The thermometer may not connect to the app the first time you try. The best course of action is to force the app to close and disconnect the bluetooth connection. There are slight differences between how this is done using an Android device or an Apple one:  

Android 

1. Go to ‘settings’ and choose the option ‘apps’ or ‘application manager’. You will now see a list of all your recently opened apps 
2. Tap the LD control app and choose the option ‘force stop’
3. Go to ‘bluetooth settings’ and select ‘paired devices’. Select the probe by clicking the cog
4. Select the option ‘forget’ ’
5. Reopen the app and try to connect the probe

Apple

1. Quickly tap the home button (the button at the centre bottom of your screen) twice, icons of all the apps you have open will appear. If you have a newer device, which doesn’t have a home button, you swipe from the bottom of your screen upwards and briefly leave your finger on the screen 
2. Swipe the LD Control icon upwards. The app is now closed
3. Ga naar de bluetooth-instellingen en klik op ‘verbonden apparaten’
4. Select your probe by clicking the circle with the exclamation mark 
5. Choose ‘forget’ 
6. Reopen the app and try to connect the probe

If you open the app again and try to connect the thermometer, it should work. If this is not the case, there may be a problem with your thermometer.