Hey Scotty change my engine in my car i got a Mazda 6 2006 and a light keep popping up on my dash (AT) and my check engine light is on also and it says something about timing chain.i don’t know why because if the chain it wouldn’t start right ?? If not could you tell me why it works like that when I recently change my transmission fluid

you have more than one problem, the AT light means a transmission code has set and the check engine light means other engine problems exist. Find a better mechanic first, and if it is internal engine wear, get rid or it fast, those are not worth rebuilding for the cost