The TXV being installed on most A/C systems is a very simple device that only has a couple of moving parts. They are very reliable and will operate trouble free for years if installed and maintained properly. Overheating the TXV while installing and obstructions are some causes of TXV failures.
Like with any type of metering device, we need to confirm proper airflow before trouble-shooting. The simplest way to do this is by checking the TD or temperature difference across the evaporator. With an accurate thermometer, check the return air temperature and the evaporator leavening air temperature. The difference between the two is the Evaporator TD. This TD should be 20 degrees or less. If the TD is higher than 20, we have insufficient airflow that must be addressed before trying to troubleshoot any other part of the evaporator including the TXV. A low TD, less than 14 degrees, can be caused by a starved evaporator or a heavy load.
The next thing that must be confirmed is the bulb location and installation. The bulb should be securely attached to the suction line at the evaporator outlet. The bulb should be mounted to the suction line at the 2 or 10 o'clock position for suction lines smaller than 7/8". For suction lines, 7/8" and larger the bulb should be mounted at the 4 or 8 o'clock position. The bulb must securely be attached to a clean suction line and insulated as to not be affected by air temperature. Most A/C evaporators require an externally equalized TXV. If the equalizer line is not connected, the valve will not operate properly. The equalizer line should be attached to the suction line near the sensing bulb
Now we can troubleshoot the TXV. The TXV does not operate on temperature but on superheat, so what is the evaporator superheat? Attach an accurate electronic thermometer securely to the suction line at the evaporator outlet. With an accurate low-pressure gauge measures the suction pressure and convert it to saturated suction temperature. Subtract your saturated suction temperature from your suction line temperature. The difference between the two is your evaporator superheat. Normally this evaporator superheat should be between 8 and 12 degrees. High superheat indicates a starved evaporator caused by such things as low charge, obstructed liquid line or a heavy load. A low superheat indicates overfeeding the evaporator caused by such things as bad bulb location, loose bulb or low air flow. Either condition can be caused by a malfunctioning TXV, but that doesn’t happen often.
Some TXVs are adjustable while many OEM valves are not. Either way, readjusting the TXV is the last thing we ever want to do. If you have an adjustable TXV valve that is out of adjustment, only adjust it by superheat. Do not try to adjust a non-adjustable TXV. TXV superheat should not be adjusted by adding or removing refrigerant to or from the system. A low charge can cause high superheat, but never charge by evaporator superheat, only by subcooling.