Frost formation on an automotive AC system's Expansion Valve (Thermal Expansion Valve, TXV) signals a critical disruption in thermodynamic equilibrium. This phenomenon stems from localized temperatures plunging below freezing at the valve orifice, primarily triggered by abnormal refrigerant conditions or component failures.

I. Refrigerant Issues: Core Causes.

1. Incorrect Refrigerant Charge

• Low Charge: System leaks cause evaporation temperatures to drop below -10°C, allowing moisture to freeze at the valve.

• Overcharge: Excess liquid refrigerant floods back to the valve, intensifying supercooling. 

2. Refrigerant Contamination

• Moisture: Incomplete vacuuming or saturated desiccant enables ice formation. 

• Debris: Metal particles create blockage-induced cold spots.

II. Component Failures: Contributing Factors.

1. Expansion Valve Malfunction 

• Stuck-open valves cause excessive refrigerant flow and temperature crashes.

2. Receiver-Drier Failure

• Saturated desiccants release moisture into the system.

3. Airflow Deficiencies 

• Clogged evaporator or blower failures disrupt heat absorption.

III. Diagnostic Protocol  

1. Visual Inspection: Check for oil stains (leaks), airflow, and filter condition.  

2. Pressure Tests: Measure high/low-side pressures.  

3. Refrigerant Analysis: Verify purity and moisture content.  

4. Component Checks: Inspect receiver-drier and valve strainer.

Summary

Over 70% of expansion valve frosting cases originate from refrigerant issues (low charge/moisture). Component failures typically amplify these problems. Resolution requires:① Fixing leaks & replacing receiver-drier.② Precise refrigerant recharge.③ Addressing valve/airflow faults.