The low pressure light comes on although the tire pressure is correct

I'm with Tom. Don't go back there!! This might be a pain in the butt, but have you checked the tire pressure every day? If it's intermittent, it could be that the air has seeped/leaked around the nail and that's why it's low. I was wondering if you have separate low pressure light for each wheel, or one light for all 4. Usually the light comes on when there is 5-7 pound difference all around. The simplest thing to do is take it to someone like Direct Tire or someone who has the diagnostic tools to analyze your problem. Good luck, and hope this helps!