Re: 4.7 Main Cat
From my understanding there were 2 exhaust setups for the WJ's:
1. Federal emissions compliant. These had one main cat after the Y pipe with a before and after O2 sensor.
2. California emisions compliant. This setup has a pre-cat on each side of the Y pipe with a before and after O2 sensor on each side. Then there is a main cat after the y pipe with no sensors around it.
I would be willing to bet that a previous owner cut out the main cat and just put in straight pipe. This will not throw a code since the main cat has no O2 sensors on it. It is also probably illegal, but if they were getting away with that I'm guessing your area doesn't do emissions testing.
My Jeep: 2001 Grand Cherokee Limited, 245/65R17 Falken Rocky Mountain ATS, K&N drop in air filter, Bilstein HD's. De-badged, eagle-eye LED Tails & LED 3rd brake light, Addco front and rear sway bars, Superchips tuned & TM snip, interior 6k LED's, Gen3 ccfl projectors with Volt 6k HID lows, akebono brake upgrade with R1 Concepts premium rotors.