I have a $5 dongle from eBay that plugs into the OBD port to read the trouble codes with a free phone app. First time I used it it said “Oxygen sensor#2”. I bought the $15 part from eBay and installed it. That would have easily been a $100 or more repair.
I also replaced my starter. That was $100 rather than $300 at the shop.
I’ve replaced spark plugs, and headlamps where wiring shorted and burned, so I had to solder on new wire leads and sockets. Those jobs were rather difficult only because the engine compartment is so crammed and confined.
The detailed instruction videos people place on YouTube are fantastic. Their generosity is wonderful.