gravl

Greetings, wise and experienced sages. Perhaps you can steer me to a diagnosis. 90 4runner (6 cyl, manual trans, 4wd) one day suddenly refused to start (turns over fine). Got the dreaded code 12 from the dashboard morse code blinker. Plugged a used ecu (89661-35351, same as original) in, and it started right up. Slightly rough idle at first (has had that for a little while), but smoothed out when warm. Shut it down, pulled efi fuse to see if light resets, and it again refuses to start, with code 12 (which seems to mean everything from your cat has worms to a fish died in the gas tank). Is this just a used ecu deciding to die, or can there be some silent assassin of ecus hiding under the seat, or is there some other cause and the ecus are really ok? This is like the Titanic, everything worked fine until it sank.
Thanking you in advance
Bob