1991 FJ 80 with 3FE. it has had a code 21 since I have owned the truck. Code 21 is o2 sensor or main o2 circuit. I have replaced the o2 sensor with a denso OEM sensor and pulled the efi fuse to clear the codes. Within 5 seconds of restarting the truck the CEL is back on with the same code 21. So, anybody got an idea of what would be causing this?? the wiring harness by the o2 sensor looks clean and free of burn marks, dirt, etc etc.... so where do I look? any help is appreciated

Thanks