Problem causing alternator to stop working

2001 2.2L 4cyl. S10 60,000miles 5spd Service engine soon light came on a week ago. It went off. Then it came back on a couple days later. Then my voltmeter on the instrument panel started reading about 12V. replaced bad alternator. Service engine soon light went off, the truck felt like it had more power than before. Drove to get gas. Light came back on. Next time I started it up, light went off, drove 30 miles. Now voltmeter only measuring 12 volts again. I ran AC, lights and radio the whole way, battery still reads 12V. Any idea what is going on? Thanks.

get it tested, the alternator might be faulty.
Your battery will slowely go dead and leave you stranded

It was the new remanufactured NAPA alternator. Thank you.

I didn't say that it was used, and I don't even see anything wrong with used stuff... unless it's a pain in the but to change. Like my starter then it's either new or the best re-built so I hopefully don't have to pull it out for a long time.

It could still be a faulty alternator even when it was profesionally re-built.