Im having the same issues with my new s21 as a lot of other users. S21 cannot show Android Auto on my car display. It worked before in my car with usb to usbc and my Huawei (!!) S20. After changing to Samsung S21 Ultra, suddenly it does not work. What a bomber, im soo disappointed. Thought Samsung would be better integrated than Huawei.... and I see currently no solution for this problem. Samsung...PLEASE help us ??
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There is nothing to delete - the phone is not connected yet to the car. It just say, please connect via USB to the car - but the phone is already connected to the car via USB. And nothing happens from here. Nothing shows on the car display.
That is the strange part, because this is an original Samsung USB-C to USB cable I use (1 m length). And it worked very well before with my Huawei S20 and Android auto on the car display.
Additionally I have a 3,5 jack to usb-c adapter i used with my Huawei S20 and my old headset with jack. This is also no longer possible with the new Samsung S21. Something about that it's not compatible.
So I think maybe the S21 has "another" USB standards/interface "protocol".
2 days ago, I had a 1 hour chat with DK Samsung support. he asked me to delete my network settings on the phone at first, this did not solve the problem. At the end he told me to contact Google support. Even though i told him MY PHONE IS NOT CONNECTED to the cardisplay. Anyway, I got an idea. I connected the S21 with the old Samsung cable i used before with my Huawei - to and old PC. Cable is USBC to USBA. This did not work - S21 was not seen by the PC. Then I used the cable in the S21 box, which is as USBC to USBC, and connected this to a new PC - then the S21 was seen right away. I then asked the support guy - please advice where to bye a S21 supported USBC to USBA cable, because I think this might be the problem. he could not advice, because Samsung does not sell such a cable. In my car I have USBA (the old kind of USB connection). Then I started to search the internet for cables (what a jungle) !! After a lot of qualified (or not) guesswork from my side, I decided to spend 150 Euros on various connectors and adapters (crazy situation). I got the cables yesterday. I bought only short lenght cables - this was recommended in other goups with the same problem.... I tried also to see if I could find a "quality" cable (whatever that is, looking to the price, gold coated connectors or or or - who knows...). BUT one of these cables actually worked !! Suddenly Android Auto appeared on the cardisplay !! Hurraaayyy ! Now, I start to be happy with my new phone, even though some of the excitement went lost, due to all these problems 😞
So solution is, bye a new cable - and hopefully you can find a person, who can help you bye the right cable (and only one cable), the first time.
Enjoy and good luck with your Samsung S21 Ultra.....
A bought a 25 cm usbc 3.1 to usba 2.0 cable that works fine for me. Not that costly. BUT of course, I also bought some other cables, with same connectors, that did not work that well.