Daikin Boiler Error Codes: Full Guide to Causes and Fixes 2026
Quick Answer: The most common Daikin boiler error codes are U4-65 and U4-66 (thermostat and CAN-bus communication faults), 8A-46 (freeze protection mode), HJ-09 (low system pressure), and 11-66/11-67 (flame loss). HJ-09 can be resolved by repressurising the system. A reset is worth attempting for U4 and 8A codes in appropriate conditions. All ignition, sensor, gas valve, and PCB fault codes require a Gas Safe registered engineer. Understanding ideal boiler error code meanings can help homeowners troubleshoot minor issues before calling in an expert. Familiarizing yourself with these common codes can lead to quicker resolutions and potentially save on repair costs. However, for persistent problems or more complex fault codes, it’s always best to consult a qualified technician.
Daikin is a Japanese manufacturer better known in the UK for its air conditioning and heat pump products, but also produces a range of gas condensing boilers under the Daikin Altherma brand. Like all boiler manufacturers, Daikin uses a fault code system to communicate problems on the control panel. Daikin’s codes are structured differently from most UK boiler brands — using alphanumeric combinations rather than simple letter or number sequences — but the diagnostic principle is the same. Boiler manuals for all Daikin models can be downloaded directly from the Daikin website by selecting the relevant model.
Common Daikin Boiler Error Codes
U4-65 — Defective OpenTherm Room Thermostat
The U4-65 code indicates a problem with the connection between the boiler and the room thermostat, specifically where the thermostat uses the OpenTherm communication protocol. This can be caused by a wiring fault between the thermostat and the boiler, a thermostat that has lost power or developed a fault, or a communication error between the two devices.
Attempting a boiler reset is the recommended first step, as a temporary communication dropout can trigger U4-65 and may clear without any further action. If the code returns after resetting, checking the thermostat has power — including replacing batteries if it is a wireless model — is a practical second step before calling an engineer. If neither action resolves U4-65, a Gas Safe engineer should inspect the wiring and thermostat connection.
Can you fix it yourself? Attempt a reset and check thermostat power. Call a Gas Safe engineer if U4-65 persists.
U4-66 — CAN-Bus Timeout
The U4-66 code indicates a timeout on the CAN-bus — the internal communication network that connects the boiler’s control components. A timeout occurs when one component on the network fails to respond within the expected time, which can be caused by a wiring fault, a failed component, or an electronic control board issue. As with U4-65, a reset is worth attempting as a first step. If the fault returns, a Gas Safe engineer should diagnose the CAN-bus communication system.
Can you fix it yourself? Attempt a reset. Call a Gas Safe engineer if U4-66 returns.
8A-46 — Freeze Protection Mode
The 8A-46 code indicates the boiler has activated its freeze protection mode, triggered when the flow temperature sensor reads below 1°C. This is a protective function rather than a fault — the boiler shuts down non-essential functions to protect the system from freezing damage during very cold conditions.
If outdoor temperatures have genuinely been below freezing, the 8A-46 is operating as intended. Switching on the frost protection setting within the boiler’s controls — which keeps the system at a minimum safe temperature during cold periods — is the most effective way to prevent this code from reappearing. If the temperature is above freezing when 8A-46 appears, a faulty flow temperature sensor may be producing an incorrect reading, in which case a reset is worth attempting and a Gas Safe engineer should investigate if the code returns.
Can you fix it yourself? Enable frost protection mode to prevent recurrence. Attempt a reset if ambient temperatures are above freezing. Call a Gas Safe engineer if 8A-46 appears without a cold weather cause.
HJ-09 — Low System Pressure
The HJ-09 code is the most straightforward Daikin fault code and indicates that the system water pressure has dropped below the minimum required for safe boiler operation. It is the only Daikin fault code that most homeowners can resolve without a Gas Safe engineer.
Repressurising the system via the filling loop — following the method described in the Daikin boiler manual for the specific model — and restoring pressure to the recommended level will clear HJ-09 in most cases. If pressure drops again shortly after being restored, a leak in the system is the likely cause and a Gas Safe engineer should locate and repair it.
Can you fix it yourself? Yes — repressurise via the filling loop to the manufacturer’s specified level. Call an engineer if pressure drops repeatedly.
