HELP!!! Tranny or NOT

[rickmatic]

First time posting. Thanks in advance for all responses.

 

OK. I have a 2003 Honda Civic EX, 1.7 vtec, Automatic. I was driving home from work one day... and all of a sudden while going around 30mph my car just neutralize itself (shift to neutral on its own. i stepped on the gas and rpm would go up but my car wouldnt move) while driving. i immediately stopped and shut off my car. i had to move so i turned it on put it on drive drove it for 5yards then it neutralized itself again. i turned it off and turned it on again a few times until i pulled into a gas station and got it towed.(check engine light turned on) i had my mechanic check my car and he suggested to repair or swap the transmission. i swapped it for a used transmission with 44k miles from 150k. my mechanic went to pick it up for me and drove it to my job which is about 10miles away with no issue. when i tried driving it, i shift to drive traveled about 5yards and neutralize. so i was pissed but im stumped and so is my mechanic he gave me 2 more choices either the used transmission happen to have the same problem my old one did(which i doubt) or your ecu needs rebooting or reflashing. so my question is does anyone have any idea what can be wrong? i know there might be alot to check but im willing to check it all if that keeps me from swapping the transmission again.

 

PS.. i should mention that between the time the transmission messed up and getting it swapped i would turn my car on and drive it around the block perfectly. but was to scared to drive it any further.

 

 

PLEASE... Any responses would help and thanks again for reading. i appreciate anyone who takes the time.