When troubleshooting an air conditioning system and you notice frost on the suction line, what would be the problem?

• A. Undercharged refrigerant
• B. Overcharged refrigerant
• C. Dirty air filter
• D. Open refrigerant line

Answer: (C)

Frost on the suction line means the evaporator is getting too cold, which happens when heat isn’t being transferred from the air to the refrigerant efficiently. A common reason for this is restricted airflow across the evaporator, often caused by a dirty air filter. When the filter is dirty, the blower can’t push enough air through the coil, so the coil temperature drops toward freezing and moisture in the air freezes on the coil and lines.

The typical fix is to clean or replace the air filter and ensure the airflow path isn’t blocked. Once airflow is restored and the coil isn’t overcooled, the frost should melt and normal operation can resume. If frost continues after the filter is addressed, other issues like refrigerant charge or a fan problem would be investigated next, but the most immediate and common cause in this scenario is a dirty air filter.