So my car was reading code P0135 O2 sensor heater circuit malfunction (Bank 1 Sensor 1) so I researched and everyone said to change the o2 sensor in the header so I did with the universal bosch that wired up no problem very easy clear directions. Cleared the code and now this morning it threw this code P0421 Warm up catalyst below threshold(Bank 1) does that mean i got a messed up O2 sensor or what?

 

that's a code for your first cat... i believe there was a recall on it. if not throw some non-foulers on your second O2 sensor and you should be okay.