How to fix a gm code 42?

1993 Grand Am 3.3L V6 (6-204) - Solved intermittent "Code 42 - electronic spark timing" issue by replacing Mass Air Flow sensor, after checking all wires in harness (and replacing some) between the ECM and ICM, checking all connections, and replacing all the other sensors. All work prior to replacing MAF sensor improved performance but did NOT solve intermittent Code 42 problem. Never got a "MAF sensor" Code 34.