Here's my take on the video ...
I see from the schematic of the OBDII that there's 4 LEDs and these are driven from the OBD controller IC. Two indicate status (Tx, Rx) of the UART comm port (your Arduino comm) and one (LED4) seems to be to indicate a reset is being done (and also to reset any non-volatile memory that may be present) as well as indicating comm to the car, and the last (LED3) seems to indicate comm from the car and perhaps also serves as an interrupt acknowledged indication (guessing from the names).
Your vid shows the UART Tx and Rx lines flash and I think this is due to the ATZ reset message being received by the OBD IC. It might be that the OBD then also sends some response message, that you don't get to see ... now. Then there's the Knight Rider ripple of all the LEDs, which I think is the OBD controller saying it's doing a reset. My conclusion from this is that the comm from your Arduino to the OBD is working (yea !) but I don't think the OBD is connecting to your car (crap !).
That the UART LEDs flash every so often when you disconnect some ground wire ... I think this is just noise tricking the OBD into thinking it's getting a message (probably one it thinks is invalid). I note you can see the ATZ reset message sent by the Arduino and I'll assume (since you don't mention anything) that you
don't see any other messages on the PC monitor when the ground is removed and the UART status LEDs are flashing. This lack of visible messages from the Arduino tells me that it's not sending anything to the OBD and so the LED that indicates this (messaging) is happening at the OBD end is due to noise. I'll also guess that probably that any message sent by the OBD to the Arduino is not getting received by the Arduino (or it's deemed invalid at the Arduino end). My conclusion ... I'd leave the ground attached for further testing. The 9 pin DB on the OBDII board would seem to carry a ground from the car to the OBDII board. You'll need a single ground reference from the OBDII board to the Arduino for comm btw the two to work (I believe this is the wire you removed). I assume you're powering the Arduino of the PCs USB port so disconnecting the PC is not an option right now. Just be sure the PC isn't tied to the car in any other way and there should be no ground loop problems but but leave a common ground reference for eveything to talk to everything else.
Right now I'd have just the PC, the Arduino and LCD tied together and try to get a simple "Hello World" added to the Tutorial sketch working to show that sketch isn't missing something to talk to the LCD. I'm going to look at that now.
BTW if you use the full editor you'll find a code tag button that will allow you to paste code in a nice scrolling box with indentation preserved. Makes it mucho easier to read.
Code: Select allThis is in the code box.
See the indentation !
Rejoice !!
EDIT : I note from the tutorial page ...
This command will cause the OBD-II-UART board to reset. On the board you'll see some LEDs flash, and then you'll receive the start-up prompt in the terminal window.
Does the simple tutorial code send the "start up prompt" to the LCD ? If not, yours should. That'll verify 2 way comm btw the Arduino and OBDII as well as a working LCD display !