On Android 7.1.1, the Nexus 6P and 5X have an in-car Bluetooth issue unresolved since October
For most users, the Nexus 6P and Nexus 5X either won't connect to in-car Bluetooth systems, or they connect, but phone calls can not be made via the car's audio system. This seems to happen with most car models, regardless of maker.
Google may be planning to finally fix this Bluetooth bug with the launch of Android 7.1.2. Unfortunately, there is no word on when this new version of Nougat is expected to arrive, though we hope it will happen in the near future.
sources: Android Issue Tracker, Google Product Forums
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