For an aircraft at or above FL180 with an invalid Mode C readout, what should you verify first?

In the context of aircraft operating at or above Flight Level 180 (FL180), an invalid Mode C readout indicates that the transponder is either not functioning properly or is not sending accurate altitude information. The first step to rectify this situation is to ensure that the aircraft's altimeter setting is correct, which is standardized at 29.92 inches of mercury when the aircraft is operating above FL180.

Verifying the altimeter setting helps confirm whether the altitude being reported by the aircraft aligns with the altitude level assigned by air traffic control. If the altimeter is set incorrectly or fails to reflect the standard setting, it can lead to a miscommunication regarding the aircraft's actual flight level, potentially resulting in unsafe scenarios, including proximity to other traffic.

The other options, while relevant to flight operations, do not directly address the immediate issue of the invalid Mode C readout. Ensuring the pilot's flight plan or current location might provide context about the flight's intended route or position but does not resolve altitude reporting issues. Similarly, the aircraft's heading alone does not aid in correcting the transponder's altitude reporting. Thus, confirming the pilot’s altimeter setting at 29.92 is the most critical first step in this scenario.