1981 Celica GT Hatchback

I own a 81 Gt H/B. A few months ago the sensor light came on on the dash and I cannot get it to go off. I thought that it was the O2 sensor and I replaced it but it did not cure the problem. I got ahold of a Haynes or Chilton manual and have followed their advice but cannot get the light to go out. Anybody out there have any ideas on how to get the light to go out. The car runs fine with it on. Thanks