What is the first step for handling an aircraft below FL180 with an invalid Mode C readout?

The first step for handling an aircraft below FL180 with an invalid Mode C readout is to issue the correct altitude setting and confirm altitude. The primary concern in such a situation is ensuring that the aircraft maintains safe vertical separation from other traffic. By issuing the correct altitude setting, air traffic control can help mitigate the risk of altitude deviations that may arise from an invalid Mode C readout. This step serves to verify whether the aircraft is operating at the communicated altitude and to ensure that the pilot is aware of the required altitude to maintain safety.

This action also promotes clear communication between air traffic control and the pilot, ensuring both parties are aligned on altitude expectations. Confirming the altitude allows the controller to assess whether any further actions are necessary, such as reassigning altitude or providing traffic advisories if the aircraft is climbing or descending incorrectly.