The No. 4 fuse on my ’91 245 keeps blowing. The problems began when I had a new O2 sensor installed. After the installation, I got several error codes indicating the O2 sensor was at fault. I would reset the black box, but the check engine light would return by the next afternoon. It would always come on in the afternoon (when hot) and after I had made a short stop for gas, etc. After living with this (and a very rough running car) for about a month, I discovered that the No. 4 fuse was blow. I replaced it. Everything was fine for about a week, until I had to sit in traffic and the check engine came on with the O2 code. Replaced fuse and reset box, everything OK for about 2 weeks; then the scenario repeated itself. Now the fuse blows as soon as I start the car. I checked the white connecters in the foot well, they were OK. Any suggestions?

Thanks for the help!