Chosen Solution
The laptop’s keyboard responds when I press the power button. It also responds when I press Alt upon startup, to choose the startup disk. However, it stops being recognised at all time, after the system boots. Also, the trackpad fails to be recognised at all times: before AND after boot. What is wrong here? Is it a hardware problem? EDIT I cleaned the connectors and here is the result: the keyboard WORKS all the time. But the trackpad never works. Both external mouse and keyboard work. However, The trackpad cable should not be to blame because it worked normally with the previous logic board that was in this laptop. NOTE I just found out both Thunderbolt ports are not recognised, and the wifi card is not working (but Bluetooth works). Don’t know if it’s linked to the trackpad in any way
Most likely the trackpad cable, common failure in this model. If the trackpad is not recognised, only the power button on the keyboard will work when in an OS, no other keys will work. Try a trackpad cable first, take it from there
