How do you reset the anti theft on a 2004 Pontiac Montana?

You need an auto locksmith to reset. Cheapest way is to check on Craig's under services, search auto locksmiths, find one who is mobile and can do anti theft reset. It will take about 10 mins and they will come to you. Usually about 80-100.00 they can also check system for a problem since they rarely screw up but module or key halo could be going out. Chip in key pretty much bullet proof.