Mine is pretty old and was sold by NAPA years ago. It has a magnetic sensor head with a hose on the end you shove up the tail pipe. I have been watching e-bay for meters so I could mount a 02 sensor in the head pipe, but just haven't found one I like yet. Mine works like this one,
http://www.ebay.com/itm/Vintage-Sea...9395d8&item=301697570264&vxp=mtr .
