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When the Automatic Temperature Control (ATC) system is responsive to the ambient and in-vehicle temperature sensors but does not react to the sunload sensor, the most likely cause is a faulty sunload sensor.

The sunload sensor plays a crucial role in adjusting the climate control settings based on the amount of sunlight entering the vehicle. Its primary function is to detect solar radiation and adjust the temperature control system accordingly to maintain a comfortable environment. If the ATC system functions properly with other sensors, it indicates that those components are operational. However, when the system fails to take sunload readings into account, it suggests that this specific sensor is not transmitting accurate information to the ATC.

In contrast, issues with the system programmer or the ambient and in-vehicle sensors would typically affect the entire climate control system, leading to more widespread problems, while a defective sunload sensor would result specifically in the system's failure to adjust based on direct sunlight, which is consistent with the symptoms observed.