The answer to 'why is my central air blowing cold air' is that there are a few common causes, including a refrigerant leak, a problem with the compressor, or a clogged air filter. Symptoms may include the air not getting as cold as it should, the unit running constantly, or higher than normal energy bills.
If your central air is blowing cold air, the first thing to check is the air filter. A clogged filter can restrict airflow and cause the system to freeze up. Replacing the filter is an easy DIY fix. You'll also want to make sure the outdoor unit is clear of debris and the coils are clean. More complex issues like a refrigerant leak or compressor problem will likely require a professional HVAC technician to diagnose and repair.