Grout does not fail on its own. How to read cracks, hollow-sounding tile and dark caulk lines, and which of them mean water is already in the wall.
Grout is not waterproof and it was never meant to be. It is a filler between tiles. The waterproofing sits behind the tile, and grout is only the part you can see. That is why cracked grout matters: not because the crack itself leaks, but because of what usually caused it.
A crack is a symptom, not the problem
Cement grout is hard and brittle. It cracks when something behind or beneath it moves. In Los Angeles the usual causes are a flexing subfloor, tile set directly over drywall in a wet area, missing movement joints on a long run, and thinset that never fully bonded. Re-grouting a moving floor buys a few months. Fixing what moves buys years.
The hollow tap test
Tap across the tile with a coin or a knuckle. Bonded tile sounds dull and dense. A hollow, drummy ring means the thinset has released and there is air under that tile. One hollow tile in a doorway is normal wear. A hollow patch three or four tiles wide is a bond failure, and the field around it will follow.
Where the joint should be sealant, not grout
Every change of plane, so wall to wall, wall to floor, tub to tile, should be flexible sealant rather than rigid grout. Those joints move with temperature and load, and grout placed in them cracks within a year, every time. If the corners of your shower are grouted, the cracks you are looking at were predictable, and the fix is to cut them out and seal them correctly.
Four signs water is already behind the tile
Grout that stays noticeably darker for hours after the shower is off, which means the wall behind it is holding water. Caulk that turns black no matter how often it is cleaned, because the growth is behind the bead rather than on it. A soft or springy spot in the shower floor. And a stain, a paint bubble or a musty smell on the far side of that wall, often in a hallway closet.
Any one of those and the question stops being cosmetic. It becomes a moisture and framing question, and the tile has to come off to answer it honestly.
Floors: cracks that run in a straight line
A crack that travels tile to tile in a straight line is almost always the substrate rather than the tile, usually a plywood seam or a slab crack telegraphing upward. On wood-framed floors it normally means deflection: joists spanning too far, or blocking that was never installed. That is a floor structure repair first and a tile job second. Reset the tile without addressing it and the same line comes back in the same place.
What is worth repairing and what is not
Worth repairing: isolated cracks over a solid, non-hollow field; corner joints converted from grout to sealant; a handful of loose tiles with sound substrate behind them. Not worth repairing: hollow tile across more than roughly a fifth of the area, any shower wall that reads wet, or grout that has already been patched twice. At that point a tile and surface refresh costs less than a third repair plus the water damage it hides.
One test to run before you call anyone
Dry the shower completely, run it for ten minutes, then stop and come back three hours later. If any grout line is still clearly darker than the rest, mark it with tape. That mark tells whoever opens the wall exactly where to start, and it is the difference between a targeted repair and an exploratory one. If you would rather not open anything yourself, send us the photos and we will tell you which of the two it looks like.