Daikin JA-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin JA-00 fault code mean?
The JA-00 code indicates that the high pressure sensor (also called the high pressure transducer) on the master outdoor unit has developed a fault. The outdoor unit's control board continuously monitors the voltage signal produced by this sensor. When that signal falls outside the expected range — due to an open circuit, a short circuit, or an outright sensor failure — the system raises the JA alarm and stops operating to protect the refrigerant circuit. The '-00' suffix identifies the fault as belonging to the master unit in a multi-unit or VRV arrangement, distinguishing it from JA-01, JA-02 and JA-03 codes which point to slave units.
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
- Faulty or failed high pressure transducer Common
The high pressure sensor itself has degraded or failed outright. The outdoor PCB reads a signal outside the permitted voltage window and raises the JA code. This is the single most common root cause and almost always requires the sensor to be replaced.
- Loose, corroded or damaged sensor connector Common
The wiring connector between the high pressure sensor and the outdoor PCB can work loose over time, corrode in the exposed outdoor environment, or suffer damage from vibration. A poor connection produces an unreliable or absent signal that the board interprets as a sensor fault.
- Open or short circuit in the sensor wiring loom Sometimes
A break or short in the cable running from the sensor to the PCB produces the same out-of-range signal as a dead sensor. Wiring faults can result from rodent damage, chafing against sharp sheet-metal edges, or water ingress into the outdoor unit.
- High and low pressure sensors swapped Sometimes
If the outdoor unit has recently been serviced or the sensors have been handled, there is a risk that the high pressure and low pressure sensors were reconnected to the wrong terminals. The PCB will see an incorrect voltage and log a JA fault even though the sensor itself is undamaged.
- Defective outdoor unit PCB Rare
If the sensor and all its wiring test within specification yet the fault persists, the outdoor control board itself may have failed and is no longer reading the sensor signal correctly. PCB faults are less common but should be considered once all other causes are ruled out.
How to fix it
- Switch the heat pump off at the controller and leave it powered down for two to three minutes, then restart DIY safe
A transient electrical glitch can occasionally cause a spurious sensor fault. A clean restart clears the fault memory and allows the system to re-read the sensor. If JA-00 returns immediately or within a few minutes, the fault is genuine and the following steps apply.
- Visually inspect the outdoor unit for obvious physical damage or debris DIY safe
Walk around the outdoor unit and check that no leaves, ice or debris are blocking the casing or air paths. Check that the unit has not been struck or flooded. Report any visible damage to your engineer — it may be relevant to diagnosing the fault.
- Check the system's electrical supply and consumer unit DIY safe
Confirm the dedicated MCB or fuse for the heat pump has not tripped. If it has tripped, reset it once. If it trips again, do not reset it a second time — call an engineer. Also confirm the outdoor unit's local isolator switch is in the ON position.
- Do not attempt to open the outdoor unit or touch any internal components heat pump engineer
The high pressure sensor is mounted inside the outdoor unit on the refrigerant circuit. Accessing it requires removing panels, working near live electrical terminals, and potentially disturbing refrigerant pipework. This must only be done by a qualified engineer.
- Arrange for an MCS-certified heat pump engineer to inspect and test the sensor and its wiring heat pump engineer
A qualified engineer will remove the outdoor unit panels safely, locate the high pressure transducer (marked X17A on Daikin wiring diagrams), disconnect the connector and check it for corrosion or looseness, then measure the sensor's output voltage against the manufacturer's specification. They will also verify that the high and low pressure sensors are connected to the correct terminals.
- Have the engineer replace the high pressure sensor if it fails testing heat pump engineer
If the sensor output voltage is outside specification or the sensor does not respond correctly, it must be replaced with a compatible Daikin part. The refrigerant circuit does not need to be opened for this repair on most models, but the work still involves electrical components inside the outdoor unit and must be carried out by a qualified engineer.
- If the sensor tests satisfactory, have the engineer assess the outdoor PCB heat pump engineer
A sensor and wiring loom that both test within specification point to a faulty outdoor control board. PCB replacement requires an engineer who is familiar with Daikin outdoor unit wiring and should be sourced through a Daikin-authorised parts channel to ensure compatibility. Any work involving the refrigerant circuit — for example if a sensor is found to be leaking — requires an F-Gas registered engineer.
Parts you may need
- Daikin high pressure sensor / transducer (outdoor unit) · from £65
- Sensor wiring loom / connector repair kit · from £20
- Daikin outdoor unit PCB (control board) · from £320
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–£500, depending on the underlying cause.
Frequently asked questions
Is the Daikin JA-00 fault dangerous — can I keep using the heat pump?
The heat pump will have locked out automatically when JA-00 appeared, so it will not be running. You should leave it off until the fault is investigated. The code itself relates to a sensor rather than a refrigerant leak or electrical short, but the system cannot safely regulate high-side pressure without a working sensor, so operating the unit in a fault state could damage the compressor.
How much does it typically cost to fix a Daikin JA-00 fault in the UK?
For most households, the repair falls between £150 and £500. A wiring or connector issue at the lower end can be resolved for around £150–£200 including the engineer's callout. A sensor replacement (part and labour) typically comes to £200–£400. If the outdoor PCB needs replacing, costs rise to £500–£800 or more. A replacement compressor — which is rarely the cause of a JA-00 fault — can exceed £1,000, but that scenario is very unlikely with this particular code.
Will a Daikin JA-00 fault be covered under warranty?
It may be, depending on the age of your system and the cause of the fault. Daikin typically offers a two to five year warranty depending on the model and whether the system was registered. If the sensor has failed due to a manufacturing defect and the system is within the warranty period, the repair should be covered. However, faults caused by improper installation (for example sensors swapped at commissioning) or physical damage are usually excluded. Contact a Daikin-authorised engineer before arranging any independent repair, as using an unauthorised engineer can void the remaining warranty.
What is the difference between fault codes JA-00, JA-01, JA-02 and JA-03 on a Daikin system?
All four codes relate to the same underlying fault — a malfunction of the high pressure sensor — but on different units within a multi-unit or VRV installation. JA-00 (or a plain JA) means the fault is on the master outdoor unit. JA-01, JA-02 and JA-03 identify the fault on the first, second and third slave outdoor units respectively. On a single-unit heat pump installation you will only ever see JA or JA-00.