I recently installed a new alternator, has worked great for a month now (Constant 13.7). I was on the highway doing 65, when i accelerated to pass another car i felt a complete loss of power, felt like it was going to stall but didn't. After it happened i got a code 34 and my voltage is only 11.9 Vots. I tested the alternator, and it is still charging but i can't get over 12 volts.

 

Have you load tested the battery?