In replacing the catalytic convertor of 6.0l and 8.1l engines one should begin by raising and supporting the vehicle, and then support the transmission by using a correct transmission jack. Remove the transmission mount to transmission support nuts, lift the transmission off of the support, take out the transmission support cross member bolts and the cross member. Depending on which side is to be replaced one removes (the exhaust muffler) nuts on the muffler, or loosens the exhaust pipe clamp. Remove the necessary exhaust system hangers to disengage the pipes and employ a jack stand to hold the exhaust in place. Pull off the connector position assurance (CPA) retainers and oxygen sensor electrical connectors (1, 2). Loosen the right nuts for the exhaust manifold pipes and remove the nuts for the left exhaust manifold pipes. Lower the transmission a little with the use of the transmission jack, take out the Catalytic Converter, and move its hanger out of the bracket of the exhaust pipe hanger. If you are carrying a 6.0l engine with regular production option (RPO) y91, then take the oxygen sensors when restorating the Catalytic Converter (chances are you may have to remove the Catalytic Converter again). When installing, apply anti-seize compound gm p/n 12377953 or type to the old oxygen sensors and install them, tightening them to 42 nm (31 ft. Lbs.). When the vehicle is either equipped with a 6.0l engine, and the engine is rpo y91, then if the exhaust seal is new, it must put that on the exhaust manifold and then the Catalytic Converter must be installed, doing so by sliding the hanger into the exhaust pipe hanger bracket. Restart the replacement of a new exhaust seal to the exhaust manifold and to the Catalytic Converter. Put the left exhaust manifold pipe nuts into place and tighten the right exhaust manifold pipe nuts to 50 nm (39 ft. Lbs.). Aerate the transmission slightly using transmission jack, reconnect oxygen sensor electrical connectors (1,2) and attach the cpa retainers. Mount the exhaust pipe/Catalytic Converter to the vehicle and the exhaust system hangars. Depending on the side being replaced either install the exhaust muffler nuts and tighten to 40nm (30ft. Lbs.), or install a new exhaust pipe clamp and tighten to 44nm (33ft. Lbs.). Reinstall the transmission support coss memberand the bolts tight 95 nm (70 ft. Lbs.), lower the transmission on the support, and install the transmission mount to transmission support nuts, tight 40 nm (30 ft. Lbs.). Lastly, take off the support from the transmission, and lower the car.