Daikin 89-02 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin 89-02 fault code mean?
Fault code 89-02 appears on Daikin Altherma heat pumps when the refrigerant-side temperature of the brazed-plate heat exchanger inside the indoor (hydro) unit drops below 0°C during space heating operation. The system detects that the heat exchanger is at serious risk of freezing — or is already beginning to freeze — and shuts down to prevent damage. This is distinct from code 89-01, which covers freeze-up during cooling or domestic hot water production; 89-02 is specific to space heating mode. The unit will not restart until the underlying cause has been identified and resolved, and a manual reset has been carried out.
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 water flow or insufficient system pressure Common
If the heating circuit water cannot circulate freely and at adequate pressure, the plate heat exchanger cannot absorb enough heat from the water side. This starves the refrigerant of warmth, causing it to drop to freezing temperatures. Common culprits include a partially closed isolation valve, a blocked inline strainer or filter, a failing circulation pump, or system pressure that has dropped too low (typically below 1 bar). This is one of the most frequent triggers for 89-02 in UK installations.
- Refrigerant undercharge or leak Common
If the sealed refrigerant circuit has lost charge — even slowly over time through a minor leak — the refrigerant evaporates at an abnormally low pressure and temperature inside the plate heat exchanger, which can quickly cause freezing. A refrigerant shortage is one of the primary causes listed in Daikin's own service documentation for this fault code. Only an F-Gas registered engineer can legally diagnose and rectify this.
- Faulty heat exchanger thermistor (R2T sensor) Sometimes
The R2T thermistor monitors the plate heat exchanger temperature and feeds that data to the control board. If the sensor has drifted out of calibration or developed a fault, it may report falsely low temperatures and trigger the 89-02 alarm even when conditions are normal, or it may fail to protect the unit adequately when temperatures genuinely fall. This is a relatively straightforward part swap for an engineer but does require proper diagnosis first.
- Faulty or stuck four-way valve Sometimes
The four-way reversing valve controls the direction of refrigerant flow and determines whether the system operates in heating or another mode. If this valve sticks or fails to switch correctly when heating mode is demanded, refrigerant can flow in the wrong direction through the heat exchanger, creating localised cold spots and triggering the freeze-up alarm.
- Fouled or dirty plate heat exchanger Sometimes
Scale, sludge, or debris build-up on the water side of the brazed-plate heat exchanger reduces its ability to transfer heat between the water circuit and the refrigerant. With heat transfer impaired, the refrigerant runs progressively colder, eventually dipping below freezing point. This is more common in older systems, hard-water areas, or installations that have never had an inhibitor added to the heating circuit.
- Faulty hydro PCB (A1P) Rare
The indoor unit's main printed circuit board manages freeze-protection logic and controls how the system responds to sensor readings. A faulty PCB may misinterpret sensor data, fail to activate protective responses at the right time, or incorrectly manage pump and valve operation. PCB faults are less common but should be considered once other causes have been ruled out.
How to fix it
- Check the system water pressure gauge DIY safe
Look at the pressure gauge on your indoor unit or heating system manifold. It should typically read between 1 and 2 bar when the system is cold. If it reads below 1 bar, low system pressure may be restricting water flow and triggering the fault. You can top up pressure using the filling loop on your heating system — consult your system's filling loop instructions and aim for around 1.5 bar. If the pressure drops again quickly, there may be a leak that needs professional attention.
- Inspect the outdoor and indoor units for obvious blockages DIY safe
Walk around the outdoor unit and clear away any leaves, debris, snow, or ice that may have built up around the casing or air inlet grilles. Restricted airflow around the outdoor unit affects overall system performance. Indoors, check that all radiator or underfloor heating valves are open and that any inline strainers or filters on the heating circuit are not visibly blocked.
- Check the fault code on the controller and attempt a reset DIY safe
Note the fault code displayed (89-02) and the time it appeared. Turn the heat pump off at the controller, wait at least three minutes, then power it back on and attempt a restart. If the fault clears and the system runs normally, monitor it closely over the following hours. If the code returns, a reset alone will not solve the underlying problem and an engineer should be called promptly.
