Not sure on iPhone, but on Android I leave the mini ELM327 plugged into my OBDii port. Anytime I want to connect, I open the app. Bluetooth connects and after a few moments I start seeing data. Mine defaults to a screen I made that watches sensors I care about. Quite a few sensors available. Can even monitor instant MPG. Another button in the app allows me to see the error codes and reset them. I think the free version allows some sensors and to read only the error codes. The Pro version adds plenty, more gauges, deleted codes, track performance and whatever else. To make the screen I was talking about, you go to a sceen in the app and long press. A pop up shows up and you select the gauge style you want, then the sensor. You can press and hold them to drag them around to get yhe layout you want.
Torque — OBD2 Performance and Diagnostics for your Vehicle
Torque — OBD2 Performance and Diagnostics for your Vehicle