait for the boiler to restart and check if the error reappears

Parts commonly replaced

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

Related codes

Full explanation for E1-64
E1-65

Your boiler's internal electronic self-monitoring system has detected a technical fault and has shut down as a safety precaution.

High DIY-safe £150-450

What to check first

  1. Locate the Power/Reset button on the front control panel
  2. Press and hold the Reset button for 2 seconds
  3. Wait for the boiler to complete its restart sequence to see if the central heating resumes

Parts commonly replaced

  • Main Control PCB
  • Gas Valve
  • SCOT Electrode

Related codes

Full explanation for E1-65
E1-66

Your boiler is struggling to automatically adjust its internal settings for the burner to stay lit and is attempting to restart itself.

Medium DIY-safe £100-250

What to check first

  1. Reset the boiler using the front control panel
  2. Wait for the boiler to complete its automatic recalibration sequence
  3. Ensure the gas supply to the property is active

Parts commonly replaced

  • Gas valve
  • Ionisation electrode
  • Control PCB

Related codes

Full explanation for E1-66
E1-67

Your boiler has lost its internal settings and needs to be restarted to recalibrate itself.

High DIY-safe £80-250

What to check first

  1. Locate the reset button on the boiler control panel
  2. Press and hold the reset button for at least 2 seconds
  3. Wait up to 10 minutes for the boiler to complete its automatic calibration cycle

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Gas Valve

Related codes

Full explanation for E1-67
E1-68

Your boiler has lost track of the basic flame sensor settings required to run safely and needs to be restarted to recalibrate itself.

High DIY-safe £120-350

What to check first

  1. Locate the power or reset button on the front control panel
  2. Press and hold the reset button for 5 seconds
  3. Wait for the boiler to complete its restart sequence to see if the error clears

Parts commonly replaced

  • Ionisation electrode
  • Burner controller PCB
  • Gas valve

Related codes

Full explanation for E1-68
E1-69 / E1-70

Your boiler’s internal computer has encountered a software error while checking its settings, which has caused the system to stop working for safety.

High DIY-safe £120-450

What to check first

  1. Locate the power switch or fused spur for the boiler
  2. Turn the power off for 30 seconds
  3. Turn the power back on and press the 'Reset' button on the display panel
  4. Monitor the boiler to see if the error screen clears

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Parameter Plug

Related codes

Full explanation for E1-69 / E1-70
E1-71

Your boiler has experienced a fundamental electronic control failure that has caused it to lock down for safety.

High Engineer only £250-500

Parts commonly replaced

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

Related codes

Full explanation for E1-71
E1-72

Your boiler has detected an internal communication error with its ignition system and has locked itself out for safety.

High DIY-safe £180-450

What to check first

  1. Locate the power button/reset icon on the boiler's control panel
  2. Press and hold the reset button for 5 seconds
  3. Wait for the boiler to restart; if the code persists or returns immediately, you must call a Gas Safe engineer

Parts commonly replaced

  • Main PCB (Control Board)
  • Flame Sensing Electrode

Related codes

Full explanation for E1-72
E1-73

Your boiler's internal computer has encountered a software glitch or a technical hiccup and needs a system restart to try and clear the error.

High DIY-safe £350-£600

What to check first

  1. Locate the Reset button on the boiler control panel
  2. Press and hold the Reset button for 2-3 seconds
  3. Wait for the boiler to complete its startup sequence and check if the code clears

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for E1-73
H9-01

Your boiler is struggling to understand the outdoor temperature, which may cause your heating to run less efficiently or at the wrong temperature.

Low DIY-safe £90-160

What to check first

  1. Locate the reset button (usually a circular arrow or 'R') on the front control panel.
  2. Press and hold the reset button for 2 to 3 seconds.
  3. Wait for the boiler to restart; if the code reappears, a professional is required to check the wiring.

Parts commonly replaced

  • External temperature sensor
  • Sensor wiring harness

Related codes

Full explanation for H9-01
Showing 31–40 of 46
HC-01

Your boiler has a fault with the thermometer that measures hot water temperature, meaning your hot water may be unreliable although your heating should still work.

Low Engineer only £90-160

Parts commonly replaced

  • DHW NTC Temperature Sensor
  • Connecting Cable Harness
Full explanation for HC-01
HJ-08

Your boiler has too much water pressure and needs some drained out to reach a safe operating level.

Medium DIY-safe £0-120

What to check first

  1. Check that the filling loop valves are fully closed
  2. Bleed water from a radiator using a bleed key to reduce pressure
  3. Monitor the pressure gauge until it reaches approximately 1.5 bar
  4. Restart the boiler

Parts commonly replaced

  • Filling loop valve
  • Expansion vessel
  • Pressure relief valve

Related codes

Full explanation for HJ-08
HJ-09

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

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver flexible hose) underneath or near the boiler.
  2. Slowly open the one or two taps/levers on the filling loop until you hear water moving.
  3. Watch the pressure gauge and close the taps firmly once the needle reaches 1.5 bar.

