Replaced alternator. Now the car fails smog-catalyst not on, even after 1000 mi.

135,000 on the car. 1,000 on the new alternator.

Pretty low miles. Have a competent aftermarket shop scan it for DTCs , and let them know the situation. They can also monitor all the systems (if they have up-to-date software) for sensors/readings out-of-range , but not bad enough to set a code.