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P0421 = Warm Up Catalyst Efficiency Below Threshold (Bank 1)
This means the HO2 (Oxygen) Sensor downstream of the catalytic converter on bank 1, is detecting that the converter is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.
If your installing a new head gasget, the torque spects and pattern will come with it. If not and you are installing a new head gasget you have to use new head bolts, the short bolts witch go in next to exhaust manafold are 43 ft. lbs. and the longer ones in the center of the head are 46 ft. lbs. start with the one in the middle by exhaust and next one in center of head next to it and do the rest in a circle pattern, after you got them all torque than you have to turn each one 1/4 turn, thats why you have to use new bolts that 1/4 turn stretches the bolts. If you don't understand this go to auto zones web sight thats where i found it. I just did one 2 weeks ago.
go to the local auto parts store and purchase a Chiltons Repair Manual on your car, it has all the torque specs listed for this. Or you could try the library and see if there is one there for your car and just look it up for free..... just write it all down.