Parts commonly replaced

  • Filling loop valve
  • Pressure sensor
  • Expansion vessel

Related codes

Full explanation for HJ-09
HJ-10

Your boiler has lost the ability to accurately measure the water pressure, which prevents it from operating safely to heat your home.

High Engineer only £140-£260

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness

Related codes

Full explanation for HJ-10
J6-01

Your boiler is getting too hot too quickly because water isn't flowing through the system properly or the pressure has dropped too low.

High DIY-safe £100-£250

What to check first

  1. Check the pressure gauge and top up the water to 1.5 bar if it is low
  2. Ensure at least two or three radiator valves (TRVs) are fully open
  3. Restart the boiler to see if the error clears

Parts commonly replaced

  • Flow Temperature Sensor (NTC)
  • Circulation Pump
  • External Filling Loop
Full explanation for J6-01
J6-20

Your boiler has detected that the water returning to the unit is too hot, which has caused it to shut down to prevent damage.

High DIY-safe £120-280

What to check first

  1. Ensure all radiator valves are fully open to allow water to circulate.
  2. Check that the central heating system pressure is between 1.0 and 1.5 bar.
  3. Press the Reset button on the boiler control panel to clear the lockout.

Parts commonly replaced

  • Return temperature sensor (NTC)
  • Circulation pump
  • Main Heat Exchanger (if blocked)

Related codes

Full explanation for J6-20
J6-21

Your boiler has detected that the water coming back from your radiators is hotter than the water going out, which usually means there is a blockage or the internal sensors are confused.

Medium DIY-safe £100-280

What to check first

  1. Check that all radiator valves (TRVs) are fully open
  2. Ensure the boiler pressure is between 1.0 and 1.5 bar
  3. Reset the boiler to see if the temporary sensor error clears

Parts commonly replaced

  • Flow temperature thermistor
  • Return temperature thermistor
  • Circulation pump

Related codes

Full explanation for J6-21
JJ-64

Your boiler is not receiving enough electrical power from your home's mains supply to operate safely.

High DIY-safe £90-250

What to check first

  1. Check if there is a power cut or brownout in your local area
  2. Ensure your fuse box / consumer unit has not tripped
  3. Turn the boiler off at the fused spur switch, wait 30 seconds, and turn it back on to reset the electronics

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • External Voltage Regulator
  • Electrical Wiring
Full explanation for JJ-64
U2-01

Your boiler has detected that the electrical supply coming into the unit is currently too high, which could cause damage to internal parts if left unchecked.

Low Engineer only £80-250

Parts commonly replaced

  • PCB (Main Controller)
  • Voltage Regulator
Full explanation for U2-01
U4-65

Your boiler is still providing heat and hot water, but it has lost communication with your smart room thermostat, meaning you cannot control the temperature remotely.

Low DIY-safe £80-180

What to check first

  1. Check that your room thermostat has power and functional batteries.
  2. Ensure the thermostat bridge or receiver is plugged in and switched on.
  3. Inspect the wiring connection between the thermostat wall plate and the boiler for any loose cables.

Parts commonly replaced

  • OpenTherm Communication Cable
  • Smart Thermostat receiver
  • Room Thermostat

Related codes

Full explanation for U4-65
Showing 41–46 of 46
U4-66

Your boiler's internal components have stopped communicating with each other, meaning the system cannot send or receive the signals needed to run.

High Engineer only £150-£450

Parts commonly replaced

  • Wiring harness
  • Main PCB (Printed Circuit Board)
  • User interface display board
Full explanation for U4-66
U4-67

Your boiler has experienced a communication error that has stopped it from working, and it requires a manual power reset to clear the internal memory.

High DIY-safe £100-250

What to check first

  1. Locate the fused spur switch or main power switch for the boiler
  2. Turn the power 'Off' and wait for at least 30 seconds
  3. Turn the power back 'On' and wait for the boiler to restart

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Wiring connection harness

Related codes

Full explanation for U4-67
UA-64

Your boiler's computer memory card is experiencing a communication error or has failed during an internal update, causing the system to stop working.

High Engineer only £120-450

Parts commonly replaced

  • Boiler Chip Card (BCC)
  • Main PCB (Printed Circuit Board)

Related codes

Full explanation for UA-64
UA-65

Your boiler's main control board is currently unable to communicate with its internal memory chip and needs a software or hardware synchronisation.

High Engineer only £120-450

Parts commonly replaced

  • Boiler Chip Card (BCC)
  • Printed Circuit Board (PCB)
Full explanation for UA-65
UA-66

Your boiler's internal memory chips are having trouble communicating with each other, meaning the control board doesn't recognise the boiler's specific settings.

High Engineer only £180-450

Parts commonly replaced

  • Boiler Chip Card (BCC)
  • Main PCB (Printed Circuit Board)

Related codes

Full explanation for UA-66
UA-67

Your boiler is unable to start because it cannot detect its internal memory chip, which contains the essential programming it needs to operate.

High Engineer only £120-250

Parts commonly replaced

  • Boiler Chip Card (BCC)
  • Main PCB (Printed Circuit Board)

Related codes

Full explanation for UA-67