What causes third gear not to engage in automatic transmission? First and second gears pull strong.
Most times it's worn or burned bands within the transmission. Have it looked at by a reputable trans. shop in your area, your most probably going to need a rebuild. Good Luck!
I'd say that a place to look would be at the 3-4 accumulator piston inside the valve body of the transmission. The spring can break preventing it from shifting. Messy job, but not much different than installing a transmission shift kit.
Search Chevrolet Silverado 1500 Questions
Chevrolet Silverado 1500 Experts