Just got done putting on a Flowmaster Exhaust on my buddies 91 K2500. We also took off the cat. When we were done we took it for a drive. My buddy liked the way it ran and better performance. But i thought it should have had better throttle responce. So I checked the timing and put a scanner on it. When checking the O2 sensor out I kept getting "02 sensor not ready" and the computer kept staying in "Open Loop" mode and would not close the loop to operate off the 02 sensor. The truck was at operating temp (195 degrees) The 02 was putting out 400- 500mv 02 cross counts seemed about right. Think the 02 is bad? I put in a new 02 a year ago and the truck doesn't get many miles put on it. Or by removing the Cat maby the 02 sensor is not heating up enough to start working. /forums/images/graemlins/confused.gif The 02 is located on the exhaust pipe down stream from the Y about a foot in front of where the Cat used to be. Any thoughts on this is appreciated.