Quick answer
Daikin error code U0 means refrigerant shortage. The usual cause is a leak, most often at a flare joint, though a closed stop valve or an expansion-valve or sensor fault can mimic low charge. Do not keep running it: a system short of refrigerant overheats the compressor. Turn it off, look for ice indoors or oil stains at the outdoor connections, and call a licensed refrigeration mechanic.
Daikin error code U0 means the system has detected a refrigerant shortage. The control watches how the compressor is running, and when the readings match a system that is low on charge, it stops and posts U0. Refrigerant is not used up like fuel, so a shortage on a system that once worked means it has leaked out somewhere, or something is behaving as if it had.
Why U0 is a turn-it-off code
Do not run a Daikin system that is showing U0. Refrigerant does two jobs: it moves heat, and the refrigerant returning to the outdoor unit helps keep the compressor cool. Short of refrigerant, the compressor runs hot, and the compressor is the most expensive part of the system to lose. The control shuts down to prevent that, and clearing the code to squeeze out another hour of heating or cooling works against the protection.
A small leak is rarely an immediate hazard to people in the house, but it is not something to ignore either; whether a refrigerant leak is dangerous covers the health and safety side. Newer equipment uses R-32, which is mildly flammable, so a suspected leak indoors is one more reason to leave the system off.
What you can look for without touching the refrigerant side
- Switch the system off at the remote, and at the outdoor disconnect if you know where it is.
- Open the indoor unit's front panel and look at the coil behind the filters. Frost or ice on part of the coil is a typical sign of low charge.
- At the outdoor unit, look at the copper connections where the line set meets the unit. An oily, dust-collecting stain on or below a flare nut marks a leak, because escaping refrigerant carries compressor oil out with it.
- Follow the line set along the wall and look for kinks, rubbed-through insulation or damage from a trimmer or ladder.
- Note when the system was installed and whether it ever worked properly. A U0 in the first weeks points to the installation; a U0 years later points to a joint that has been seeping.
That is the limit of homeowner work. Do not tighten the flare nuts, open the valve caps or connect gauges.
Causes of a Daikin refrigerant shortage code, most likely first
- A refrigerant leak at a flare joint. Ductless systems are connected on site with flared copper fittings, and a flare that was made slightly off, under-tightened or over-tightened can seep for months before the charge falls far enough to trip U0.
- A leak elsewhere in the circuit, such as a line set that has rubbed through or a coil.
- A closed or partly closed stop valve at the outdoor unit. The refrigerant is in the system but cannot circulate. It turns up after an install or a service visit where a valve was not fully reopened.
- An electronic expansion valve or sensor fault that mimics low charge. A valve that sticks, or a thermistor that reads wrong, gives the control the same picture as a shortage while the full charge is still in the pipes.
Daikin's service manual lists U0 as refrigerant shortage for the FTXS and FDXS indoor units paired with RXS outdoor units. Code tables are not identical across every Daikin series, so treat the manual and labels that came with your unit as the reference.
Who is allowed to fix U0 in Ontario, and what the repair involves
Always call for this one. Handling refrigerant in Ontario requires an Ozone Depletion Prevention (ODP) card, and the work belongs to a licensed 313A or 313D refrigeration and air conditioning mechanic. The proper repair has a fixed order: find the leak, fix the leak, pressure-test with nitrogen, pull a vacuum, then weigh in the correct charge for the unit and its line length. Adding refrigerant without finding the leak is a top-up that will leak out again.
- Diagnostic visit: $90–$150, folded into the repair when you proceed.
- Refrigerant leak repair and recharge: $300–$800+, since finding and sealing the leak costs more than the refrigerant.
- Expansion valve, thermistor or coil replacement: quoted in writing after the leak search, because the part and the labour vary by model.
If the leak is in a coil on an older unit, the quote may come close to the value of the system. Whether to repair or replace a heat pump sets out how to weigh that.
ZK Mechanical finds and repairs the leak before any refrigerant goes back in, and puts the price in writing first. Book heat pump repair or read about refrigerant leak repair; related codes are in the Daikin error code guide. The shop line is (647) 801-1252, and the contact form reaches the same people.