Why does my 2001 Lincoln Navigator down shift, when going from 2nd to 3rd

It probably is the Transmission. The computer controls the transmission BUT - if there's a failure, the transmission usually defaults to a gear that will let you move the car at the very least. If you're concerned about being over sold - get a second opinion. I haven't seen this before but I have seen two similar failures: One was a lincoln with a wiring loom that was left loose and grounded out on the exhaust manifold - It would let you cruise at 70mph for a minute or two and suddenly shift into second gear! The other was a Bronco that would shift 1 - 2 and lock up reverse and second - a dead and sudden stop no matter what! Don't let just one opinion make your decision.

Aamco advertises: "Half the people that come in don't need a new transmission!" Why don't they just say "Half the people that drive in here we stick it to them for a new transmission whether they need it or not" Listen to Tracy. Second, even third opinion. It's "gear hunting" and could possibly be fixed.

I have a Lincoln navigator and repairs I put it in reverse and it doesn't work and goes forward and set up backing do you think of the transmission or by sensor