Daikin 89-01 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin 89-01 fault code mean?
The 89-01 code indicates that your Daikin heat pump has detected a freeze-up condition at the brazed plate heat exchanger — the component responsible for transferring heat between the refrigerant circuit and the water-filled heating system. To protect the heat exchanger from physical damage caused by ice formation, the unit shuts itself down completely. This is a manual-reset lockout, meaning the system will not restart on its own; you must clear the fault via the user interface once the underlying problem has been resolved. If your unit previously showed an 89-02 warning code on three separate occasions, that repeated warning will have escalated into this full 89-01 lockout.
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 or insufficient flow Common
If the water pressure in your heating circuit has dropped below the minimum required level, or if flow through the plate heat exchanger is restricted — perhaps by a partially closed valve, a blocked filter, or an underperforming circulation pump — the refrigerant can cool the heat exchanger far below freezing. This is the most frequently identified trigger for an 89-01 on Daikin Altherma systems. Target system pressure is typically around 1.5–2 bar when cold.
- Refrigerant shortage or leak in the sealed circuit Common
When the refrigerant charge is lower than it should be, the evaporation temperature inside the heat exchanger drops dramatically, causing the water side to freeze. A refrigerant deficit can stem from a slow leak at joints, a valve, or due to gradual permeation over time. Only an F-Gas registered engineer can diagnose and address this — they will check operating pressures, temperature relationships and inspect for leaks before adjusting the charge.
- Faulty heat exchanger thermistor (R2T) Sometimes
The R2T sensor monitors the temperature of the plate heat exchanger. If this thermistor has drifted out of calibration, developed an open circuit, or is reporting artificially low temperatures, the control system may trigger a freeze-up alarm even when actual conditions are normal. An engineer will check resistance values against Daikin's reference table to confirm whether the sensor is at fault.
- Faulty hydro PCB (A1P) Sometimes
The hydronic control board manages the water-side circuit, including pump operation and sensor inputs. A failing A1P board may misinterpret thermistor data or fail to command the circulation pump correctly, leading to the conditions that trigger this code. Engineers check whether the board is receiving correct supply voltage before condemning it.
- Dirty or fouled plate heat exchanger Rare
Scale build-up or debris accumulation inside the brazed plate heat exchanger can restrict water flow and reduce heat transfer efficiency. Over time this can create localised cold spots that approach freezing temperatures during high-demand or cold-weather operation. This is more likely in hard-water areas or systems that have never been chemically cleaned.
How to fix it
- Check and restore system water pressure DIY safe
Look at the pressure gauge on your heating system or heat pump hydrobox. If it reads below 1 bar, the system is likely undercharged with water. Most Daikin Altherma systems should sit at approximately 1.5–2 bar when cold. Top up via the filling loop carefully, watching the gauge, until you reach the correct pressure. Also visually inspect visible pipework for any signs of leaking joints. Do not overpressure the system — stop at 2 bar.
- Clear any debris from around the outdoor unit DIY safe
Ensure the outdoor unit has clear airflow on all sides. Remove any leaves, dirt, or debris that may have accumulated near the unit. In winter, check that ice or compacted snow is not blocking the air intake grilles or the fan outlet. Restricted airflow can contribute to abnormal operating conditions inside the refrigerant circuit.
- Attempt a manual reset via the user interface DIY safe
Once you have addressed anything visible (such as low pressure), navigate to the fault or alarm screen on your Daikin controller and perform a manual reset. If the code clears and the unit starts, monitor it closely over the next hour to confirm it does not re-trip. If the fault returns promptly, or if you could not identify an obvious cause such as low pressure, do not keep resetting — doing so without fixing the root cause risks damaging the heat exchanger.
- Have an engineer inspect the thermistor R2T and hydro PCB A1P heat pump engineer
A qualified heat pump engineer will check the resistance of the R2T thermistor against Daikin's reference values at a known temperature, confirming whether it is reading accurately. They will also check that the A1P hydro PCB is receiving the correct supply voltage and is responding to inputs correctly. A faulty thermistor is a relatively straightforward and lower-cost fix; a failed PCB will require replacement with a genuine Daikin part.
- Have an F-Gas registered engineer check the refrigerant circuit heat pump engineer
If water pressure and flow are confirmed to be adequate and the sensors appear healthy, the next step is a refrigerant circuit investigation. An F-Gas registered engineer will connect gauges to check suction and discharge pressures, assess superheat and subcooling values, and carry out a leak test. If a shortage is confirmed, they will trace and repair any leak before recharging the system — simply topping up without finding the leak is a temporary fix at best.
- Consider a system flush or heat exchanger clean if fouling is suspected heat pump engineer
If your system has hard water, has never been chemically flushed, or if an engineer identifies restricted flow despite adequate pump operation, a powerflush or chemical clean of the plate heat exchanger may be recommended. This work involves isolating and partially draining the system and should be carried out by a professional.
- Contact an MCS-certified heat pump engineer if the fault persists heat pump engineer
If the steps above do not resolve the 89-01 code, or if you are not comfortable diagnosing the cause, contact an MCS-certified heat pump engineer — ideally one who is Daikin-accredited to protect any remaining warranty. Refrigerant work must be performed by an engineer holding F-Gas certification. Be ready to tell them the fault code, how many times it has occurred, and any 89-02 warnings that preceded it.
Parts you may need
- Daikin R2T plate heat exchanger thermistor · from £45
- Daikin A1P hydro PCB (hydronic control board) · from £320
- Inline system filter / strainer (22mm or 28mm) · from £35
- Filling loop assembly (for topping up system pressure) · 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–£600, depending on the underlying cause.
Frequently asked questions
Can I reset the Daikin 89-01 fault myself and carry on?
You can perform the manual reset from the user interface, and if the cause was simply low water pressure that you have already corrected, the system may run normally afterwards. However, if the fault returns — or if you cannot identify a clear cause like low pressure — you should not keep resetting it. Repeated freeze-up events can cause permanent damage to the brazed plate heat exchanger, which is an expensive component to replace. Call an engineer before the fault escalates.
How much does it typically cost to fix a Daikin 89-01 fault in the UK?
Most people with this fault pay somewhere between £150 and £600, covering a callout, diagnosis, and the most common fixes such as thermistor replacement or resolving a water pressure or flow issue. If the cause turns out to be a refrigerant leak, costs will generally be higher — leak repair plus recharging typically adds £300–£700 to the bill depending on the severity. A full PCB replacement sits at the higher end of the typical range. In rare cases where a compressor has been damaged by prolonged freeze-up, costs can reach £1,500 or more, but this is uncommon if the lockout is addressed promptly.
Why does the plate heat exchanger freeze, and is it a design flaw?
The plate heat exchanger freezes when the refrigerant evaporating inside it becomes cold enough to freeze the water passing through the water-side channels — typically when something disrupts the normal balance of heat transfer. That can be too little water flow, too little refrigerant, or a faulty sensor giving the system incorrect information. It is not a design flaw; Daikin's control system is actually detecting the freeze-up and shutting down to prevent damage. The 89-01 lockout is the system working as intended — the underlying cause is what needs addressing.
Will this fault be covered under my Daikin warranty?
It may be, depending on the root cause. Daikin offers 5–7 year parts and labour warranties through certified installers. If the freeze-up was caused by a manufacturing defect in the thermistor or PCB, this would typically be covered. However, if it was caused by low system pressure due to poor installation, an unreported leak, or lack of maintenance, warranty cover is less likely. Contact your installer or Daikin directly to discuss your specific situation. If you are on Daikin's Gold maintenance package, you may have additional cover for callouts and parts.