Daikin U0 Fault Code: Refrigerant Shortage Detected
The U0 fault code indicates that your Daikin system has detected symptoms consistent with a refrigerant shortage. Internally, the unit monitors suction pressure, superheat levels, and the behaviour of the Electronic Expansion Valve (EEV). When these readings suggest the refrigerant charge is too low — or that flow is being restricted — the system locks out to protect the compressor from running without adequate lubrication and cooling. Daikin positions U0 as an early protective lockout, typically triggering before more severe faults such as E4 or F3 appear.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the Daikin U0 fault code mean?
The U0 fault code indicates that your Daikin system has detected symptoms consistent with a refrigerant shortage. Internally, the unit monitors suction pressure, superheat levels, and the behaviour of the Electronic Expansion Valve (EEV). When these readings suggest the refrigerant charge is too low — or that flow is being restricted — the system locks out to protect the compressor from running without adequate lubrication and cooling. Daikin positions U0 as an early protective lockout, typically triggering before more severe faults such as E4 or F3 appear.
Common causes
- Refrigerant leak Common
Refrigerant does not deplete through normal use — if levels are low, a leak is almost certainly present. Common leak points include flare nut connections at the indoor or outdoor unit, braised joints, and Schrader service valves. At the outdoor unit, look for oily or greasy residue around the copper pipe connections, or frost forming on the thinner discharge pipe — both are tell-tale signs of escaping refrigerant.
- Faulty or stuck Electronic Expansion Valve (EEV) Sometimes
The EEV controls how much refrigerant flows through the system. If the valve sticks in a partially closed or fully closed position, the unit may behave as though it is short of refrigerant even when the charge level is correct. The system cannot bring suction pressure into the acceptable range and triggers U0 as a precaution.
- Incorrect installation or loose flare connections Sometimes
U0 appearing on a newly installed or recently serviced system often points to an installation issue rather than a genuine leak. Flare connections that were not tightened correctly, pipe runs that were not properly evacuated before commissioning, or a charge that was not weighed in to the manufacturer's specification can all produce this fault.
- Restricted or blocked refrigerant pipework Rare
A partial blockage within the refrigerant circuit — caused by debris, moisture freeze, or a clogged filter drier — can restrict flow enough to mimic the low-charge symptoms that trigger U0.
- Faulty suction temperature or pressure sensor Rare
If the sensor reporting suction conditions to the PCB is giving inaccurate readings, the control board may incorrectly conclude that refrigerant is short and lock out the system even when the charge is within specification.
How to fix it
- Check for visible signs at the outdoor unit DIY safe
Without touching any refrigerant components, inspect the copper pipework at the outdoor unit. Ice or frost forming on the thinner pipe, or an oily, greasy residue around the flare nut connections, both suggest refrigerant is escaping. Note what you find and pass this information to your engineer — it helps them locate the leak faster.
- Reset the boiler once or twice only DIY safe
Locate the reset button on your Daikin indoor controller or remote. Hold or press it as described in your user manual. If the unit restarts and runs normally, monitor it closely. If U0 returns within a short time, do not continue resetting — repeated resets on a refrigerant-short system risk compressor damage.
- Do not attempt to top up refrigerant yourself Gas Safe engineer
Handling refrigerants in the UK requires F-Gas certification. It is illegal and dangerous to purchase or handle refrigerants such as R410A without the appropriate credentials. Do not connect gauges, open service valves, or attempt a regas — this work must be carried out by a qualified, F-Gas certified HVAC engineer.
- Book an F-Gas certified HVAC engineer for leak detection and recharge Gas Safe engineer
A qualified engineer will connect calibold manifold gauges to read suction and liquid pressures, superheat, and subcooling, comparing results against the manufacturer's charging chart for the current ambient temperature. They will use an electronic leak detector or bubble solution on all joints, flares, and service cores to find the source of any leak. The correct repair sequence is: find and fix the leak first, then fully evacuate the system to specification, then weigh the correct charge of refrigerant back in. Simply topping up without locating the leak is illegal, wasteful, and guarantees the fault will return.
Parts you may need
- Electronic Expansion Valve (EEV) assembly · from £120
- Suction temperature sensor · from £35
- R410A refrigerant (per kg) · from £45
- Filter drier / liquid line drier · from £25
- Flare nut repair kit / copper fittings · from £20
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–£400, depending on the underlying cause.
Frequently asked questions
Can I just top up the refrigerant to clear the U0 fault?
No — and in the UK it is also illegal to do so without F-Gas certification. More importantly, topping up without finding the leak only masks the problem. Refrigerant does not get consumed; if levels are low, there is a leak somewhere. Adding more gas without repairing the leak means the fault will return, often within weeks, and the ongoing refrigerant loss puts the compressor under strain each time. The correct approach is leak detection first, repair, then a full evacuation and weighed recharge.
Could the U0 code damage my compressor?
Yes, if the system is run repeatedly while short of refrigerant. Refrigerant carries the oil that lubricates the compressor; low refrigerant means reduced lubrication and elevated operating temperatures. Daikin designed U0 to lock the system out before serious damage occurs, but resetting it multiple times and allowing it to run in a fault condition removes that protection. Compressor replacement on a Daikin split or multi-split system typically costs £600–£1,500 or more including labour — far exceeding the cost of prompt refrigerant circuit repair.
How long does a refrigerant leak repair and recharge take?
For a straightforward single-split system with an accessible leak, an F-Gas engineer can usually complete leak detection, repair, evacuation, and recharge in two to four hours. More complex multi-split or VRV systems with concealed pipework can take longer. Most engineers charge by the hour (typically £50–£80/hr in the UK), so the total bill depends on how quickly the leak is found and how much pipework needs attention.
Is the U0 code covered under Daikin warranty?
It depends on the root cause. Manufacturing defects — such as a faulty EEV or sensor supplied with the unit — are generally covered under Daikin's standard warranty (typically one to five years depending on the product and whether it was registered). However, refrigerant leaks caused by installation errors or physical damage are usually excluded. If your system is still within the warranty period, contact your Daikin-approved installer or a Daikin service centre before arranging independent repairs, as unauthorised work can void cover.