Daikin 89-03 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin 89-03 fault code mean?
The 89-03 code appears on Daikin Altherma heat pumps when the evaporator temperature (Te) drops too low during a defrost cycle — specifically when the heat exchanger thermistor R2T reads below 6°C at a point in the cycle when temperatures should be recovering. It is a warning rather than a full lockout, meaning the unit can continue operating, but it signals that something is preventing the defrost from working as it should. Left unaddressed, if the warning triggers three times within 30 minutes, the system escalates to the more serious 89-01 freeze-up error, which shuts the unit down entirely. Acting on 89-03 promptly is the most cost-effective way to avoid that outcome.
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
- Low system water pressure Common
If the hydraulic circuit pressure has dropped below the recommended level (typically around 1.5–2 bar), water flow through the plate heat exchanger is reduced. This makes it harder for the unit to transfer heat efficiently during defrost, causing the evaporator temperature to remain lower than expected. Check the pressure gauge on your unit or controller — a reading below 1 bar warrants a top-up.
- Restricted or insufficient water flow Common
Partially closed valves, a poorly balanced system, an underperforming circulation pump, or air trapped in the hydraulic circuit can all limit how much water moves through the heat exchanger. Reduced flow means less heat is delivered to the refrigerant side during defrost, keeping evaporator temperatures low and triggering the warning.
- Faulty heat exchanger thermistor R2T Common
The R2T thermistor monitors evaporator temperature on the hydrobox. If it has drifted out of calibration, developed a poor connection, or failed outright, it may report a falsely low temperature during defrost even when the actual heat exchanger condition is fine. This is one of the first components an engineer will check and test.
- Refrigerant shortage or leak in the sealed circuit Sometimes
A low refrigerant charge reduces the system's ability to absorb and transfer heat. During defrost, this can prevent the evaporator from warming up at the expected rate, causing the thermistor to read below the threshold. A leak test and refrigerant charge check by an F-Gas registered engineer is required to investigate this.
- Faulty hydrobox PCB (A1P) Sometimes
The hydro PCB A1P processes signals from the thermistor and controls the defrost logic. If it is malfunctioning — due to a firmware issue, a damaged component, or a poor connection — it may misinterpret sensor data or fail to manage the defrost cycle correctly, resulting in the 89-03 warning being logged incorrectly.
- Fouled or scaled plate heat exchanger Sometimes
Scale build-up from hard water, or debris accumulation within the hydraulic circuit, can reduce the heat transfer efficiency of the brazed-plate heat exchanger. This effectively insulates the refrigerant from the water circuit, making defrost less effective and lowering measured evaporator temperatures.
How to fix it
- Check the system water pressure gauge DIY safe
Locate the pressure gauge on your heat pump controller or on the unit itself. A healthy reading is typically between 1.5 and 2 bar. If it reads below 1 bar, the system is likely undercharged with water. Most systems have a filling loop — consult your installer's handover notes or your controller manual for how to top up pressure safely. Re-pressurise slowly to around 1.5 bar.
- Inspect the outdoor unit for debris or ice build-up DIY safe
Walk around the outdoor unit and check that the air inlet and outlet grilles are clear of leaves, garden debris, overgrown vegetation, or compacted snow. These obstructions interfere with airflow and can stress the defrost cycle. Clear any blockages by hand — do not use sharp tools or pressure washers near the unit.
- Check for any tripped MCBs or power interruptions DIY safe
Go to your consumer unit and confirm the MCB or circuit breaker labelled for the heat pump is in the on position. A partial power interruption can sometimes cause erratic sensor behaviour. If a breaker has tripped, reset it once — if it trips again immediately, do not reset it further and call an engineer.
- Restart the heat pump from the controller DIY safe
Because 89-03 is a warning that can auto-reset once conditions improve, a controlled restart from the user interface is worth attempting after addressing obvious issues like low pressure or debris. Follow your controller's restart procedure. Monitor the unit over the next few hours to see whether the warning returns.
