Your vehicles Haynes or Chilton (or manufacturers) Repair Manual and a OBDII reader / scan tool. I found my repair manuals at a used book seller for dirt cheap and Harbor Freight had the scan tool for cheap. It was even cheaper w/ a discount coupon. Together the cost doesn't equal the service charge or one hour labor charge of most mechanics. Along with the appropriate tools I am able to diagnose and repair pretty much any problem on board. Afterward I'm able to clear the trouble code and make a detailed record of the vehicle's status for future reference.