Troubleshooting CDI and Heading Alignment in FSX G1000 GPS Approaches

I understand you're encountering issues with GPS approaches in FSX using the Cessna 172 equipped with the G1000. Your challenge regarding the absence of CDI during the approach and incorrect heading alignment can indeed be perplexing. Let me guide you through some steps to potentially resolve this:

1. CDI Mode Selection: It's crucial to ensure that your CDI (Course Deviation Indicator) is set to GPS mode. This can be achieved by pressing the CDI button on the G1000 until GPS mode is selected. This step is critical for the G1000 to guide you correctly during GPS approaches.
2. Approach Activation: In FSX, you must actively load the approach at your intended destination. This involves using the PROC (Procedure) button on the G1000, choosing your approach, and selecting the appropriate transition. For enhanced situational awareness and preparedness for any changes, it's advisable to load the approach from a fix until you're closer to the airport.
3. Approach Execution: Once the approach is loaded and activated, the G1000 should display a magenta line leading to the final approach fix. Ensure you're on the correct intercept course for the approach. Engaging the NAV mode on the autopilot can aid in following the GPS course.
4. Checking for Bugs and Updates: Be aware of potential bugs in the G1000, especially in earlier versions of FSX. Ensure that your simulator is updated to the latest version. For additional features and bug fixes, consider using mods like the Working Title G1000 mod, which enhances G1000 functionality, including automatic CDI switching and improved autopilot guidance.
5. Direct-To Navigation: If not already in use, try the "Direct-To" button (D->) on the G1000 to navigate directly to your first waypoint or the initial approach fix. This might help realign the aircraft with your intended flight path.

Should these steps not resolve the issue, I'd recommend providing more specific details or screenshots of your G1000 screen during the approach phase for more precise diagnosis. Furthermore, engaging with flight simulation forums and communities can offer additional insights and solutions from experienced simmers who may have faced similar challenges.

Safe flying and let me know if you need further assistance!