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Use Your Senses First
A good technician uses their senses before they use diagnostic tools. Is your suction line abnormally cold? Make sure the evaporator coil isn't frozen, and inspect for obvious airflow issues like a dirty filter or evaporator coil.
Is your liquid line abnormally warm to the touch? It could be a dirty condenser, condensing fan issue, or overcharge.
Listen for abnormal motor and compressor noises, watch for signs of corrosion, and take note of oil for possible leaks.
Smell for signs of burning lacquer, which can signal burned motors or controls.
Listen for a blower that sounds like a train engine. (If it's an ECM, it could be an airflow restriction.)
Train your senses to spot abnormalities, and you will save time and catch issues before you need to pull out tools for confirmation.
This all isn't to say that you shouldn't use diagnostic tools, just that you will save a lot of time if you use the best tools of all first, the natural kind.