Daikin J6-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin J6-00 fault code mean?
The J6-00 code signals that your Daikin heat pump has detected an abnormality in the outdoor unit's heat exchanger thermistor — the temperature sensor that monitors conditions on the refrigerant-side of the outdoor coil. The system has either lost the signal from this sensor entirely (open circuit) or is receiving an implausible reading (short circuit). Without reliable data from this thermistor, the unit cannot manage heat exchanger temperatures correctly, so it raises an alarm and may lock out. The '00' suffix means no additional sub-category applies — this is the base-level thermistor fault.
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 outdoor heat exchanger thermistor Common
The thermistor itself has developed an internal open or short circuit, producing a reading that falls outside the expected resistance range. This is the most frequent root cause — thermistors can degrade over time due to moisture ingress, vibration or simply age, and once they drift outside tolerance the PCB flags the fault.
- Loose, damaged or disconnected wiring connector Sometimes
The connector linking the thermistor to the outdoor unit's main PCB can work loose due to vibration, or the wire insulation can become brittle and crack, particularly in exposed outdoor locations. A poor connection can produce the same open-circuit signal as a failed sensor without the sensor itself being faulty.
- Outdoor unit main PCB failure Rare
If the PCB's thermistor input circuit has failed, it will misread or lose the signal from a perfectly good sensor. This is the least common cause but should be considered if a new thermistor and confirmed-good wiring still produce the J6-00 code.
How to fix it
- Switch the heat pump off at the indoor controller, then isolate it at the outdoor unit isolator or the relevant MCB in the consumer unit. Leave it fully de-energised for at least five minutes before restoring power and restarting. DIY safe
A brief power cycle can clear transient sensor glitches caused by a voltage spike or a momentary connector issue. If the J6-00 code does not return after restart and the unit runs normally, monitor it over the next day or two. If it reappears, the fault is persistent and needs investigation.
- Check around the outdoor unit for any obvious physical damage — crushed cables, rodent damage to wiring, or heavy ice build-up on the coil — and clear any debris from the unit's surroundings. DIY safe
While you should not open the outer casing yourself, a visual inspection of the visible wiring entering the unit and the general condition of the outdoor coil is worthwhile. Rodents occasionally chew sensor cables, and this can be spotted from the outside. Do not attempt to defrost ice with sharp tools — allow a normal defrost cycle or call an engineer.
- If the fault persists after a reset, contact an MCS-certified heat pump engineer to carry out a full thermistor diagnostic. heat pump engineer
The engineer will access the outdoor unit, locate the heat exchanger thermistor connector on the main PCB, and check for secure seating and any visible wire damage. They will then measure the thermistor's resistance with a calibrated multimeter across the temperature range and compare results against Daikin's specification chart. This pinpoints whether the sensor, the wiring or the PCB input is at fault.
- Have the engineer replace the outdoor heat exchanger thermistor if resistance readings confirm it is out of specification. heat pump engineer
Thermistor replacement is a straightforward task for a qualified engineer — the sensor is clipped or strapped to the refrigerant pipework inside the outdoor unit and connects via a plug-in harness to the PCB. No refrigerant circuit work is required for this step, but the outdoor unit casing must be removed, which should only be done by a competent engineer.
- If a new thermistor and fully intact wiring still produce the J6-00 fault, have the engineer assess and, if necessary, replace the outdoor unit main PCB. heat pump engineer
PCB replacement requires careful matching of the replacement board to the exact model variant. The engineer will check Daikin's parts database for the correct board, and may need to carry out a configuration or address-setting procedure after fitting. This is the most involved repair for this fault code and also the most expensive.
Parts you may need
- Outdoor heat exchanger thermistor (Daikin) · from £35
- Thermistor wiring harness / connector · from £18
- Outdoor unit main PCB (model-dependent) · 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–£400, depending on the underlying cause.
Frequently asked questions
Is it safe to keep running my Daikin heat pump with a J6-00 fault?
For a short period — a few days while you arrange an engineer visit — the risk is generally low, provided the unit is not going into repeated lockout and your home is still being heated adequately. However, because the outdoor heat exchanger thermistor helps the system manage refrigerant temperatures, running for extended periods without it could allow conditions that stress the compressor. We'd recommend booking an engineer promptly rather than leaving it for weeks.
How much does it typically cost to fix a Daikin J6-00 fault in the UK?
For most people, the repair is a thermistor replacement, which including parts and a call-out typically costs between £180 and £400. If the fault turns out to be the outdoor unit PCB rather than the sensor, costs rise considerably — PCB replacement on a Daikin outdoor unit is commonly £450 to £900 all-in, depending on the model and the board's availability. It is worth checking whether your unit is still within Daikin's manufacturer warranty (three years on residential units, provided annual servicing by a qualified engineer is evidenced) before committing to repair costs.
Do I need an F-Gas engineer to fix a J6-00 fault?
Not necessarily for the thermistor replacement itself — that repair does not involve opening the sealed refrigerant circuit, so it does not legally require an F-Gas registered engineer. That said, the outdoor unit casing must be removed, and an MCS-certified heat pump engineer is the right person for the job regardless. If diagnostic work reveals a refrigerant-side issue alongside the thermistor fault, any work on the refrigerant circuit must be carried out by an F-Gas registered engineer.
Can the J6-00 fault clear itself, or will it always need a repair?
Occasionally a J6-00 is caused by a connector that has worked slightly loose, and a power reset — combined with the vibration of the unit restarting — may reseat it well enough that the fault does not recur. In that scenario it can appear to clear on its own. However, if the fault returns even once after a reset, the underlying cause (degraded thermistor, damaged wire or PCB issue) will not resolve without hands-on attention from an engineer. Don't assume a single clear means the problem has gone away.