wolfsdad

Bought this Pathfinder with 75k and now has 165k. The service engine light has been intermittant on/off since we bought it. Nissan service dept said emission repair (no details) $500. The light would come on any time offbrand gas was used, and will go off when major brand(shell, exxon,chevron) is used. I was told by service dept that the computer memory gets cleared out when the light goes out and so cannot trouble shoot unless the light is on. I expected the memory to hold the most recent faults as other cpu systems do. Gas mileage has dropped from 22mpg to 15 recently. What can I do find the source of this problem. Any help appreciated.