11-66 and 11-67 — Flame Loss
These two codes both indicate flame loss — 11-66 when the flame signal is lost during the ignition sequence and 11-67 when the flame goes out during normal operation. Flame loss can be caused by a gas supply interruption, a failing gas valve, a faulty ignition electrode, or a blocked flue restricting the combustion air supply.
Confirming that other gas appliances in the property are operating normally rules out a gas supply issue. A single reset attempt is appropriate if the gas supply is confirmed. If either code returns after one reset, a Gas Safe engineer should inspect the ignition and gas delivery system before the boiler is restarted.
Can you fix it yourself? Check gas supply and attempt one reset. Call a Gas Safe engineer if the fault persists.
Full Daikin Boiler Error Code Reference
Gas Valve and Ignition Codes
| Code | Fault | DIY Fix? |
|---|---|---|
| 10-64 | Gas valve circuit fault | No — engineer required |
| 10-65 | Gas valve current fault | No — engineer required |
| 11-64 | Ignition error | Check gas supply, attempt reset — then engineer |
| 11-65 | Flame stabilisation error | No — engineer required |
| 11-66 | Flame signal lost | Check gas supply, attempt reset — then engineer |
| 11-67 | Flame loss during operation | Check gas supply, attempt reset — then engineer |
| E1-64 | Flame detected before burner operates | No — engineer required immediately |
Ion Control and SCOT System Codes
| Code | Fault | DIY Fix? |
|---|---|---|
| 12-64 | Ion control deviation too great | No — engineer required |
| 12-65 | Ignition SCOT actuator error | No — engineer required |
| 12-66 | Ion base value below minimum limit | No — engineer required |
| 12-67 | Ion base value exceeds maximum limit | No — engineer required |
| 12-68 | Ion base value differs from previous reading | No — engineer required |
| 12-69 | Offset adaptation reached limit | No — engineer required |
| E1-65 | Internal SCOT system error | No — engineer required |
| E1-66 | Calibration condition error | No — engineer required |
| E1-67 | Calibration missing | No — engineer required |
| E1-68 | Ion base value exceeds maximum limit | No — engineer required |
| E1-72 | SCOT flame amplifier fault | No — engineer required |
Fan and Overheating Codes
| Code | Fault | DIY Fix? |
|---|---|---|
| 13-64 / 13-65 | Fan speed error | No — engineer required |
| 16-64 | Flue temperature overheating | No — engineer required |
| 1J-64 | High limit thermostat overheating | No — engineer required |
| J6-01 | Flow temperature sensor overheating | No — engineer required |
| J6-20 | Return temperature sensor overheating | No — engineer required |
| J6-21 | Return temperature higher than flow temperature | No — engineer required |
Temperature and Sensor Codes
| Code | Fault | DIY Fix? |
|---|---|---|
| 80-01 | Temperature sensor error | No — engineer required |
| 81-01 | Flow temperature sensor fault | No — engineer required |
| 81-65 | Hot water temperature sensor fault | No — engineer required |
| 8A-46 | Freeze protection mode active | Enable frost protection — reset if above freezing |
| 8H-65 | Steep rise in flow temperature | No — engineer required |
| H9-01 | Outside sensor fault | No — engineer required |
| HC-01 | Hot water temperature sensor fault | No — engineer required |
Pressure Codes
| Code | Fault | DIY Fix? |
|---|---|---|
| HJ-08 | System pressure too high | Bleed a radiator — engineer if recurring |
| HJ-09 | System pressure too low | Yes — repressurise via filling loop |
| HJ-10 | Water pressure sensor fault | No — engineer required |
Electrical Supply Codes
| Code | Fault | DIY Fix? |
|---|---|---|
| JJ-64 | Supply voltage below minimum limit | Check consumer unit — contact electricity provider |
| U2-01 | Supply voltage above maximum limit | Contact electricity provider |
PCB and Control Card Codes
| Code | Fault | DIY Fix? |
|---|---|---|
| E1-69 / E1-70 | Parameter CRC fault | No — engineer required |
| E1-71 | EK lockout failure | No — engineer required |
| E1-73 | Printed circuit board error | No — engineer required |
| UA-64 | Boiler chip card blocked | No — engineer required |
| UA-65 | PCB update required | No — engineer required |
| UA-66 | Boiler chip card ID fault | No — engineer required |
| UA-67 | Boiler chip card missing | No — engineer required |
Communication Codes
| Code | Fault | DIY Fix? |
|---|---|---|
| U4-65 | Defective OpenTherm room thermostat connection | Attempt reset, check thermostat power — then engineer |
| U4-66 | CAN-bus timeout | Attempt reset — engineer if recurring |
| U4-67 | Remote reset supervision fault | No — engineer required |
Fault Code Detail: Key Secondary Codes
E1-64 — Flame Detected Before Burner Operates
This code is a significant safety lockout — it indicates the boiler’s flame detection system is registering a flame when the burner has not yet been instructed to fire. This most commonly indicates a gas valve that is not fully closing when the boiler is in standby, allowing gas to pass through and ignite. This is a potential safety concern that requires immediate inspection by a Gas Safe engineer. The boiler should not be reset and restarted without professional investigation.
