Tire pressure warning light on an 08 Scion tc?

First of all I just found out from the shop that the tires were inflated to 50psi. The mechanic asked me did I do it? Hah I was like hell no I never touched them so they must have been that high when I bought it. So anyway he brought them down to 42 and then of course I noticed the tire pressure light went on and so I took it to an air pump and deflated them to 32 in the front and 29 in the back like it says to do when you open the door. The light was still on after that so I called the mechanic and he said he recommends them be at 42 and so I did that and the light is still on. WTF? How do I fix it?