What could cause the catalyst I/M monitor not to "ready", yet not cause a CEL or MIL? I hooked a scanner up to my Explorer and the only issue was the catalyst I/M monitor staying in a "not ready" state. No codes and all other monitors were fine. The truck has sluggish acceleration, and seems to loose steam going up hills and so forth. The idle has also gotten rougher. I was getting a "popping" noise from around the exhaust at one point, which I assume was a backfire, but that has now gone away. I've tried removing the rear cats but that didn't change anything. I've also swapped out the coil packs and MAF for a day with known good ones. And I checked the fuel pressure which was low, but still within the recommended range.