8H-65 — Steep Rise in Flow Temperature
A steep and rapid rise in flow temperature indicates that heat is being generated faster than it can be dissipated through the heating circuit — most commonly caused by a pump failure, a closed radiator valve, or very low system pressure. This is an early warning sign of an overheating event and should be investigated by a Gas Safe engineer before the boiler develops a more serious fault.
HJ-08 — High System Pressure
High pressure above the maximum operating level can be reduced by bleeding a radiator to release a small volume of water from the system. However, if high pressure recurs without the system having been overfilled, a faulty expansion vessel or a pressure relief valve that is not opening correctly is the likely cause, and a Gas Safe engineer should investigate.
UA-64 to UA-67 — Boiler Chip Card Faults
The UA series of codes relate to the boiler chip card — a programmable card that stores the boiler’s configuration and operating parameters. A blocked card, a missing card, an ID mismatch, or a PCB that requires a firmware update will all trigger codes in this range. All require a Gas Safe engineer with Daikin product experience to resolve, as incorrect chip card configuration can affect all aspects of boiler operation.
JJ-64 and U2-01 — Electrical Supply Faults
These codes indicate the electrical supply to the boiler is outside the acceptable voltage range — too low for JJ-64 and too high for U2-01. Neither is a boiler fault in itself. Checking the consumer unit for tripped breakers and contacting the electricity network operator if the supply appears abnormal are the appropriate first steps. A Gas Safe engineer should only be involved if an internal boiler wiring fault is confirmed to be causing the reading. In some cases, the Vokera boiler error code solutions can help pinpoint issues related to specific error codes. Understanding these error codes is crucial for homeowners to troubleshoot before seeking professional assistance. By following the provided guidelines, one can ensure their boiler operates efficiently and safely. Further investigation may be necessary if you continue to experience issues, and in such cases, you may want to consult ferroli boiler error code solutions for specific troubleshooting guidance. Familiarizing yourself with these codes can help identify potential problems early on. If all else fails, it’s essential to seek professional assistance to ensure the safe and efficient operation of your boiler. In addition to these initial checks, it’s important to follow keston boiler troubleshooting steps to ensure the unit operates safely and efficiently. This may involve inspecting the boiler’s pressure settings, examining the flue system for blockages, and verifying the status of safety devices. Engaging with customer support can also provide guidance tailored specifically to your model problems or error codes.
FAQ
What does the Daikin HJ-09 fault code mean?
The Daikin HJ-09 fault code indicates the system water pressure has dropped below the minimum required for safe boiler operation. It is the most straightforward Daikin fault code and the only one most homeowners can resolve themselves. Repressurising via the filling loop to the level specified in the Daikin boiler manual will resolve HJ-09 in most cases. If pressure drops repeatedly after topping up, a leak in the heating system is the likely cause and a Gas Safe engineer should investigate.
Can I fix Daikin boiler faults myself?
HJ-09 low pressure can be resolved by the homeowner via the filling loop. Enabling frost protection mode resolves recurring 8A-46 activations during cold weather. Attempting a reset is appropriate for U4-65, U4-66, and 8A-46 in appropriate conditions. Checking the gas supply and attempting one reset is worth trying for 11-64, 11-66, and 11-67 before calling an engineer. Checking the consumer unit for tripped breakers is appropriate for JJ-64 and U2-01. All other Daikin fault codes require a Gas Safe registered engineer.
