I have a 06 2.4l.I have recently installed a volt gauge. Most of the time its reading 14.5-15.5. On some occasions it will read 13.5-13.8.I have been addressing codes p102 and p0053 but haven't figured it out yet. I had the mass airflow and tcm changed by Chevrolet and o2 sensors changed earlier this year. Is 15.5 you much voltage? Could that be causing those codes?

 

 

15.5 Volts is okay, it means that something is commanding the alternator to a 90% duty cycle. Like a low battery or the headlights etc.

P0053 could be a loose/broken wire to the #1 O2 sensor
P0102 Means there is an open circuit in one of the wires to the MAF.