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just read up my notes

yr ecu has 2 plugs x1 73 pins
x2 80 pins

the plug yr missing is the x1 grey plug that plug runs wires for these sensors

which u will need plugs for aswell

speed snesor
mas air flow sensor
tcc for auto box
rear o2 sensors
accel pedal sensor

and also has wiring for starter relay
fuel pump
powers for ecu

so its heaps different compared to ls1 gen 3