If there's one thing I've learned tinkering on cars since before I could drive one, it's that being more informed saves sleepless nights and stacks of cash. No one should have to spend a $130 diagnostic fee to diagnose a check-engine light telling them that their fuel cap needs to be replaced.
This handy Handheld Car OBD II Scanner / Code Reader will only cost you $14.99 after coupon code 4ZWJ5FLA at Amazon and is all but guaranteed to save you several times that cost while you own it. Even if you feel like you have no mechanical ability, reading your car's code and doing a quick Google search will empower you with information before you speak to a mechanic.
You may even surprise yourself. Jobs like replacing an oxygen sensor, mass airflow sensor, and even ignition wires are easy to perform and can save you hundreds of dollars in labor.