Daikin H1-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin H1-00 fault code mean?
The H1-00 code on Daikin Altherma heat pumps indicates a problem with the optional indoor ambient temperature sensor (thermistor R6T) or, on some models, a communication breakdown between the indoor and outdoor units. The system has detected that the sensor reading is outside its expected operating range — returning an implausible value or no signal at all. On most Altherma Monobloc models the heat pump will continue running in a limited capacity, but the fault should be investigated promptly to avoid incorrect operation or further component damage.
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 degraded thermistor R6T Common
The optional ambient temperature sensor (thermistor R6T) is the most common culprit. Over time, thermistors can drift out of specification, develop an internal open circuit, or fail outright, causing the controller to report H1-00. The unit may show a fixed reading such as 0 °C regardless of actual conditions.
- Loose, corroded or damaged wiring at the sensor connector Common
Vibration and thermal cycling can work connectors loose on the hydro PCB A1P. Corrosion from moisture ingress can also increase resistance in the circuit to the point where the controller treats the sensor as faulty even if the thermistor itself is intact.
- Power supply disturbance or voltage imbalance Sometimes
An imbalance greater than approximately 4 % across supply phases, a brief voltage dip, or a power surge can corrupt controller logic or temporarily disrupt communication between the indoor and outdoor units, triggering a transient H1-00. If the code clears after a reset and does not return, a power event is a likely explanation.
- Faulty hydro PCB (A1P) or noise filter PCB Sometimes
If the sensor and its wiring both check out correctly, the fault may lie in the PCB receiving the sensor signal. A failed hydro PCB A1P or a damaged noise filter PCB can misread a healthy sensor and generate a false H1-00.
- Incorrect or incompatible replacement sensor installed Rare
If the system has recently been serviced and a thermistor was replaced, fitting the wrong specification part can push readings outside the accepted range and immediately trigger H1-00.
How to fix it
- Check whether the unit is still in warranty DIY safe
Daikin typically offers a 5-year warranty on Altherma heat pumps when registered. If your system is under 5 years old, contact your installer or Daikin UK before spending anything — parts and labour may be fully covered. Keep your installation paperwork to hand.
- Perform a controlled power reset DIY safe
Switch off the heat pump at the indoor controller, then turn off the dedicated isolator or circuit breaker for both the indoor and outdoor units. Leave everything off for at least 10 minutes to allow the PCB capacitors to fully discharge. Restore power and check whether H1-00 reappears. A one-off power event can cause a transient fault that clears cleanly this way. If the code returns within a short time, a reset alone will not resolve the underlying issue.
- Check the outdoor unit for obstructions DIY safe
While the unit is resetting, go outside and check that the outdoor unit's air intake and discharge are free from leaves, debris, or compacted snow and ice. Poor airflow can cause sensor readings to behave unexpectedly. Clear any blockages by hand — do not use sharp tools or a pressure washer near the unit.
- Verify system water pressure DIY safe
Check the system pressure gauge on the indoor unit. Daikin Altherma systems typically operate between 1.0 and 2.5 bar when cold. If pressure is low, top up via the filling loop following the guidance in your user manual. Low pressure is not a direct cause of H1-00 but rules out a simultaneous secondary problem.
- Inspect the sensor connector and wiring — engineer task heat pump engineer
An MCS-certified engineer should visually inspect the wiring loom and connector at thermistor R6T and at the hydro PCB A1P for signs of corrosion, chafing, or loose pins. A multimeter resistance check of R6T against Daikin's published temperature-resistance table will confirm whether the sensor is within specification or has failed.
- Test and replace thermistor R6T if required — engineer task heat pump engineer
If resistance readings are outside specification, the engineer should replace thermistor R6T with the correct Daikin-approved part for the model. After fitting, they should clear the fault code and confirm the sensor reads a plausible ambient temperature before returning the system to normal operation.
- Inspect the hydro PCB A1P and noise filter PCB — engineer task heat pump engineer
If the thermistor and wiring are both healthy, the engineer should check that the hydro PCB A1P is receiving the correct supply voltage and inspect the noise filter PCB for signs of burn marks, swollen capacitors, or physical damage. PCB replacement should only be carried out by a qualified engineer using manufacturer-approved components.
- Verify the incoming power supply quality — engineer task heat pump engineer
The engineer should confirm that the mains supply voltage and phase balance meet Daikin's specifications (typically within 4 % imbalance for three-phase supplies). If supply quality is poor, an electrician should investigate upstream before the heat pump is returned to service.
- Contact an MCS-certified heat pump engineer if the fault persists heat pump engineer
If H1-00 returns after a reset and the above homeowner checks have been completed, book a visit from an MCS-certified heat pump engineer experienced with Daikin Altherma systems. Any work involving refrigerant circuit components also requires an F-Gas registered engineer. You can find qualified engineers via the MCS installer database at mcscertified.com.
Parts you may need
- Daikin thermistor R6T (optional ambient sensor) · from £35
- Daikin hydro PCB A1P (Altherma Monobloc) · from £280
- Noise filter PCB · from £120
- Wiring loom / sensor connector repair kit · from £25
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 keep using my heat pump while H1-00 is showing?
On most Daikin Altherma Monobloc models the system continues to operate when H1-00 is active, because R6T is an optional rather than a critical sensor. However, running with a faulty ambient sensor means the heat pump may not be optimising its output correctly, which can reduce efficiency and increase running costs. It is worth getting the fault diagnosed within a few days rather than ignoring it indefinitely.
H1-00 appeared after a power cut — do I need an engineer?
Not necessarily straight away. A power surge or brief outage can occasionally latch a fault code without any hardware actually failing. Perform the controlled power reset described above (switch off the isolator, wait 10 minutes, restore power). If H1-00 does not return within a normal heating cycle, the issue was almost certainly a transient power event. If the code comes back, call an engineer.
How much does it typically cost to fix H1-00 on a Daikin Altherma?
Most people with this fault pay somewhere between £150 and £450 all in. A wiring or connector repair sits at the lower end (around £150–£200 including the callout), while a thermistor replacement typically comes to £200–£350 including parts and labour. A hydro PCB replacement is the more expensive scenario, usually £400–£600 once parts and labour are combined — and in rare cases where a main PCB or additional components are needed, costs can climb higher still. Always ask for a written quote before work begins.
Is H1-00 covered under my Daikin warranty?
Daikin Altherma heat pumps typically carry a 5-year warranty when properly registered after installation. If your unit is within that period and the fault is due to a manufacturing defect rather than physical damage or incorrect installation, the repair should be covered at no cost. Contact your original MCS-certified installer in the first instance — they are responsible for warranty claims with Daikin. If you are unsure who installed your system, Daikin UK customer support can advise.