Are you talking about a V8? Bank one is left side (drivers side) #1 piston is front of engine. My experience with O2 and Cats is go to a muffler shop off beaten path. Don't go to a big chain outfit. Ask a mechanic where he would take his car. They will be honest and check your cat for free. Cats wear out, O2 sensor should last around 60,000 mi. unless your fuel control system is bad and sends too much fuel contaminating cats and O2 sensors. But a 2006 should not need either of these things. I believe by Federal law cats are warranted for 7-10 years? Ask around. I bought my own OBDII scanner. You can read your own "check engine" light. You'll know if you're being led astray!