I have an 02 TRD 4x4 and have 150k miles. I am finally getting around to adding headers and a magnaflow. My brother suggested that while I'm at it I replace the catalytic conveter as well. He says they clog up and sap torque (which is what I'm after). BUT, I don't want to waste money on a replacement if it's not necessary. How can I tell what condition it is in?