I need help...girl that don't know anything about trucks...with my 05 that has 89k miles with a V6. When I start it up in the morning after sitting all night it runs rough and has a lack of power when going up hills. It down shifts to maintain speed up the hill. After about 15mins of running it doesn't have any problems with running smoothly.

It's had the oil and spark plugs changed recently. I took it to Autozone to have the codes read and they were unable to get the machine to read my truck....kept saying to plug it in. I don't know if this is related but my gas mileage has gone down alot as well.

Does anyone have any ideas what the problem could be? I'm at a complete lose! Thanks in advance.