A high A/C compressor compression ratio means the compressor is working against an excessive pressure difference between the discharge and suction sides. Common causes include refrigerant overcharge, poor condenser airflow, an inefficient or dirty condenser, restrictions that raise high-side pressure, and evaporator airflow problems that lower suction pressure. Incorrect oil balance can also increase drag and reduce heat transfer. Diagnosis compares pressures to ambient conditions and verifies airflow, charge accuracy, and restriction points before repairs.