Check engine light flashing

[custom10nut]

I have a 91 C3500 Cheyenne (No Frills), and last year I had a no start issue,
Starting with the Fuel filter, then fuel pump, I replaced everything in the fuel system and rebuilt the TBI. Also I replaced the ignition system components, and still no start. After talking with a few heavy mechanics, we summized that the ECM was bad.
I ordered a replacement and installed it.
It started and runs fine, but from the first time, the chech engine light seems to go through a cycle of “Quick Flash” then on to a slower fash, then wil come on steady, then sometimes it will flash randomly..
One other thing is that it seems to run really rich when I first start, like it’s getting too much fuel, but runs fine after warming up.
All the electrics on the TBI are hooked up correctly. It flashes a “Bad O2sensor, but I’ve put two on it and nothing’s changed.
I was told it could be caused by the ECM not being properly set.
Anyone have any insight?