I had a car fail because of low idle rpms, fixed it by adjusting the base idle set screw and it passed, not only did it just pass but the HC numbers were close to zero. Huge improvement.

Next I had another car that failed emissions testing, changed the spark plugs and fresh new engine oil change and there was a dramatic improvement at the next emission test (passed as well). There was no hint of poor idle or anything to indicate the spark plugs were bad, but they were easy to change and cheap to replace.