My wife said our '07 Sonata produced a check engine light and the mechanic said it was a code P0187 - system too lean at idle. He took care of it without charging but I am interested in learning a little more about it.

 

Was the code a P0187 (which is a fuel temperature sensor B malfunction) or a P2187 (which is the system too lean at idle)? If the later, then "unmetered" air was entering the engine causing it to run lean at idle. The problem was likely as simple as a loose vacuum line.