As the saying goes: Trust your friends, but cut the cards. So even if you trust your auto mechanic, consider using the $90 CarMD to make sure you're not being charged for unnecessary repairs.
Just a little bigger than a standard cell phone, CarMD plugs into the On-Board Diagnostics II port of any 1996 or later model car or truck. (In most vehicles, the OBD II port is under the dashboard.) The unit gave my wife's 2001 Honda Civic the green light just minutes after I unpacked it. The device shows a green check mark if your car passes CarMD's diagnostics, a yellow question mark if it finds a pending problem or inconclusive results, and a red X if the vehicle needs service. CarMD says that a yellow or red light means that a car might fail a state emissions test.