- Engineer to check water flow rate and circulation pump operation heat pump engineer
An MCS-certified engineer will measure actual water flow through the plate heat exchanger and verify that the circulation pump is delivering adequate volume and pressure. They will also inspect and clean any strainers or filters and confirm that all isolation valves are fully open. Restricted flow is one of the most common causes of this fault and is usually straightforward to resolve.
- Engineer to inspect the refrigerant circuit and check for undercharge heat pump engineer
Using calibrated pressure and temperature gauges, the engineer will assess whether the refrigerant charge is correct for prevailing conditions. If operating pressures and superheat readings suggest an undercharge, a leak search will be carried out before any refrigerant is added. All refrigerant handling in the UK must be performed by an F-Gas registered engineer — it is a legal requirement.
- Engineer to test the R2T thermistor and four-way valve heat pump engineer
The engineer will measure the resistance of the plate heat exchanger thermistor (R2T) and compare it against Daikin's reference values for the current temperature. A faulty sensor will be replaced. They will also verify that the four-way reversing valve switches correctly between modes and is not sticking in the wrong position, which can cause localised freezing in the heat exchanger.
- Engineer to inspect and test the hydro PCB (A1P) heat pump engineer
If all other components check out, the engineer will assess the indoor unit's main control board, verifying that it is receiving correct power supplies and that its outputs are functioning as expected. A faulty PCB may need to be replaced. This is a specialist diagnostic step and requires access to Daikin's service documentation and test procedures.
- Carry out a final reset and extended test run once repairs are complete heat pump engineer
After any repair, the engineer will perform a manual reset through the controller and run the system under normal load for a sufficient period to confirm that the 89-02 code does not return. Operating temperatures, pressures, and flow rates should all be logged to provide a baseline for future reference. If the fault reappears, further investigation is needed before the system is left running.
Parts you may need
- R2T plate heat exchanger thermistor (sensor) · from £35
- Brazed-plate heat exchanger (hydro unit) · from £280
- Hydro PCB A1P (indoor unit control board) · from £420
- Four-way reversing valve · from £190
- Circulation pump (heating circuit) · from £120
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
Is it safe to keep using my heat pump while fault code 89-02 is showing?
No — you should not continue running the system with this fault active. The 89-02 code indicates the plate heat exchanger is at or below freezing point during heating operation. Continuing to run the unit risks ice forming inside the heat exchanger, which can crack the plates and cause an expensive coolant-water cross-contamination failure. The brazed-plate heat exchanger itself can cost £300–£600 or more to supply and fit, so treating this fault promptly is far cheaper than ignoring it. Switch the unit off and call an MCS-certified engineer as soon as possible.
Could low boiler — sorry, low system pressure really cause this fault on a heat pump?
Yes, absolutely. Unlike a gas boiler, a heat pump relies on a continuous, well-pressurised flow of water through the plate heat exchanger to keep the refrigerant warm enough to operate safely. If system pressure drops too low — typically below 1 bar — water flow becomes insufficient and the refrigerant temperature can fall below 0°C, triggering 89-02. Check your pressure gauge and top up via the filling loop if needed. If the pressure keeps dropping, you likely have a small leak somewhere in the heating circuit that should be investigated by an engineer.
How much does it typically cost to fix a Daikin 89-02 fault in the UK?
For the most common causes, expect to pay roughly £180–£350 for a thermistor (R2T sensor) replacement including parts and labour, or £350–£600 for a refrigerant leak repair plus recharge. If the circulation pump needs replacing, budget around £200–£400 fitted. More serious repairs — such as a PCB replacement (£450–£900) or, in the worst case, a damaged plate heat exchanger — will cost considerably more. Compressor replacement following freeze damage can run to £1,400 or above, which is why acting quickly on this fault matters.
Does the Daikin warranty cover an 89-02 freeze-up fault?
It depends on the cause. Daikin's standard residential warranty covers manufacturing defects for three years (five years commercial), provided the unit has had annual servicing by an F-Gas certified engineer and records are kept. If the freeze-up was caused by a factory-faulty sensor or PCB, a warranty claim is likely valid. However, if the cause is a refrigerant leak, low water pressure, a blocked strainer, or lack of servicing, these are generally not covered. Always contact Daikin or your approved Daikin Service Partner first before arranging any repair under warranty — attending without prior authorisation can make the visit chargeable even if the unit is within its warranty period.