Why Slabs Crack Here
San Diego County has a lot of expansive clay. The County soils report says this soil takes on water and swells, then shrinks again. That push and pull can crack, tilt or lift a slab over time.
Tree roots, poor base prep, missing joints and heavy loads add to it. Freeze and thaw is not a cause here, so do not let anyone sell you a fix for it.
Repair or Replace
Thin hairline cracks that stay flat are often only a looks problem. They can be cleaned and sealed. A crack that is wide, growing or has one side higher than the other points to movement under the slab.
When a slab has sunk or lifted, or is broken in many pieces, replacing that section is often the lasting fix. Ask the contractor to explain why they suggest one or the other, and get it in writing.
Safety First
A raised edge on a walk or driveway is easy to trip on. Mark it and get it looked at soon, even if the rest of the slab is fine.
If the problem is on the public sidewalk, check with the city before you hire anyone. Who repairs it and what permit applies are city questions.
What to Ask at the Visit
Ask what caused the damage, what the fix will be and whether the cause will be dealt with too. A patch over a moving base tends to crack again. Ask how they will match the color and texture of the old slab.