.........I have read in the C4, C5 and current model forums about headers making haywire with the rear O2 sensors. I have Melrose pipes on my car and afterwards the check engine lite too. The following worked for me without any aftermarket parts or appointments with a tunner. The goal was to prevent the rear O2 sensors from taking a sniff of the exhaust stream............ Go to your local hobby shop or Ace Hardware. What you are looking for is the K&S metal display. Pull out a foot long brass tube next size smaller than the dia. of the pick-up end of the sensor. Take home and cut tube to the length of the pick-up (around 3/4 inch) and then split the tube lengthwise with a tin snips. Expand and slip over the sensor. The tension and the fact that your O2 sensors insert into the exhaust pipe at around the 4 and 8 o'clock position will prevent them from rattling off. The brass tubes are thin walled and once on the sensor, will pass thru the threaded bung mount on your pipes. The split is facing downstream after re-installation. I done this months ago and have no pics but if you grasp the idea of sleeving over your sensors to isolate the pick-up side, your PO420 code days are over!