Which factor could contribute to high pressures in an AC system?

High pressures in an AC system can indeed be attributed to a dirty condenser coil. When the condenser coil is coated with dirt, dust, or debris, it impedes the coil’s ability to release heat. This reduced heat exchange efficiency causes the refrigerant to remain in the condenser coil for a longer period, leading to an increase in both the temperature and pressure of the refrigerant. As the pressures rise, it can ultimately cause the system to operate inefficiently and run a higher risk of damage.

In contrast, low outdoor temperatures would generally contribute to lower pressures, as the cooling effect on the refrigerant would be enhanced, allowing the system to function properly. An unplugged power source would stop the entire system from operating, hence there wouldn't be any pressure build-up. Low refrigerant levels would typically result in lower pressures because there is less refrigerant in the system to absorb and release heat. Therefore, a dirty condenser coil is a critical factor in contributing to high pressures in an AC system.