I have a Hyundai Sonata 2000 had fail Nox emission (Gross Polutted). The car has no Check Engine Light on. But I found there was 2 DTCs Code from OBDII that were P0134 Oxygen sensor circuit No Activity. P0154 Oxygen sensor circuit No Activity. I looked up under Hyundai Diagostic book and it said (Check for Contaminated or crossion). I did replaced both upstream Oxegen sensor before CAT, and the code are still there. I even check the voltage from the oxygen connector, but did not get anything. Do you have any ideas how to fix it?. Bao