Apart from some "killer commands", the ECU might have been destroyed electrically. The OBDII-Interface can implement several different data buses with different voltage levels. If the bus lines get connected to +12V, this could destroy something.

Is it okay to leave an OBD II scanner on a car?

From their web site FAQ: You can leave the Bluetooth models plugged in if you are driving the vehicle frequently. Depending on the battery condition, if you leave your car sitting for days to weeks, we recommend that you remove the OBD2 device. ... According some other users, it does draw power when the car is off.

Can I start my car with OBD2?

3 Answers. Ultimately the answer is yes you can. To qualify this answer, though, you can only control some things, but not all things, then only on some vehicles. You can do simple things through the port, such as turn the A/C pump on, or any other thing which is controlled by the computer (PCM).