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Phone key - does the app need to be running (iPhone X)

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As a data point. I have an iPhone X and I can say I rarely open the app (rather use the Stats app). It’s been many days between drives lately and I’ve had no issues. I do have background refresh enabled for both and have both in control center of that makes any difference.
 
I have an M3 AWD 2018 and my iPhone 8 plus was running fine as a key for several months until there was a particular software upgrade and it stopped functioning. Tried all the many suggestions on the forum of disconnecting, resetting up, etc. and was able to get it to work. BUT several software upgrades later, it stopped working again--unless the app was running in the background. My wife's iPhone 8 did encounter these challenges,

The other day, I tried to open the door not realizing that the app was not running in the background (I prefer not to have it running constantly as it does of course sap the iPhone battery.) The Model 3 went to the default of asking me to swipe my card key, but before I pulled it out to do so, on a hunch I swiped the entire iPhone itself across the B pillar key reader and--voila!--open sesame!

Since then, my iPhone continues to automatically open the Model 3 without the app running and without swiping, as it used to. I would certainly recommend trying this for similar problems anyone might encounter, and to avoid the nuisance of battery drain and/or turning the app on and off.
 
I have an M3 AWD 2018 and my iPhone 8 plus was running fine as a key for several months until there was a particular software upgrade and it stopped functioning. Tried all the many suggestions on the forum of disconnecting, resetting up, etc. and was able to get it to work. BUT several software upgrades later, it stopped working again--unless the app was running in the background. My wife's iPhone 8 did encounter these challenges,

The other day, I tried to open the door not realizing that the app was not running in the background (I prefer not to have it running constantly as it does of course sap the iPhone battery.) The Model 3 went to the default of asking me to swipe my card key, but before I pulled it out to do so, on a hunch I swiped the entire iPhone itself across the B pillar key reader and--voila!--open sesame!

Since then, my iPhone continues to automatically open the Model 3 without the app running and without swiping, as it used to. I would certainly recommend trying this for similar problems anyone might encounter, and to avoid the nuisance of battery drain and/or turning the app on and off.
What 'battery drain' ? My Tesla app used 3% of my iPhone SE battery over the last 24hrs. If you are seeing much more than this it doesn't sound right.
 
Dont force close the app and it should be fine. It is absolutely fine to leave apps running in the background

You can force close the Tesla App. That’s what Background App refresh is for. It will start the app back up in background with no visible UI.

You also need Location Always enabled. iOS is pretty aggressive since iOS 13 encouraging users to turn that stuff off. But if you want it to work all the time you need both background app refresh on and location always. And you can put the app in background or force close it. It might not restart in background immediately though.

You are correct that Force Killing on Android is s bad idea. It should still work but eventually it will leak all the bluetooth connections and require a reboot to function again. Because Android is brain dead.

I write Mobile apps for a living that have similar requires to the Tesla App.
 
Most probably one of the iOS updates has changed your “always allow” setting for GPS or iOS 14 disabled Bluetooth access initially for me and Tesla app - I never have the app open (it’s always force closed) and have no problems.

It also needs to be logged in - if the app randomly logs out for some reason the phone key stops working.