How can I be certain that my transmission has went out and not a computer proble

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did you have to tow it in? Are there multiple squares around multiple gears? does it drive at all or idk not leave 1st gear? if any of those things apply you may simply have a bad trans range sensor. $1400- $2900 is absurd btw, I'd go on ebay or go to a junkyard keeping in mind you can source a transmission for your car from a mazda tribute or mercury mariner as well for probably about $100-$150. Then you've got to find a mechanic who isn't a dickhead and will actually install it for you since there are lots of places that will tell you they won't because it's from a junkyard or ebay and they didn't get to charge you extra $ on top of their parts cost. Secret truth in auto mechanics- they don't make their money on labor, they make their profit by charging 2X to 3x and beyond on parts.