I have an 03 sonata 2.4 w/ 122k mi on it..I am getting a p0421 code and I am using excessive amounts of oil ( three to five qts between oil changes ) I am wondering if the downstream 02 is getting fouled by the oil..or is the oil use from the code? car ISNT smoking or leaking on the ground???!!!

 

P0421 = Warm Up Catalyst Efficiency Below Threshold (Bank 1)

Basically the computer is seeing that the signal from the O2 before the catalytic converter is different from the signal from the O2 after the catalytic converter. So either one (or both) of the O2 sensors are bad or the catalytic converter itself is bad. And yes, if it's using that much oil, it could have fouled the sensors or ruined the catalytic converter.