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Who's the (battery) juice thief?

First, the alternator (and relatively new battery) needed changing because the battery was being drained by it while the car was parked and off. The mechanic told me to make sure the lights and other stuff were turned off even though the lights turn off automatically when turning off the car. Then a few days later the oxygen sensor needed to be replaced and a new alternator was installed for who know what reason. Now, about a year later, the new battery is being drained only when the car is on! And I need a new alternator! It is still under warranty until September 20. Help!

Replacement alternators often are rebuilds and often they do a cheap job of rebuilding them. A proper rebuild is fine, but the usual is to just replace what failed and not everything that is worn.

I guess the culprit is the alternator after all. Thank you. I will pass the information on to my bewildered mechanic who is changing it before the expiration date of the warranty. Subaru parts told me they no longer had new alternators available for my year model. I will just have to take my chances like you imply.

Dealer supplied alternators are usually properly rebuilt, but even then, you can get a bad one or two. Good Luck.