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First try a simple solution (below), then if this doesn't work you'll need to go to a more drastic measure of removing a section of the patio & placing proper flashing & or seal at the main house, drain rock to below frost & drainage.
The joint between the patio & slab should be cleaned of all previous sealants. To do this well you may need to use a masonry grinding wheel. The crack should be cleaned to a depth at least equal to the width of the joint. If it is deeper,insert polyethylene backer rod in the joint to make it the same depth as it is wide. Then fill the joint with an elastomeric caulk. Polyurethane is a good choice. With some types of caulk, it is necessary to coat the edges of the joint with a primer for good adhesion. Follow the directions of the caulk manufacturer.