The TXV, or metering device.

Between the outdoor unit and the indoor coil, something has to drop the refrigerant from high pressure to low pressure and meter how much gets through. That is the metering device.

The simple version is a fixed orifice: a precisely sized hole. The better version is a TXV, a thermostatic expansion valve, which senses the temperature of the refrigerant leaving the coil and opens or closes to keep the flow right as conditions change.

A TXV is why a system holds up better across a wide range of outdoor temperatures, which in a Valley summer is worth having.

How a failing one behaves

A TXV that is stuck mostly closed starves the coil. Suction pressure drops, the coil gets too cold, and it frosts. From the driveway that looks exactly like a system low on refrigerant.

A TXV that is stuck open floods the coil. Suction pressure runs high, the system cools poorly, and liquid can make it back toward the compressor, which is the expensive version.

Both can also come and go, because the sensing bulb responds to conditions. A system that works in the morning and misbehaves in the afternoon is worth measuring at both times rather than diagnosing once.

Why it matters to you

This is the most common honest misdiagnosis in the trade. The symptoms overlap almost perfectly with a low charge, and the fix for a low charge is to add gas, which is quick and billable.

The way to tell them apart is superheat and subcooling readings taken together, not pressure alone. If somebody added refrigerant to your system and it froze again a week later, a metering device problem is worth ruling out before more gas goes in.

That is readings and arithmetic, and you should get a copy of them.