Refrigeration Cycle Diagnostics: What Each Component Tells You

The short field answer:
A refrigeration system talks through pressure, temperature, and state change. The evaporator tells you about load and feeding. The metering device tells you whether the coil is being starved or overfed. The compressor tells you about mass flow, compression, and protection issues. The condenser tells you about heat rejection, liquid quality, and charge behavior.

This is the most useful exam mindset: do not diagnose “the whole unit” first. Diagnose the cycle in order. If suction is wrong, ask whether the evaporator load is wrong, the metering device feed is wrong, or the compressor is not moving vapor correctly. If head pressure is wrong, ask whether the condenser is rejecting heat properly or whether liquid is stacking in the condenser. That is the difference between guessing and troubleshooting.

Good exam logic:
High superheat does not automatically mean undercharge. It can also mean restriction or underfeeding. High head pressure does not automatically mean overcharge. It can also mean dirty condenser, poor condenser airflow, or non-condensables. The component tells you where to look; the readings tell you why. (Copeland Webapps)