Daikin J5-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin J5-00 fault code mean?
The J5-00 fault code points to a problem with the suction pipe thermistor on the outdoor unit. This sensor monitors the temperature of the refrigerant as it returns to the compressor. When the heat pump's control board detects that the thermistor's resistance is outside its expected range — whether due to an open circuit, a short circuit, or a degraded sensor — it raises the J5-00 alarm and shuts down to protect the system. The '00' suffix identifies this as a fault on the primary circuit. Because the suction pipe thermistor feeds critical data into the heat pump's operating logic, an invalid reading can cause the unit to behave erratically or refuse to run at all.
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
- Degraded or failed suction pipe thermistor Common
The thermistor is an electronic component that slowly degrades with age, heat cycling, and exposure to the elements. Over time its resistance characteristics shift until it can no longer deliver an accurate reading, causing the control board to flag an abnormality. This is the single most frequent root cause of a J5-00 alarm.
- Open or short circuit in the thermistor wiring or connector Common
The wiring harness running from the suction pipe thermistor to the outdoor unit PCB can suffer corrosion at the connector pins, chafing on a sharp edge, or simply work loose over years of vibration. Either an open circuit (broken connection) or a short circuit (wires touching) will produce an out-of-range resistance reading and trigger the fault.
- Faulty outdoor unit PCB Sometimes
If the thermistor and its wiring both test within specification, the problem may lie with the input circuit on the outdoor unit's main control board. Moisture ingress, insect damage, or general component wear can corrupt the readings the PCB takes from the sensor, causing a false J5-00 alarm.
How to fix it
- Restart the heat pump from the indoor controller DIY safe
Switch the unit off at the controller, wait 60 seconds, then power it back on. A transient electrical disturbance can occasionally cause a spurious thermistor fault. If the J5-00 clears and does not return within a normal heating cycle, monitor the unit closely over the next 24 hours. If the fault reappears, proceed to the next steps.
- Check the outdoor unit's electrical isolator and consumer unit MCB DIY safe
Go to your consumer unit and confirm the MCB for the heat pump circuit has not tripped. Then check that the external isolator adjacent to the outdoor unit is fully switched on. Reset any tripped breaker once only. If it trips again immediately, do not reset it — call an engineer.
- Clear the area around the outdoor unit DIY safe
While you are outside, check that leaves, debris, or compacted snow are not blocking the unit's grilles or airflow paths. This will not clear the J5-00 fault directly, but a restricted unit runs hotter and can accelerate sensor wear. Ensure at least 30 cm of clear space around all sides.
- Check system water pressure DIY safe
Look at the pressure gauge on your indoor unit or buffer vessel. The system should normally read between 1.0 and 2.5 bar when cold. If pressure is low, top up via the filling loop following your installer's guidance. Low pressure is not a direct cause of J5-00 but can produce co-occurring faults that complicate diagnosis.
- Contact an MCS-certified heat pump engineer to inspect the thermistor and wiring heat pump engineer
A qualified engineer will access the outdoor unit, locate the suction pipe thermistor (labelled R2T in Daikin service documentation), and measure its resistance — typically expected around 10–12 kΩ at 25 °C. They will also inspect the wiring harness and connector for corrosion, chafing, or loose pins. If resistance is out of range, the thermistor is replaced. This work involves opening the outdoor unit casing and should not be attempted by a homeowner.
- If the thermistor tests within specification, have the engineer assess the outdoor unit PCB heat pump engineer
When the sensor and wiring check out correctly, the fault lies in the control board's input circuitry. The engineer will inspect the PCB for signs of moisture damage, burnt components, or corroded tracks, and replace the board if necessary. PCB replacement on Daikin outdoor units typically requires the unit to be fully powered down and the replacement board to be configured to match the system.
Parts you may need
- Daikin suction pipe thermistor (R2T) · from £35
- Thermistor wiring harness / connector pigtail · from £20
- Outdoor unit main PCB (A1P) · from £280
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–£380, depending on the underlying cause.
Frequently asked questions
Can I keep using my heat pump while the J5-00 fault is showing?
It is not advisable. With an invalid suction pipe temperature reading, the control board cannot manage the refrigerant circuit correctly. Running the unit in this state risks the compressor operating outside safe conditions, which could turn a straightforward £150–£200 thermistor repair into a far more expensive compressor replacement. Switch the system off and call an engineer promptly.
Is J5-00 likely to be covered under my Daikin warranty?
If your heat pump is still within its manufacturer warranty period and has been serviced annually by an MCS-certified engineer as required, Daikin should cover the thermistor or PCB replacement. Most manufacturers include a parts and labour warranty of 2–5 years (longer with product registration). Keep records of every annual service visit, as warranty claims can be declined if you cannot demonstrate a regular service history.
How much does it typically cost to fix a Daikin J5-00 fault in the UK?
For the most common repair — thermistor replacement including engineer callout and labour — expect to pay in the region of £150 to £300. If the outdoor unit PCB turns out to be at fault, the cost rises to roughly £350–£600 depending on the specific board and your engineer's rates. In rare cases where a PCB is no longer available new, sourced parts or a specialist repair service may alter the cost. Compressor replacement is not associated with this fault code.
How can I prevent the J5-00 fault from coming back?
An annual heat pump service by an MCS-certified engineer is the single most effective measure. During a service the engineer checks all thermistor connections, cleans contacts, inspects wiring for early signs of corrosion, and verifies resistance readings before they drift out of range. Keeping the outdoor unit clear of debris and ensuring the unit is not regularly exposed to standing water or ice buildup also helps prolong sensor life.