Which of the following is NOT a valid step in troubleshooting a faulty LCD panel?

The reason "Replace LCD assembly immediately" is the correct choice for the invalid step in troubleshooting a faulty LCD panel is that it represents a premature action in the troubleshooting process. In any systematic troubleshooting approach, especially for hardware issues, it's essential to thoroughly diagnose the problem before taking significant actions like replacing components.

Effective troubleshooting should begin with confirming the issue through various checks, such as running ePSA tests to identify potential hardware failures, checking for software updates that might resolve display issues, and connecting the system to an external display to determine if the problem originates from the LCD panel itself or if it's a broader issue related to the graphics hardware.

By following these methods first, a technician can avoid unnecessary replacements, which can save time and resources. Replacing the LCD assembly without completing these essential diagnostic steps may lead to further complications or expenses, especially if the LCD was not the actual cause of the problem. Thus, replacing the LCD assembly immediately is not a valid part of a logical troubleshooting sequence.