Final answer:
When servicing an air dryer, the desiccant filter is typically replaced. The condenser is the component in an air conditioner that converts gas back into liquid.
Step-by-step explanation:
When servicing an air dryer, the component that often requires replacement is the desiccant filter. In most air dryers used in automotive air conditioning systems, the desiccant filter is responsible for removing moisture from the compressed air. It's important to change this filter to ensure the air dryer continues to operate effectively. As for the component of an air conditioner that converts gas into liquid, it is the condenser. The compressor pressurizes the refrigerant gas and pumps it into the condenser where it releases heat and changes back into a liquid form.