Daikin A5 fault codewhat it means and how to fix it
Freeze-up protection or high pressure control. A filthy indoor filter is a legitimate published cause, which makes this the one on the list worth checking yourself.
Take care
Which Daikin appliance is showing A5?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
A wall-mounted air conditioner with a remote control
This page is yours. A5 is freeze-up protection or high pressure control, and a blocked indoor filter is a published cause.
Switch off, let any ice melt, clean the filters and check the airflow, then run it again.
An Altherma heat pump with radiators or a hot water cylinder
Same headline fault, different machine. An Altherma has no indoor filter, and Daikin’s first checks for it are water pressure, flow, stop valves and air.
Use the Daikin A5 page in the heat-pumps section of this site instead.
What A5 means
A5 sits under Daikin's INDOOR UNIT heading and covers high pressure control or the freeze-up protector. It is on this site as a caution rather than a stop because, almost uniquely among Daikin's codes, the causes Daikin publish for it include ones you can do something about: a clogged indoor filter, dust on the indoor coil, or air that cannot get through the unit. Those causes are not in the owner's manual, which prints the code and its name and nothing else; they are in Daikin's service manuals, which are written for engineers. Cleaning the indoor filter, though, is a task the owner's manual does give you, and on this code it is the first thing to do rather than the last.
The mechanism is worth understanding because it explains why the symptom looks so much worse than the cause. The indoor coil gets cold by design, and it stays above freezing because a steady flow of warm room air passes over it. Restrict that airflow and the coil temperature falls until it starts making ice rather than water. Ice further blocks the airflow, which drops the temperature further, and the process runs away with itself. Freeze-up protection exists to stop the system before that does damage, so what you are seeing is a safety response to an airflow problem rather than a component failure.
What restricts the airflow is usually the filter, and filters get ignored because they are behind a flap and out of sight. It can also be the coil itself furred with dust behind the filter, the fan running slowly, a blocked or crushed outlet, or furniture pushed against the unit. All of those produce the same result. The reason this matters financially is that a restricted indoor coil can present as something much more expensive. Mitsubishi, on their own equipment, list a blocked indoor coil alongside lack of refrigerant and faulty expansion valve operation as causes of an abnormal low-pressure sensor reading. Refrigerant work is certified and costly. Filters are free. If the filters have not been cleaned, the fault has not been diagnosed.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| A blocked or filthy indoor filter | A clogged air filter of the indoor unit is the second cause on Daikin’s service list, and the form the problem usually takes. Free to check and free to fix, so it comes first. | Common |
| A dust-clogged indoor coil | Dust accumulation on the indoor heat exchanger, in Daikin’s words: behind the filter, and what happens after years of running without cleaning one. Restricts airflow the same way but needs a proper clean rather than a rinse. | Common |
| Obstructed airflow at the unit | Daikin call this short-circuited air, and their remedy is to provide sufficient air passage. Furniture pushed against the unit, a blocked outlet grille or curtains across it all do it. | Occasional |
| A slow or failing indoor fan | Produces the same restriction without anything visibly blocking it. Daikin publish a separate fan motor code, so this may show as that instead. | Occasional |
| A genuine refrigerant or pressure fault | The case that needs certified work. Daikin’s own list ends with a defective indoor heat exchanger thermistor and a defective indoor PCB rather than refrigerant, and the code’s name covers high pressure control; all of it is engineer work, reached after the airflow causes have been eliminated rather than before. | Occasional |
What to try yourself
Switch the system off and let any ice on the indoor coil melt before you assess anything. Running it iced makes everything worse and tells you nothing.
Open the front flap and remove the filters. They come out without tools, and this is explicitly something the manufacturer expects you to do.
Wash them, dry them fully, and refit them. Refitting damp filters puts the moisture straight back into the airflow you are trying to fix.
Look at the coil behind the filter. A grey felted surface means the coil itself needs cleaning, which is a service job rather than a rinse.
Check nothing is obstructing the intake or the outlet — furniture, curtains, or anything resting on top of the unit.
Run it again and see whether the code returns. If it does with clean filters and clear airflow, retrieve the code and call an engineer.
Do not go beyond the filter cover. The coil, the fan and the sensors sit alongside mains wiring, and the pipework behind them is part of a sealed refrigerant circuit that is certified work to open.
Get an F-Gas certified heat pump engineer to look at your Daikin A5
We do not pass enquiries to a repair firm, so there is no form here. When you book an F-Gas certified heat pump engineer, tell them the make, the model and the exact code or message on the display, and ask whether they carry parts for this make — that is what turns one visit into a repair rather than a quote.
What it costs to fix
Typical repair cost
£0–£400
Time on site
45 minutes
Nothing if it is the filters, which is the outcome worth pursuing first. A coil clean is a service charge, and a genuine pressure or refrigerant fault is substantially more because it is certified work.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Daikin fault codes
- U0Get it checkedRefrigerant shortage. The circuit is sealed, so if it is short it has leaked — and this is certified work by law, not a top-up.Related: same brand
- U4Get it checkedThe indoor and outdoor units have stopped talking. Daikin’s own cause list starts with the mains supply and the inter-unit cable, so one breaker cycle is worth a try before anyone is called.Related: same brand
- E6Get it checkedThe compressor failed to start. It sits in Daikin’s outdoor unit group alongside the overload and pressure codes, and all of them are certified work.Related: same brand
- 00Usually simple to fixNot a fault. Daikin publish 00 as NORMAL, and it is also what shows while you are stepping through the diagnostic.Related: same brand
Own a Daikin air conditioner? Save it and its codes come first next time.
Related guides
Common questions about Daikin A5
Can a dirty filter really cause this?
Yes. Daikin’s service manuals list a clogged air filter of the indoor unit, dust on the indoor heat exchanger and short-circuited air as causes of the freeze-up protection this code covers, and the first remedies in their own troubleshooting flow are to clear the air passage and clean the filter. It is counterintuitive because the symptom — a frozen coil and a system that has shut down — looks far more serious than the cause. Clean filters first, every time.
There is ice all over the indoor unit.
That is the freeze-up this code is protecting against. Switch the system off and let it melt naturally rather than chipping at it or aiming heat at it, then find out why the airflow was restricted. Running it again without fixing the airflow just refreezes it.
How often should the filters be cleaned?
Daikin’s figure is once in about every two weeks, and their manual says clogged filters cause inefficient operation and waste energy. More often in a dusty room or one with pets. It takes a couple of minutes and it prevents a category of fault that otherwise ends in a call-out.
It still shows A5 with clean filters.
Then you have eliminated the cheap cause, which is worth having done. What remains is the coil itself needing a proper clean, the fan, or a genuine pressure or refrigerant fault — and the last of those is certified work. Retrieve the code and hand it over.
I have a Daikin Altherma heat pump showing A5, not an air conditioner.
Same code and the same headline, high pressure control or freeze-up protection, but an Altherma has no indoor filter to clean. Daikin’s Altherma checklist for A5 starts on the water side: system pressure, flow, stop valves and air in the circuit. This page is written for a wall split; the Daikin A5 page in our heat-pumps section is the one for an Altherma.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.