What indicates a problem in the cockpit differential pressure?

The presence of a red CKPT PX warning light in the cockpit effectively indicates a problem with the differential pressure. This warning is crucial as it is designed to alert pilots to a potential issue with cabin altitude pressure or seal integrity, which can compromise safety if not addressed. When the red CKPT PX warning light illuminates, it signifies that the cockpit pressure has fallen outside of the acceptable limits, requiring immediate attention from the crew. This is especially critical in flight, where unregulated pressure can lead to hypoxia or other physiological issues for the pilots.

The other options are indicative of normal operation or lesser issues: a green CKPT PX light generally signifies that the system is operating within the normal parameters, while a yellow system warning may hint at a cautionary issue that does not require immediate action. A blue altitude indicator reflects the current altitude and does not pertain to pressure issues directly. Therefore, the red CKPT PX warning light's specific association with differential pressure problems makes it the correct choice in this context.