This is the most common mistake.
Without a , you are blindly probing a computer that costs thousands of dollars. One wrong connection can fry the unit. Toyota Ecu Pinout Pdf
Toyota releases mid-year updates. A 1998 Supra ECU pinout may not fit a late-1999 Supra if Toyota changed the OBD-II protocol. Always cross-check the ECU part number (e.g., 89661-14xxx). This is the most common mistake