service booster light

had a 03 chevy silverado 4x4 2500 come into the shop with a service booster light on.....test drove it and the pedal felt fine....im assuming you need a scan tool to look at this problem......my best guess is its the power assist coil plugged into the boost itself....so pretty much i wanna know what the hell this is all about and since when was there a service booster light lol