Daikin A5-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin A5-00 fault code mean?
The A5-00 fault code indicates that the indoor heat exchanger has exceeded its safe operating limits — either overheating during a heating cycle (high-pressure protection) or entering freeze-up protection during a cooling cycle. The '-00' sub-code denotes the primary detection event. When the system's control board detects that refrigerant pressures or heat exchanger temperatures have moved outside their acceptable range, it locks out operation and displays this code to prevent 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
- Dirty or clogged air filter Common
A heavily loaded air filter is the single most common trigger for A5-00. When airflow across the indoor heat exchanger is starved by a thick layer of dust and debris, the coil cannot transfer heat effectively. This drives refrigerant pressures and temperatures outside their normal operating window, tripping the protection logic. Daikin recommends inspecting filters every two months — many callouts turn out to be nothing more than a neglected filter.
- Ice build-up on the indoor heat exchanger coil Common
If the heat exchanger coil partially or fully ices over — often as a downstream consequence of restricted airflow or low refrigerant — heat transfer collapses entirely. The control board interprets this as a freeze event and shuts the system down. You may notice water dripping from the indoor unit as the ice thaws after shutdown.
- Low refrigerant charge due to a leak Common
A slow refrigerant leak — commonly from a loosened compression fitting or a small crack in the pipework — reduces the system's ability to absorb or reject heat. The resulting abnormal pressures trigger both freeze-up and high-pressure protection. Under UK F-Gas regulations, any confirmed refrigerant leak must be repaired and the circuit recharged by an F-Gas registered engineer.
- Faulty heat exchanger thermistor Sometimes
The thermistor attached to the indoor heat exchanger continuously reports its temperature to the PCB. If this sensor drifts out of calibration, develops a high resistance, or fails outright, the control board may receive a reading that falsely indicates an overheating or freezing condition, logging an A5-00 even when the refrigerant circuit is healthy.
- Faulty or stuck expansion valve (EV) Sometimes
The electronic expansion valve meters refrigerant flow into the indoor heat exchanger. If the EV coil fails or the valve sticks open, too much refrigerant floods the coil, causing the pressures and temperatures to deviate sharply from their target range. A stuck-closed valve produces the opposite problem — a starved coil that quickly freezes. Either scenario can register as A5-00.
- Dirty evaporator or condenser coil (outdoor unit) Sometimes
Fouling on the outdoor coil raises system pressures and forces the indoor coil to compensate, eventually pushing it into protection. Compacted dirt, leaf debris, or in winter a blanket of snow and ice around the outdoor unit can all contribute. This is distinct from the indoor filter issue — the coil itself needs a professional chemical clean.
- Short-cycling due to an oversized or poorly configured system Rare
A unit that repeatedly starts and stops before completing a full heating or cooling cycle never allows the refrigerant to stabilise at its design pressures. This erratic behaviour can progressively push the heat exchanger into an A5-00 condition, particularly in systems where weather compensation or zone settings have been misconfigured.
How to fix it
- Switch the unit off at the controller and allow at least 30 minutes for any ice on the indoor coil to thaw DIY safe
Do not attempt to defrost the coil manually with any heat source. Simply let the system sit unpowered. If you can see visible ice or heavy condensation around the indoor unit, this confirms freeze-up was involved. Place some towels below the unit to catch melt water.
- Check and clean the indoor air filter DIY safe
Open the indoor unit's front panel and carefully remove the filter cartridge. If it is visibly grey with dust, rinse it under lukewarm water, allow it to dry completely (do not refit a wet filter), then reinstall it. This single step resolves a significant proportion of A5-00 callouts and costs nothing.
- Clear any obstructions around the outdoor unit DIY safe
Walk around the outdoor unit and remove any accumulated leaves, garden debris, or snow that is blocking the intake or discharge grille. Maintain at least the clearance specified in your installation manual — typically 300 mm on the intake side and 500 mm from the fan discharge. Do not reach into the fan aperture.
