Please try the following procedure(on Android please disable the 'smart lock' Bluetooth feature for the PoolLab) 1. close the app (not running in the background) 2. in fact the PoolWaterLab shows the 'Bootloader' screen press the 'pH'-button for 2 seconds to reboot the PoolLab in normal mode 3. unpair the PoolLab in the device settings of your mobile device 4. turn off Bluetooth on your mobile device and turn it back on after 10 seconds 5. restart the PoolLab app 6. reconnect the PoolLab via Bluetooth (in the app, not in the Bluetooth settings of your device) If this doesn't solve the issue try the following(force bootloader mode) 1. press and hold 'pH', 'fCl', 'TA' and 'CYA' buttons and release them simultaneously after 1 second 2. turn off Bluetooth on your mobile device and turn it back on after 10 seconds 3. restart the PoolLab app and reconnect to the PoolLab (note: now displayed as 'PoolLab OTA') Note: On Android in some cases it can help to restart your smartphone or tablet before reconnecting to the PoolLab If nothing above helps please try to remove the PoolLabs batteries for 1 minute.