What causes the yellow IDG light to illuminate?

The yellow IDG (Integrated Drive Generator) light illuminates primarily due to high oil temperature. The IDG is responsible for converting engine power into electrical power for the aircraft's systems, and it contains an oil system that cools and lubricates its internal components. When the temperature of the oil exceeds its normal operating range, it can indicate a potential overheating condition that could lead to a malfunction or failure of the IDG. The illumination of the yellow light serves as a warning to the flight crew to take necessary precautions, which may include monitoring the IDG's performance or planning for a potential disconnect from that generator if conditions do not improve.

In contrast, other options like high oil pressure or low oil temperature do not indicate a malfunction in the same way. High oil pressure is typically a sign of good health for the IDG since it ensures adequate lubrication, while low oil temperature may suggest that the IDG is not operating efficiently but does not activate the warning light. High fuel pressure is unrelated to the IDG's functionality, as it pertains to the fuel system and does not affect the oil condition of the IDG.