In my experience with ECMs, the way to check the fault codes is fairly simple. Most of the time, there is a sequence of flashes to the "Check Engine" light. I had a repair manual that deciphered the sequences for me.
The car was a 1988 Pontiac Fiero GTV6. There was a dealer diagnostic plug underneath the cover of the center console. Using a paper clip to short out two of the 6 terminals, the check engine light would first flash a code stating the the system was intact and operating properly, then any and all stored codes would flash in the order they were recorded.
My Haynes repair manual would list what the flashes meant. If I would disconnect the battery for at least a minute, these codes would be deleted. After reconnecting, I would start the motor, which would be in a 'limp home' mode until the ECM would receive enough information to start adjusting the mixture, timing, total advance, etc.
Any new codes would be stored, so after a few days, I would check them, and see where I needed to start with repairs.
Hope this helps,
Sly