my 2005 civic transmission slips but after clearing codes it doesn't for about ten minutes

i bought a 2005 h civic LX , and after few days , the transmission slipped
gears as i tried to go forward or backward it acted like N. the check engine
light went on. i went checked it and it says many possibilities are very
potential. but when i clear codes using the check device or even by
disconnecting and reconnecting the battery , it works fine for ten minutes but
afterwards the check engine sign goes on and it starts slipping. i went to a
mechanic and said he would change the whole thing and it would cost
around $1500. i'm asking , as long as the transmission goes fine with the
check engine light off , does this mean that no hard parts are damaged , i
mean some panel or sensor maybe damaged??? or i have to replace the
whole thing?. i checked the transmission oil and it's almost perfect ( amount
and status). all liquids are enough and in good condition. please help