When your air conditioning smells bad, it's a clear sign that something isn't working properly within the system. The unpleasant odor can come from various sources—mold and mildew are among the most common causes, thriving in the moisture that builds up inside AC units. Clogged drain lines, dirty air filters, or stagnant water in the drip pan can all contribute to a musty smell. In more severe cases, a foul or rotten odor could be due to a dead animal in the ductwork, while a burning smell might indicate electrical problems or overheating components. Regardless of the cause, when your air conditioning smells bad, it's essential to address the issue promptly to maintain indoor air quality and ensure the system operates safely and efficiently.