Daikin J7-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin J7-00 fault code mean?
The J7-00 fault code indicates that the outdoor unit has detected an abnormal reading from the intermediate heat exchanger thermistor (referred to as R5T on Altherma models, or the liquid pipe thermistor on split and VRV systems). The controller has received a value that falls outside the expected range — typically caused by an open or short circuit in the sensor — and has shut the unit down as a precaution. In plain terms, a temperature sensor in the outdoor unit has either stopped responding or is sending back a reading that the system cannot trust.
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
- Failed thermistor (open or short circuit) Common
The thermistor itself — sensor R5T on Altherma monobloc models — has developed an internal fault, either an open circuit (no signal at all) or a short circuit (a fixed, implausible reading). This is by far the most frequent root cause. Thermistors can fail due to age, moisture ingress into the outdoor unit, or physical damage to the sensor body or its wiring.
- Loose or corroded connector Sometimes
Even if the thermistor is still functional, a poor connection at the plug that joins the sensor lead to the outdoor main PCB can produce exactly the same fault code. Vibration from the compressor and fan, combined with UK weather conditions, can cause connector pins to loosen or corrode over time, interrupting the signal and triggering J7-00.
- Faulty outdoor main PCB Rare
In a minority of cases the thermistor and its wiring check out fine, but the outdoor main PCB is misreading or failing to process the sensor signal correctly. A defective PCB can mimic a thermistor fault. This is typically only confirmed once the thermistor and connector have been ruled out by an engineer.
How to fix it
- Check the outdoor unit for obvious obstructions DIY safe
Before anything else, take a look at the outdoor unit. Make sure the area around it is clear of leaves, debris, or snow that might be affecting airflow or have worked into the unit casing. A visual check costs nothing and rules out the simplest possibilities.
- Perform a hard reset by switching off the power DIY safe
Turn the heat pump off at the indoor controller, then switch it off at the dedicated isolator or consumer unit MCB. Leave the unit completely de-energised for at least three to five minutes to allow the control electronics to fully discharge. Then restore power and restart the system normally. Occasional false J7-00 readings can clear this way.
- Check and top up system pressure if low DIY safe
While the unit is off, glance at the system pressure gauge on your buffer tank or pipework. Healthy pressure is typically between 1.0 and 2.0 bar when cold. If it reads below 1.0 bar, top it up via the filling loop following your system's instructions. Low pressure is not a direct cause of J7-00 but is worth addressing before calling an engineer.
- Attempt an error-code reset via the controller DIY safe
If the fault code reappears after the hard reset, check your Daikin controller manual for the error-reset procedure. On many Daikin remotes this involves holding the timer-cancel button for approximately five seconds until the display flashes, then confirming. This only clears the displayed fault — it will return immediately if the underlying problem is still present.
- Have an engineer inspect the thermistor connector and wiring heat pump engineer
An MCS-certified heat pump engineer should gain access to the outdoor unit's electrical panel and inspect the connector for sensor R5T (or the liquid pipe thermistor on other models). They will check for pin corrosion, looseness, or damaged insulation on the sensor lead. This step alone resolves a meaningful proportion of J7-00 faults without needing any parts.
- Replace the thermistor if confirmed faulty heat pump engineer
If the sensor is confirmed to have failed — either open or short circuit when measured with a multimeter against the resistance-temperature curve — the engineer will replace it with a Daikin-compatible thermistor. This is the most common repair outcome for J7-00 and is usually straightforward.
- Replace the outdoor main PCB if thermistor and wiring are sound heat pump engineer
If the thermistor and its connector pass inspection but the fault persists, the outdoor main PCB is the likely culprit. PCB replacement is a more involved and costlier repair, but it is still within the scope of a routine service visit for an MCS-certified engineer.
- Book an MCS-certified heat pump engineer heat pump engineer
If the fault code has not cleared after the homeowner resets described above, contact an MCS-certified heat pump engineer as soon as reasonably practical. Sensor faults are generally low-risk in the short term compared with pressure or compressor codes, but running the unit in a degraded or locked-out state for extended periods can cause secondary issues. Any work involving the refrigerant circuit must be carried out by an F-Gas registered engineer.
Parts you may need
- Daikin outdoor thermistor / heat exchanger sensor (R5T) · from £35
- Thermistor connector/harness assembly · from £18
- Daikin outdoor main PCB (model-specific) · 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 £180–£380, depending on the underlying cause.
Frequently asked questions
Is it safe to keep using my heat pump while the J7-00 fault is showing?
In most cases the unit will lock out and stop producing heat when J7-00 is active, so continued operation may not be possible anyway. If the unit is somehow still running intermittently, a thermistor fault is lower-risk than a compressor or refrigerant-pressure code — but you should still book an engineer promptly rather than leaving it. Running a system with a faulty temperature sensor means the controller is flying blind for that part of the circuit, which can place unnecessary stress on the compressor over time.
Can I fix a Daikin J7-00 fault myself?
The only actions a homeowner should take are: clearing debris from around the outdoor unit, performing a hard power reset, checking system pressure, and attempting an error-code reset via the controller. Inspecting or replacing the thermistor or PCB inside the outdoor unit requires an MCS-certified engineer. Any work on the refrigerant or sealed circuit additionally requires an F-Gas registered engineer. Attempting internal electrical repairs yourself may void your warranty and could be unsafe.
How much does it cost to fix a J7-00 fault in the UK?
The most common repair — replacing the thermistor and/or reseating the connector — typically costs between £180 and £380 including parts and labour at UK rates. If the fault turns out to be a failed outdoor main PCB the cost rises considerably, often into the £450–£900 range depending on the specific model and parts availability. These higher-cost scenarios are less common for J7-00 specifically. Always obtain a written quote before work begins, and check whether the repair falls within your Daikin warranty period.
Will my Daikin warranty cover a J7-00 thermistor fault?
Daikin's standard residential warranty covers manufacturing defects on parts for three years from installation, provided the unit has been serviced annually by an appropriately qualified engineer and the installation was carried out by an MCS-certified installer. If your unit is within warranty and the thermistor has failed due to a manufacturing defect rather than accidental damage, the part itself may be covered — but labour is often excluded after the first year. Keep all your service records: Daikin can and do reject claims where annual servicing cannot be evidenced.