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Error Code
U0 reports a refrigerant shortage. The system has worked out that there is less gas in the circuit than it needs, usually by comparing what its sensors are telling it against what it expects. This is the code that most often leads to a leak being found, and it should not be treated as a top up job, because gas does not get used up.
Refrigerant is a sealed charge and it does not deplete with normal use, so a low level means it has escaped somewhere. The system detects the shortfall and shuts down rather than running the compressor at a temperature it cannot handle. Adding gas without finding the leak guarantees a repeat call next season.
The usual locations are flare joints, brazed connections, the Schrader valves and the coil itself. Vibration, poor brazing and corrosion each take their turn. A nitrogen pressure test, or an electronic detector on a system that still has gas in it, finds them. Some leaks only show themselves under heat.
A restriction can mimic a shortage, because the system sees less flow than expected and reports U0. A blocked filter drier or a kinked line produces the symptom without any gas having escaped. Temperature differences across components are what separate a blockage from a genuine leak.
Charge is confirmed by weight or by subcooling and superheat against the manufacturer figures. If it is short, the leak search happens before any gas goes in. Refilling a leaking system is the most expensive way to delay the real repair.
Every joint on the circuit is checked with an electronic detector, and flare connections and Schrader cores get particular attention because they are the most common failures. Where nothing shows, the system is pressure tested with nitrogen and left to stand. The drop tells you there is a leak; the detector tells you where.
Recharging is not a matter of adding gas until it cools. The correct charge depends on the line length and the manufacturer spec, and overcharging causes faults of its own. Handling refrigerant also requires licensing, so this is the one step on the list that is genuinely not a do it yourself job.
