Plain-language explanation: This page is educational and does not replace testing the actual equipment.
Why defrost is needed
In heating mode, the outdoor coil can run below freezing and collect frost. Periodic defrost keeps airflow open.
Symptoms of defrost trouble
The outdoor coil may become heavily iced, the system may remain in defrost too long, or defrost may occur too frequently.
Other causes of icing
Low refrigerant, airflow problems, sensor failures, outdoor-fan problems, drainage issues, and weather conditions can mimic board failure.
What technicians test
They may check sensors, board inputs and outputs, coil temperature, fan operation, refrigerant conditions, and defrost initiation.
Frequently asked questions
Is some frost normal?
Yes. A light frost can be normal before a defrost cycle.
Is steam during defrost normal?
Yes. Melting frost can create visible steam.
Should I remove ice manually?
Do not strike or chip the coil. Turn the system off and request service if ice becomes severe.