Electronics

If your going to do any sort of hardware troubleshooting you'll be dealing with electronics,

What you need, as in if you don't have these, don't start a project:

1. Multimeter that supports voltage, ammerage, resistance and continuity tests.
2. Various test leads, both probes and aligator clips
3. Hot Glue gun- its a perfect way to TEMPORARILY attach electornics while your working on them
4. Breadboard for prototyping

Optional:

1. Multimeter which supports mreasuring capacitors
2. Heat shrink tubing
3. Various resistors (you can get 'developer packs' of resistors in the most common ranges quite cheaply)
4. Various LEDs
5. Automotive relays
6. Solid state relays
7. Tinkercad or other hardware virtualization account/software