2006 GMC Sierra

that sounds like a internal transmission issue. i belive your truck has the 4l60e or the 4l65e. depending on the amount of miles on the vehicle and how hard its driven it is more than likely time for a rebuild. and get new clutches put in and have the hard parts inspected for damage and wear. along with the valve body components and solenoids to be serviced

Transmission failure isn't a very common problem for this year model GMC's unless it has unusually high miles or has been driven under enormous strain (like pulling trailers or hauling heavy loads). If it is worked hard regularly then you may have overheated the transmission. After you replace or rebuild the transmission you may want to consider having an after market transmission cooler installed if your rough on it. (they range from $30 - $100) - - - If that isn't the case then the problem more than likely lies in the clutch packs. The first gear clutch is not engaging and because of that your truck won’t go into any forward gear at all. Since first, third and fourth share many of the same linkages it probably won’t go into those gears at all, though, second gear and reverse share common linkages so your second gear is probably good — it just really doesn’t matter whether second works or not because if your truck can’t go into the forward gears, then, you can’t get to second, anyway. So, most likely, not only are the clutches for first, third and fourth shot, but it is also likely that the bands are shot for each particular gear. So, best case scenario: You will need to have your transmission rebuilt. The clutches & bands are probably the culprit. If you are mechanically inclined, or know someone who is, then you can simply replace worn or damaged parts and rebuild your transmission much cheaper than you can buy a new one. 2nd best option: Try to find a used transmission at a salvage yard - preferably in a vehicle that has been wrecked in the rear end (pay extra for a guarantee or warranty, if possible, just in case) *I do not recommend going to a GM dealer, unless you prefer paying too much for the same job any certified mechanic can do. A small "mom & pop" garage is your best bet if you don't know someone. My last bit of advice is: Shop around for the best prices & discounts and then NEGOTIATE! Everything is negotiable! Tell the shop of your choice that another shop gave you a cheaper quote ($xxx cheaper) but if they'll beat that price you'd rather use them... (because they're helpful, friendly, or etc) **One more thing... please don't take this the wrong way - - it may be beneficial for you to have a man with you when you go. He doesn't need to speak, he just needs to be seen. Mechanics tend to overcharge women and upsell them on parts and services they don't need because they assume women won't know the difference. (I am a woman in my early 30's who has worked in the car business over 12+ years. I tell you this from personal experience. This happens to me & most of the women I've come across through my job, every single time.) Good luck.