I have been driving a 2008 Chevy van (2500) since December. When it was brand new I was getting 400+ miles on a tank of gas (about 14-15 MPG).

I have been noticing a loss of mileage (like 340 per tank) and a noise from the transmission. I took it into the dealership and asked them to check the tranny, but they said that there was nothing wrong with the tranny because if did not show any CEL lights and the noise was normal.

My question is what codes would indicate from a tranny?? It only has like two or three wires connected to it. I think that it is slipping and losing mileage.

Input???

By the NON responses I am guessing that there are no extra sensors in the tranny to tell one that it is slipping..thus loss in mileage.

Transmissions cannot tolerate a slipping condition for very long, without total failure.
Slipping clutches will burn up within a few minutes to an hour of use.... and the slip gets progressively worse... so yo umay not have a slipping trans.

In your situation, possibly the torque converter is bad.... it no longer locks up and/or has a seized stator, etc. Such converter failures sometimes produce a distinctive noise and reduced mileage. And as far as I know, no code would be produced. Or possibly you have lost the overdrive gear.

Also, beware that sometimes such a converter failure can pump metal shavings/particles throughout the transmission. If this has happened, the trans will fail prematurely, and needs to be replaced.

I would say that it is NOT normal for a trans to start producing new noises. A transmission with lots of miles should produce the same noises as a new one. IMO any new noises indicates a failure of something.

This truck is still under warranty, right? IMO you should start making some very strong complaints to the dealer... possibly get a second opinion. But definitely ge t it remedied before the warranty runs out.