my 2004 grand prix won't go in d gear. But will go in reverse and 1-3 gears fine is this a major problem ?

If it's dark brown that is a tell tale sign of a burnt transmission. Overdrive could be out of it, very common with GM's W platform cars. Is the fluid level full? You might be able to save it with a transmission flush. Just don't drive it any more than absolutely necessary until you get it looked at by a good mechanic or transmission shop

I usually preach the evils of transmission flushes. When the fluid is good, it does far more harm than good. This one I make an exception, as TransAm77 says, if it brown, and probably has a burnt odor, then a flush might get you a few more miles.

Thanks Fordnut, I've saved quite a few transmissions with flushes. Like you said though when needed not when the fluid is red and looks new. I only use BG brand and always use the cleaner additive before the flush. Drive it 10-15 miles then flush it.

And that magically restores burnt clutches and other worn and broken parts? Just asking... :)

No munron, it won't magically restore worn and broken parts but it will help with sticky or gummed up valves and slightly burnt clutches for transmissions that work (like teelewis) just not totally like they should. Its ok to get a little education every now n then.

That's why I asked.