I just moved from a Nexus 6 to a Galaxy S8 and had an interesting problem with Android Auto when I moved over – I couldn’t get OK Google voice detection to work. This issue only occurred when the Android Auto app was open on the phone – I was able to unlock the phone with my voice, and OK Google detection still functioned when I was working in other apps.
After a bit of experimentation, I determined that the issue was the “Trusted Voice” feature in the Google app. Disabling the setting by navigating to Settings > Voice > ‘Ok Google’ detection and setting the slider for Trusted Voice to the off position resulted in OK Google working properly in Android Auto.
Since there are a number of other ways to unlock the phone and it’s not critical for me to be able to control my phone from across the room, I’m going to leave Trusted Voice off for the time being in favour of hands-free driving.