To resolve the issue with the NissanConnect app failing to start your vehicle 80% of the time, follow these steps:

1. Check Your Signal: Ensure that your vehicle is located in an area with a strong AT&T wireless signal. Limited connectivity can affect the app's performance. If your vehicle displays a "No Service" error, the telematics control unit (TCU) might be turned off. You should verify the signal strength before attempting to use the app again - learn more.

2. Update the App: Ensure that you have the latest version of the NissanConnect app installed on your mobile device. Go to your app store, check for any available updates, and install them. Sometimes, bugs or usability issues can be resolved with updates - learn more.

3. Clear App Cache: If the app continues to malfunction, try clearing the app cache on your device. This can help resolve issues that arise from corrupted data. To do this, go to your device's settings, find the NissanConnect app, and select "Clear Cache".

4. Reboot Your Device: Restart your smartphone to refresh its operating system. This simple step can often clear temporary issues that may be causing the app problems.

5. Reconnect Vehicle to App: Remove the vehicle from the app and then re-add it. Go to the app settings, choose the option to manage vehicles, and follow the prompts to disconnect and reconnect. This can help refresh the connection between the app and your vehicle - learn more.

6. Check NissanConnect Status: Sometimes, the NissanConnect services experience outages. Check community forums or local service announcements to see if there is an ongoing issue affecting app functionality - learn more.

7. Consider Factory Reset: If none of the above steps work, performing a factory reset on the app may help to resolve deeper issues. Be sure to back up any important settings before doing this.

8. Contact Nissan Customer Support: If the problem persists after trying all the above measures, contact Nissan customer service for assistance. They can provide tailored solutions based on your vehicle’s specifics - learn more.

Following these steps systematically should help to significantly decrease the failure rate of the NissanConnect app when trying to start your vehicle.