I have a 2004 Impala and was having same problem. No mechanic ever found problem as no error codes were present. Have posted and donated for a response with my problem. Now have spent over $4000 on it having ignition switch, BCM contol cluster, and new transmission (with 100,000 mile warranty). Car started shifting hard again predominently with multiple stop and goes and sometimes has trouble from dead stop going into first.
Don't intend to interfere with your post, but would be extremely interested in exactly what your mechanic did to fix your problem, so please post specifics back if it works as I and I'm sure others would be very greatful!
Monday, August 26th, 2019 AT 1:00 PM