A thermostatic expansion valve (TXV) is the metering device that controls exactly how much refrigerant flows into the evaporator coil. It senses the temperature of the refrigerant leaving the coil and modulates the flow to maintain the right amount of superheat, so the coil is fed precisely as the load changes. Most modern, higher-efficiency systems use a TXV; older and budget systems use a simple fixed orifice (piston) instead.
Because a TXV actively meters refrigerant, a system with one is charged to a subcooling target — typically about 10 to 15°F — rather than by superheat. When a TXV fails it usually sticks: stuck too closed, it starves the coil and the system underperforms and can freeze; stuck too open, it floods the coil and can return liquid toward the compressor.
The reason a TXV matters to a homeowner is that its symptoms imitate a refrigerant problem. A starving TXV looks a lot like a low charge, so a tech who reaches for the refrigerant gauges without checking the valve can 'fix' it with refrigerant the system doesn't need — masking the real fault and overcharging the system. A proper diagnosis distinguishes a failing TXV from an actual leak before any refrigerant or parts are sold.