What do you do if the emission light comes on in a Honda Odyssey?

We have a 2003. The owner's manual says to check the gas cap -- tighten it or take off and put it back on again. (This was our problem.) Owner's manual says, once problem is fixed, light will go off in 3-4 times of staring the van, but ours did not. So, I called the dealership and he said it's more like a week of driving the van before it goes off.