Daikin HC-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin HC-00 fault code mean?
The HC-00 fault code on a Daikin heat pump (typically a Daikin Altherma series) signals that the system has detected a problem with the water temperature thermistor — the sensor responsible for monitoring the temperature of water within the heating circuit. The '00' sub-code simply indicates this is the primary instance of the fault. When this sensor stops communicating correctly with the outdoor unit's control board, the heat pump cannot safely regulate water temperature and will lock out to protect itself and the system.
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
- Loose or corroded thermistor connector Common
Over time, the wiring connector linking the water temperature thermistor to the control board can work loose due to vibration from the outdoor unit, or the connector pins can corrode due to moisture ingress. This is the most frequently found cause and is often resolved by reseating or replacing the connector — a relatively quick job for an engineer.
- Failed water temperature thermistor Common
The thermistor sensor element itself can develop an open-circuit or short-circuit internally, typically after years of use or following a power surge. The outdoor unit's PCB expects a resistance reading within a defined range; if the sensor drifts significantly outside that range, HC-00 is triggered. An engineer can confirm this with a simple resistance check using a multimeter.
- Transient power event Sometimes
A brief voltage spike or brownout — such as those caused by a thunderstorm or a grid disturbance — can temporarily confuse the control board's sensor-reading circuits, generating a spurious HC-00 fault. In these cases, a full power cycle is sometimes enough to clear the code without any component being faulty.
- Faulty outdoor unit PCB Rare
If both the wiring and the thermistor test within normal parameters, the fault likely lies with the outdoor unit's main control board, which processes the sensor signal. PCB failure can be caused by age, moisture ingress into the electrical enclosure, or a power surge that damaged the input circuitry.
How to fix it
- Switch the heat pump off at the controller, then isolate it at the outdoor isolator switch and the indoor MCB. Leave it fully de-energised for at least 3–5 minutes, then restore power and restart. DIY safe
A transient electrical disturbance can cause a one-off sensor fault. A full power cycle clears the fault memory and lets the system re-check the thermistor on start-up. If HC-00 returns immediately or comes back within a short time, the fault is genuine and further investigation is needed. Do not repeat the reset more than twice.
- Check around the outdoor unit for anything that could have caused a disturbance — standing water near the electrical enclosure, obvious physical damage, or signs of pest activity near cabling. DIY safe
Look for external factors you can report to the engineer: water pooling around the base of the unit, damaged conduit, or chewed wiring insulation. Do not open any panels or touch any internal components — simply observe from the outside and note what you see.
- Contact an MCS-certified heat pump engineer to inspect the thermistor wiring and connectors inside the outdoor unit. heat pump engineer
The engineer will open the outdoor unit's electrical enclosure (with power isolated), locate the water temperature thermistor connector, and check for loose pins, corrosion, or broken wires. If a connector fault is found, cleaning or replacing the connector is typically a quick and low-cost fix.
- Have the engineer test the water temperature thermistor's resistance with a multimeter and compare the reading against Daikin's published resistance-temperature curve for that sensor. heat pump engineer
If the resistance is outside the specified range at the ambient water temperature, the thermistor has failed and needs to be replaced. Genuine Daikin Altherma thermistors are usually available from UK Daikin distributors, often next-day, and the replacement itself is straightforward once the correct part is confirmed.
- If wiring and thermistor both check out correctly, have the engineer assess the outdoor unit PCB for signs of damage and, if necessary, arrange its replacement. heat pump engineer
PCB diagnosis involves checking supply voltages, connector integrity and input/output signals. This work is strictly for a qualified engineer. PCB replacement on Daikin Altherma units is a defined procedure and the boards are available through Daikin's UK parts network, though lead times can occasionally extend beyond next-day for less common models.
Parts you may need
- Daikin Altherma water temperature thermistor (sensor) · from £40
- Thermistor wiring harness / connector repair kit · from £15
- Daikin outdoor unit main PCB (model-dependent) · 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–£450, depending on the underlying cause.
Frequently asked questions
Can I reset a Daikin HC-00 fault myself?
Yes — a single power cycle is worth trying and is safe for a homeowner to do. Switch off the heat pump at the controller, turn off the outdoor isolator and the indoor MCB, wait five minutes, then restore power and restart. If HC-00 clears and does not return, a transient power event was likely to blame. If the code comes straight back, or reappears within a few hours, the underlying fault is genuine and you'll need an MCS-certified engineer to investigate further.
How much does it cost to fix a Daikin HC-00 fault in the UK?
For the majority of cases — a loose connector or a failed thermistor — you should expect to pay somewhere between £150 and £450 all-in, covering the engineer's callout, diagnosis time, any parts and labour. If the root cause turns out to be the outdoor unit PCB, costs can rise to £600–£800 depending on the specific board and the engineer's rates. Full compressor or heat exchanger failures are unrelated to this fault code, but if an engineer finds wider system issues during the same visit, total costs could be higher — always ask for a written quote before authorising any parts.
Is Daikin HC-00 covered under warranty?
It may be, depending on the age of your system and the cause of the fault. Manufacturing defects in the thermistor or PCB are typically covered within the standard Daikin warranty period (usually 2–5 years depending on registration). Faults attributed to improper installation, physical damage, or lack of servicing are generally excluded. Check your warranty documentation and contact your installing MCS-certified engineer or Daikin UK directly before authorising paid repairs.
Which Daikin Altherma models show the HC-00 code?
HC-00 appears across a wide range of Daikin Altherma air-to-water heat pump models, including the Altherma 2, Altherma 3 R, Altherma 3 H HT and the Altherma M series, as well as older EHBH and EHVH series units. The code may appear on the indoor controller display, the Madoka round thermostat, or the Daikin online controller app. If you are unsure which model you have, the model number is printed on the data plate on the side of the outdoor unit.