EPA guidelines do not allow mismatching emission components. A California emission certified vehicle must maintain its California emission component configuration, regardless of the state in which it is registered. Installing a federal CAT on a California emission vehicle (if CAT is an exact fit) should not cause the check engine light to turn on, as both components should be identical in function except that the California version has passed CARB (California Air Resources Board) bench tests. An issue will arise if your state's smog inspection procedure requires the smog technician inspect a vehicle's CAT for proper design and installation/fit.