ECU programming error, P0602, code clears and returns right away. Should I be ?

What is your question?

3.5L engine was replaced with lower mileage motor. P0602 reported even after ECU replaced and reprogrammed. Mechanic cannot find any problems.

What makes this problem better or worse?

Nothing, seems to run fine.

How long has your 2004 Isuzu Rodeo had this problem?

Since engine replacement, 3yrs.