- Check the system pressure on the filling loop or pressure gauge DIY safe
Low water-side pressure in the heating circuit (below approximately 1.0 bar on most Daikin Altherma systems) can indirectly stress the heat exchanger. Check the pressure gauge on the unit or the buffer cylinder. If it reads below the manufacturer's recommended range (typically 1.0–2.0 bar), top up via the filling loop following your system's user guide. If the system keeps losing pressure, inform the engineer — there may be a leak on the water side.
- Reset the unit from the controller and observe whether A5-00 returns DIY safe
Once the filter is clean, ice has thawed, and the outdoor unit is clear, restore power and attempt a reset from the Daikin controller or BRC user interface (hold the reset function as described in your user manual). Run the system for 20–30 minutes and monitor whether the fault code reappears. A fault that clears and stays clear suggests the filter was the culprit. A fault that returns promptly indicates a deeper issue requiring professional attention.
- If the fault persists, arrange an inspection by an MCS-certified heat pump engineer heat pump engineer
A returning A5-00 points to one of the refrigerant-circuit or sensor faults that a homeowner cannot safely diagnose or repair. The engineer will check refrigerant pressures, test the heat exchanger thermistor, inspect the expansion valve and its coil, and verify PCB connections. Any refrigerant leak detection, recovery, or recharge must be carried out by an F-Gas registered engineer under UK regulations. Do not attempt to access the sealed refrigerant circuit yourself.
Parts you may need
- Indoor heat exchanger thermistor (temperature sensor) · from £35
- Electronic expansion valve (EV) coil · from £85
- Electronic expansion valve assembly · from £220
- Indoor unit PCB (printed circuit board) · from £195
- Indoor fan motor · from £120
- Replacement air filter cartridge · 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 £120–£420, depending on the underlying cause.
Frequently asked questions
Can I fix Daikin A5-00 myself without calling an engineer?
The only homeowner-safe steps are cleaning the air filter, clearing snow or debris from around the outdoor unit, checking that the system pressure is within range, and performing a reset. These actions resolve A5-00 in a meaningful number of cases — particularly if the filter has not been cleaned in several months. However, if the fault returns after those checks, the underlying cause almost certainly involves the refrigerant circuit, expansion valve, or a sensor, all of which require a qualified professional. Attempting to access the sealed refrigerant circuit yourself is unsafe, illegal without F-Gas registration, and will void your warranty.
How much does it typically cost to fix a Daikin A5-00 fault in the UK?
Most households with this fault pay somewhere between £120 and £420 all-in, covering a diagnostic visit plus the most common repairs such as a professional coil clean, a thermistor replacement, or an EV coil swap. Refrigerant leak repairs add cost — expect £150–£350 on top depending on the extent of the leak and the refrigerant type. At the more expensive end, a faulty expansion valve assembly or PCB can push costs higher; a compressor replacement (rare for this fault code) can reach £1,500 or more, but that is genuinely unusual for an A5-00 presentation. If your unit is still under warranty or you hold a Daikin maintenance contract, parts and labour costs may be covered in full.
Why does my Daikin keep showing A5-00 after I reset it?
A fault code that clears on reset but comes back within a few hours or days has not been fixed — the reset simply clears the log entry while the underlying problem continues. Recurring A5-00 typically means restricted airflow that persists (either a deeply fouled coil that needs a chemical wash, not just a filter rinse), a refrigerant circuit that is slowly losing charge through a leak, or a failing thermistor giving erratic readings. Each time the system hits the protective threshold, it will lock out again. An MCS-certified engineer with manifold gauges and diagnostic equipment is needed to find the root cause.
How often should I service my Daikin heat pump to avoid A5-00?
Daikin recommends inspecting and cleaning the indoor air filter every two months during regular use — this is a straightforward homeowner task. A full annual service by an MCS-certified engineer should also be scheduled, during which the refrigerant pressures, expansion valve operation, coil cleanliness, thermistor readings, and water-side pressure are all verified. Skipping annual servicing is a leading contributor to A5-00 faults and can also invalidate your warranty or MCS certification records, which matters if you receive the Boiler Upgrade Scheme (BUS) grant.