Daikin HJ-10 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin HJ-10 fault code mean?
The HJ-10 code appears on certain Daikin Altherma heat pump and Altherma H Hybrid systems. It sits within the HJ fault-code family, which Daikin uses to flag hydraulic and system-monitoring issues — the neighbouring codes HJ-08 and HJ-09 relate to high and low system water pressure respectively, while HJ-11 points to a temperature-sensor abnormality. HJ-10 most likely indicates a system monitoring or hydraulic abnormality detected by the unit's control board, though the precise definition varies between Daikin product lines and is not always published in end-user documentation. If you are unsure whether your display shows HJ-10 or a similar code, check your model's nameplate and cross-reference with Daikin UK or an MCS-certified installer before taking action.
General guidance only — not a substitute for professional advice, and not certified by an heat pump engineer. Always have your specific appliance assessed by a qualified professional. Any heat pump work must be carried out by an MCS-certified heat pump engineer. If you notice a refrigerant leak, a burning smell or water leaking from the unit, switch it off at the isolator and call an MCS-certified heat pump engineer. Refrigerant work must be carried out by an F-Gas registered engineer.
Common causes
- Low or unstable heating-circuit water pressure Common
Because the adjacent HJ-08 and HJ-09 codes both relate to system water pressure, a pressure reading outside the normal 1–2 bar operating band is the first thing to rule out. A slow leak, a recently bled radiator, or a failing pressure-relief valve can all cause the circuit pressure to drift and trigger an HJ-series alert.
- Wiring fault or loose connection on a monitoring sensor Common
The control board relies on clean signals from flow, return, and buffer-tank sensors to track system health. A corroded connector, a wire that has worked loose, or a sensor that has drifted out of calibration can produce a spurious HJ-series code without any actual hydraulic problem being present.
- Faulty or air-locked circulation pump Sometimes
If the internal or external circulation pump is not moving water at the expected flow rate — due to air-locking after a system top-up, bearing wear, or an electrical fault — the control board may log an HJ-series warning because it cannot confirm healthy circuit operation.
- Control board (PCB) fault Rare
In rare cases the main controller itself generates incorrect fault codes. This is more common on systems that have experienced a power surge or where condensation has reached the electronics over time.
- Display misread or model-specific sub-code Sometimes
Daikin's two-part code display (e.g. HJ-09 vs HJ-10) can sometimes be misread, particularly on older LCD panels or partially obscured screens. Some Altherma H Hybrid models also use boiler-side sub-codes in this numeric range that do not map directly to the main outdoor-unit fault list.
How to fix it
- Note the exact code and your model number DIY safe
Before doing anything else, photograph the display showing HJ-10 and find the model and serial number on the unit nameplate (usually on the outdoor unit casing). This information is essential for Daikin UK or an engineer to look up the precise definition for your product line.
- Check the heating-circuit pressure gauge DIY safe
Look at the pressure gauge on your indoor unit or hydraulic station. It should read between 1.0 and 2.0 bar when the system is cold. If it is below 1.0 bar, top up via the filling loop until you reach approximately 1.2–1.5 bar, then close the filling loop valve fully. If it is above 2.5 bar, do not attempt to adjust it yourself — call an engineer.
- Inspect the outdoor unit for obstructions DIY safe
Check that the outdoor unit's air intake and discharge grilles are clear of leaves, debris, or compacted snow. Restricted airflow can stress the system and contribute to control-board alerts. Maintain at least 30 cm clearance around the unit.
- Check the consumer unit for a tripped MCB DIY safe
Go to your consumer unit (fuse board) and check whether the MCB or RCD protecting the heat pump circuit has tripped. If it has tripped once, reset it. If it trips again immediately or repeatedly, leave it off and call an engineer — do not keep resetting it.
- Perform a controlled power reset DIY safe
Switch the heat pump off at the dedicated isolator or controller, wait at least 5 minutes for the control board to fully discharge, then restore power. Allow the unit to complete its start-up sequence (this can take several minutes). If HJ-10 clears and does not return within 24 hours, monitor the system closely over the next few days.
- Do not attempt further investigation if the code returns heat pump engineer
If HJ-10 reappears after a reset, avoid repeated cycling of the unit. Continuing to override a persistent fault can mask a developing problem and may cause damage to the compressor or other sealed-circuit components. Leave the unit off and arrange an engineer visit.
- Call an MCS-certified heat pump engineer heat pump engineer
Because HJ-10 is not universally documented across all Daikin Altherma variants, an engineer with Daikin product training will be able to read the full fault history from the service menu, identify whether this is a hydraulic, sensor, or PCB-related issue, and carry out any necessary repairs. Any work involving the refrigerant circuit must be performed by an F-Gas registered engineer. Check whether your system is within Daikin's 5–7 year warranty period, as covered repairs may cost you nothing.
Parts you may need
- Circulation pump (Grundfos or Wilo, Daikin-compatible) · from £120
- Pressure-differential or flow sensor · from £55
- Expansion vessel (8–12 litre, heating circuit) · from £65
- Main PCB / control board (Daikin Altherma) · from £320
- Pressure-relief valve · from £35
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£450, depending on the underlying cause.
Frequently asked questions
Is HJ-10 a serious fault — will my heat pump stop working completely?
HJ-10 is logged as a warning rather than a full lockout on most Daikin Altherma systems, so the unit may continue to operate in a reduced or safe-mode state. However, you should not ignore it. If the underlying cause is a pressure or sensor issue that goes unattended, the system can escalate to a full lockout to protect itself, potentially leaving you without heating or hot water. Get it checked promptly.
Can I fix HJ-10 myself, or do I need an engineer?
The homeowner checks — verifying system pressure, clearing debris from the outdoor unit, and performing a single power reset — are safe and worth trying first. However, if the code returns, the repair almost certainly involves sensor testing, pump diagnostics, or control-board work, all of which require professional tools and training. Any fault that touches the refrigerant circuit additionally requires an F-Gas registered engineer. In short: try the basic checks, but plan for an engineer visit if the code persists.
Could I be misreading the code — might it actually be HJ-09 or HJ-11?
Yes, this is genuinely possible. Daikin's LCD displays can be difficult to read in bright light or if the screen is ageing, and HJ-09 (low pressure) and HJ-11 (sensor fault) look similar to HJ-10 at a glance. HJ-09 is the most common code in this family and the most straightforward to resolve by topping up system pressure. Double-check the display carefully and, if in doubt, photograph it and share it with a Daikin-approved engineer or Daikin UK's technical support line.
My system is still under warranty — will this repair be covered?
Daikin typically offers a 5–7 year parts and labour warranty on Altherma systems installed by an MCS-certified, Daikin-registered installer. If your system is within that window and the fault is not caused by accidental damage or lack of annual servicing, the repair is likely to be covered at no cost to you. Contact your original installer first; if they are unavailable, contact Daikin UK directly with your model number and installation date before commissioning an independent engineer, as using a non-approved contractor can in some cases affect warranty validity.