I had that happen on a 1984 Toyota Camry "computer" looked like a coffee stain. Was caused by humidity forming on the board and caused an electrical arc. A computer guy told me to clean it with a toothbrush under the hot water tap, dry it completely with a hairdryer, let sit overnight, and try it. It worked perfectly after that. Humidity came from no seal on the case holding the board, had to seal it properly.
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63 Parisienne sport coupe (The Big GTO), black, maroon interior, 409 4 speed; former owner of a 59 El Camino, 63 Corvette SWC, 62 Chev Bel Air SC. 1963- Pontiac top selling car in Canada
Mahone Bay, NS Still not old enough to need an automatic