ok everyone, i need your help. i needed to patch a hole in my exhaust so my dad disconnected the battery and went to work. after he finished started it with the air con on. so when i turned it off the revs dropped. so i disconnected the battery to reset the ecu and ( i was told resetting the ecu meant start it, leave it in drive for a few minutes park a few minutes and takre it for a drive to teach it) now the car runs fine while driving ( well so i think) but if i put it in nuetral and rev it it as the revs drop they drop too low and it stalls. same as when i start it up it does the initial rev then nearly dies. then when i move it from park to reverse it goes up to nearly 1000 revs. what the hell is going on? can someone please help.

it will come good