While diagnosis is unlikely the number one issue is always a contact or connector. Followed by some chip death. Diagnosis was something I did years ago in a military setting where we used high powered microscopes and in a pinch the SEM (see google.) We also were known to take it to a micro-slicer machine to shave it down to the failure point micron by micron until we found it. It was all great fun and detective work but my point is that it is far cheaper to put in new parts than diagnose it.
My son spilled pop on the keyboard, no biggy dissasembled and cleaned the keyboard, I didn't take anything apart that wasn't needed, well the cleaner I used messed with the keyboard itself, it was electrical contact cleaner, but thats not the problem. Now the touchpad won't work? The first time I hooked up the keyboard the touchpad was fine, but the second time I lost all control of the touchpad? I can buy another keyboard on E-bay for next to nothing but am worried about why the touchpad quit out of nowhere?