The likely cause of garage smell in most garages is mold, mouse droppings, garbage stink, dog poo and cat urine. The dog and cat smells are easy enough to find. The mouse droppings might be in the wall. Fixing the mold smell is the most important to your health.
Visually inspect the garage, and use a moisture meter to find mold growth areas. Fix leaks everywhere including the floor, door, windows and roof. Use insulation to prevent condensation in areas where warm and cool air meets. Wipe away surface water, and use a dehumidifier to remove moister to below 50%. Use a fan to speed up drying time. Wear skin, eye and air protection when searching for and cleaning mold. Replace drywall mold whenever possible because mold penetrates fibers that are hard to clean. Use a BAC or Quat (Wet & Forget or ShockWave) based products to clean garage wood, steel and concrete. Apply mold inhibitor (Concorbium) to the garage drywall, wood, plastic, concrete, stone and carpets. Use a hygrometer to test that he dehumidifier is maintaining garage air moisture below 50%.
