My check engine light came back on again today, it only comes on after idling for 5 minutes or so. I did the key on, off, on, off, on, and it shows P0431. What's this code? BTW, I have already changed all 4 O2 sensors ( apparently I have Cali emission). I can clear the code by unhooking the battery, but the CEL will come back on eventually. Thanks in advance.