2000 Chevy Blazer service engine soon light

I have had a problem with the service engine light for about 6 months. I originally went to Auto Zone and they said that the code reading indicatedan emission problem and that I should run 93 octane gas and add Lucas fuel injector cleaner and the problem should go away. It actually did for a short time, maybe a week, then the light came back on. I have tried using 93 octane and injector cleaner to no avail. I put on a new gas cap and the light went off for a short time then came back on. I had a mechanic tell me that the problem was with the fuel tank and that it would cast 500.00 to fix as the tank would have to dropped in order to fix the problem. He also said I could continue to drive it with the light on and it would not damage the engine. I have even tried disconnecting the battery to reset the computer to no avail. I even crawled under the rear of the vehicle yesterday, dropped the spare and discovered that some sort of plug in connection was dangling, so I cleaned it up and sprayed it with wd40 and plugged it back in but it did not resolve the problem. I have spent a ton of money on this vehicle and plan on keeping it but hesitate to take any long distance trips with it with this problem. Can you please shed some light on this problem for me. Any help would be appreciated. Thanks. Brian