truck wont downshift.
When i come to a stop my truck will not down shift back to first, it will stay in second or third. took it to the dealer, said i had low voltage so i put two new batteries in it. it worked for a while then i hauled a trailer about 4 hours and it started agian. they also said there was something about a low line pressure, or something like that when they did diagnostic. havent had them check anything since then. looking for ideas or thoughts, its an 06 cummins, auto tranny.