If the car is in ECU Diagnostic mode will this make the car run rough ??? I was checking my sister's car and I can't remember if I turned the screw back to NORMAL and now she says that the car is running really rough. TIA.

 

The ECU was still in the DIAGNOSTIC setting. I turned the screw back to the normal setting (COUNTER CLOCKWISE)and the car runs fine now. Thanks !!!!!!!!