What does the Daikin 8A-46 code mean?
The Daikin 8A-46 code means the boiler has activated freeze protection mode after the flow temperature sensor detected a reading below 1°C. In genuinely cold conditions this is the boiler working as intended to prevent freezing damage. Enabling the frost protection setting within the boiler’s controls keeps the system at a safe minimum temperature and prevents 8A-46 from appearing during cold weather. If 8A-46 appears when temperatures are above freezing, a faulty flow sensor may be producing an incorrect reading and a Gas Safe engineer should investigate after a reset attempt.
What is the SCOT system on a Daikin boiler?
The SCOT system is Daikin’s Smart Combustion Optimisation Technology — an advanced combustion management system that monitors and adjusts the burner’s operation in real time using ionisation sensing. The ion-related codes (12-64 through 12-69 and E1-65 through E1-72) all relate to faults within this system. When SCOT-related codes appear, a Gas Safe engineer with Daikin product experience should carry out the diagnosis, as the calibration and parameter settings within the SCOT system require specialist knowledge to assess and correct.
What causes Daikin flame loss codes 11-66 and 11-67?
Code 11-66 indicates the flame signal was lost during the ignition sequence, while 11-67 indicates the flame went out during normal operation. Both can be caused by a gas supply interruption, a failing gas valve, a deteriorating ignition or detection electrode, or a blocked flue restricting combustion air. Checking that other gas appliances are working normally confirms the gas supply is active. A single reset attempt is appropriate before calling an engineer — if either code returns after one reset, a Gas Safe engineer should inspect the full ignition and gas delivery system.
How do I prevent Daikin boiler fault codes?
Annual servicing by a Gas Safe registered engineer is the most effective prevention strategy for Daikin boiler faults. During a service the engineer cleans the burner and ionisation components, checks gas pressure and combustion quality, inspects the flue, tests all sensors and their wiring, and confirms the SCOT system calibration is within correct parameters. Enabling the frost protection setting before cold weather prevents 8A-46 activations, and keeping the system pressure within the correct range prevents HJ-09. The Daikin boiler manual should be consulted to confirm the correct pressure range for the specific model. Understanding system boiler operating principles is essential for effective troubleshooting and maintenance. Familiarity with how different components interact can help identify potential issues before they escalate. Regular checks should include reviewing system pressures and verifying that all safety features are functioning as intended.
When should I replace a Daikin boiler rather than repair it?
A Daikin boiler under 10 years old with a single fault is almost always worth repairing. For boilers over 10 to 12 years old where multiple fault codes are recurring, or where major components such as the PCB, SCOT system, chip card, or heat exchanger require replacement, the economics of continued repair versus a new A-rated installation are worth reviewing honestly with a Gas Safe engineer. As Daikin is a less widely distributed boiler brand in the UK than Viessmann or Worcester Bosch, confirming that replacement parts are readily available for the specific model is also a practical consideration before committing to a major repair on an older unit.
Conclusion
Daikin’s alphanumeric fault code system is more complex than most UK boiler brands but provides detailed fault information across a broad range of components — from gas valve and SCOT combustion system codes through to chip card and CAN-bus communication faults. Of all the codes in this guide, HJ-09 is the only one that most homeowners can resolve independently, by repressurising the system via the filling loop. Reset attempts are appropriate for U4-65, U4-66, and 8A-46 in the right conditions, and electrical supply codes JJ-64 and U2-01 should prompt a check of the consumer unit before an engineer is called. When facing issues with your system, the Viessmann boiler troubleshooting tips can be a valuable resource. Understanding common faults and their solutions can empower homeowners to address minor problems without the need for professional assistance. Always remember to consult the user manual for specific guidance related to your model before attempting any repairs.
For all other Daikin fault codes — particularly the E1-64 flame detection safety lockout, which should be treated as an urgent safety concern — a Gas Safe registered engineer with Daikin product experience is the right call. Annual servicing and proactive frost protection configuration are the two most effective steps any Daikin boiler owner can take to prevent unexpected fault codes from disrupting heating and hot water supply.
