My car originally died on the road due to a bad alternator that killed the battery, there is now a brand new alternator and battery in the car. However, despite the battery reading 12.6 volts with a multimeter, on the dash the voltage gauge reads in the yellow hazard area. Also every couple seconds the voltage drops from 12.6 to around 8.5 when the key isnt even in the on position. With the new battery and alternator the car can start but cant run for more than 2 seconds. It is also always in diagnostic mode when I turn it on. What can I do to get to the issue?