fourfive

So, took it back to the dealer. They diagnosed it as (another) bad alternator. More specifically as a bad voltage regulator in the alternator. I just got another alternator from advanced and put it in. So far, so good. I cannot recreate the issue and it seems to be charging all the time when the car is running regardless of if the headlights are on or anthing. Fingers crossed.

Dealer did the update to the ECM. The problem is still occurring but now its different. Now, when driving with no headlights or AC etc. the voltage will stay at 14.2 when I am not giving it throttle. As soon as I give it throttle, it will drop to 12.6. Then when I parked and let it idle with no lights etc. it dropped to 12.6 and the battery light came on. Any suggestions would be greatly appreciated. I know I can just drive around at all times with the headlights on as a temporary workaround, but this is actually my wife's car who uses it for her job and drives a lot. Thanks!

Thanks for the info, at the dealership now. Requested the update but they won't do it until after they perform an "inspection" for $99 bucks. I'll post results.

Hi, new member here. I found this forum when searching for this exact same problem. My battery died twice (original on a 2015 Legacy Premium 2.5i). I tested the alternator and it was putting out between 10 and 11 volts when the headlights and all accessories are off. Once I turn the headlights or AC on, the alternator kicked up to 14.1 - 14.3 volts. I assumed the alternator was bad and replaced it along with the battery and serpentine. Now with the new battery and alternator when the headlights and accessories are off, the alternator is putting out 12.6 volts. As soon as you turn on the headlights or AC, it kicks up to 14.2. I've read about this ECM update on TSB 11-174-17R. Does anyone know if this will solve the issue or does anyone know of anything else I could try? Thanks so much!!!!