5S-FE MK2 Non-Turbo Engine Header Install Now Running Extremely Rich

5S-FE MK2 Non-Turbo Engine, header, extremely, exhaust, sensor, rich, fine, downstream, idles, runs, upstream/downstream, sensors, issue, explain, swap, cylinder, running, install, head, removed, plugged, upstream, installed, factory, manifold, blocked

During the swap of my cylinder head, I removed the factory exhaust manifold and installed an exhaust header. I plugged in the upstream o2 sensor but blocked off the downstream o2 sensor. Car idles fine but runs extremely rich. Someone please explain what my issue is and what the upstream/downstream sensors do and what I need to do.