I just finished installing my new Street RV Head. I finally got it started, valves adjusted, and it's sounding good. After driving for maybe 30 miles, I have a CEL. It's a code 21 for the O2. I was wondering if anyone else has experienced this after a a HG replacement or the like.

 

If you had a blown head gasket the O2 may have been contaminated with coolant. We used to run into that often on the 3VZEs after they blew head gaskets.

Otherwise, are you sure you got it plugged back in, and make sure that the wires are not routed so they come in contact with the exhaust.