- Have an engineer inspect and test the R2T thermistor heat pump engineer
An MCS-certified engineer will use a multimeter to check the resistance values of the R2T thermistor against Daikin's specification table. A reading outside the expected range confirms a faulty sensor. Thermistor replacement is straightforward but requires safe isolation of the unit before work begins.
- Have an engineer check the hydro PCB A1P connections and function heat pump engineer
The engineer will inspect the A1P board for signs of damage, corrosion, or loose wiring harnesses. If the board is sending or receiving incorrect signals, it may need replacement. This is an electrical component task that must only be carried out with the unit fully isolated.
- Commission a refrigerant circuit inspection and leak test heat pump engineer
If the thermistor and PCB check out as healthy, the refrigerant circuit is the next area to investigate. An F-Gas registered engineer must carry out a leak test using calibrated detection equipment. If a leak is found, it must be repaired and the circuit recharged to the correct specification before the unit is returned to service. Do not attempt any work on the refrigerant circuit yourself.
- Arrange a hydraulic circuit flush if scale or debris is suspected heat pump engineer
If hard water scale or contamination is found in the system, an engineer can perform a chemical flush of the hydraulic circuit to restore heat exchanger efficiency. This is particularly relevant in hard-water areas of England and should be followed by the addition of an approved inhibitor to slow future scale build-up.
- Contact an MCS-certified heat pump engineer if the fault persists or recurs heat pump engineer
If 89-03 keeps returning, or if it escalates to the 89-01 freeze-up lockout, do not continue attempting resets. Call an MCS-certified engineer — and ensure any refrigerant work is carried out by someone who also holds F-Gas registration. Repeated faults without proper diagnosis risk compressor damage and a significantly larger repair bill.
Parts you may need
- Heat exchanger thermistor R2T · from £35
- Hydrobox PCB A1P · from £320
- Filling loop assembly (for system pressure top-up) · from £25
- System inhibitor (hydraulic circuit corrosion protection) · from £18
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–£600, depending on the underlying cause.
Frequently asked questions
Will the 89-03 warning shut my heat pump down?
Not immediately. Unlike the 89-01 freeze-up error, the 89-03 warning allows the unit to keep running. However, if it triggers three times within a 30-minute window, the system automatically escalates to 89-01, which does cause a full shutdown requiring a manual reset. This is why it is worth investigating 89-03 promptly rather than repeatedly clearing it and carrying on.
Can I fix the 89-03 fault myself?
There are a small number of checks a homeowner can safely carry out — topping up system pressure if it has dropped, clearing debris from around the outdoor unit, and restarting the controller. However, the most likely causes (a failing thermistor, a PCB issue, or a refrigerant leak) all require an MCS-certified engineer. Refrigerant work specifically must be done by someone holding F-Gas registration. Attempting to access the sealed circuit or internal electrical components yourself would be unsafe and could void your warranty.
How much does it typically cost to fix an 89-03 fault in the UK?
For the most common repairs — a thermistor replacement or a system pressure and flow correction — most UK homeowners pay in the range of £180 to £350 including parts and labour. If the cause turns out to be a refrigerant leak requiring a repair and regas, costs typically run from £350 to £600. A hydro PCB replacement tends to sit between £450 and £900. In rare cases where prolonged operation with an undiagnosed fault has led to compressor damage, costs can exceed £1,400 — which is why addressing 89-03 early is strongly recommended.
Does my Daikin warranty cover the 89-03 fault?
It may do, depending on the root cause and your warranty status. Daikin offers a five-year parts and labour warranty on Altherma systems installed by a D1 or D2 registered installer, provided the unit was registered within 30 days of installation and has been serviced annually by a qualified engineer. If the fault is due to a manufacturing defect in the thermistor or PCB, a valid warranty claim should cover the repair. However, faults caused by low system pressure, a water leak, or scale build-up are generally considered maintenance issues and are unlikely to be covered. Keep your service records — Daikin requires evidence of annual servicing to honour warranty claims.