I recently got a 92 Chevy Scottsdale K2500. It starts, runs, but is very sluggish. The service engine light is on anytime the key is on. I've had the battery cable off for an extended time, but when I reconnect the light immediately comes back on as soon as the key is turned. If I jumper the diagnostic plug to ground it so I can read the codes it never flashes codes, even #12. It remains solid. It never flashes. Is the computer bad or is there another problem? Thanks.