I have a 2006 2500 with a 5.7 Hemi, check engine light came on, when I did the key trick it didnt give me a code. The check engine light just flashes. Prior to today, I got a code that read P0420. I replaced all four O2 sensors. Can anyone tell me what issue could be now or why it isnt giving me a code and the check engine light is just flashing?