The answer to 'evaporator coil smells bad' is that a bad smell from the evaporator coil is usually caused by mold, mildew, or bacterial growth. This can happen when the coil becomes damp and doesn't dry out completely, providing a breeding ground for microorganisms. The smell may be musty, sour, or unpleasant.
Some common causes include a blocked or dirty air filter, leaks in the HVAC system, and poor air circulation. Over time, this can lead to the coil becoming a source of indoor air pollution and potential health issues.