Which engine? Are you getting any codes? That will tell you which one(s) is/are the problem(s). They can be a PITA to replace, again, depending on which one is bad. If you have over about 75-80K on it, I'd just replace them all. I had one go bad at a little over 60K and replaced all of them. Cost me about $120 for all. Also, use the same brand that is in there now. The 5.7's run NTK's, a division of NGK. I had a problem with the fronts when I installed Bosch Sensors. Replaced them with NTK's and all was well.
06 GCO, Dk Khaki, QDII, Tint Ft & WS Visor, Flshing Hi 3rd Bk Lt, MDS "ON" Lt, Airtab Vortex Gens, Cbn Fltr, Debadged, Fumoto Drain Valve, BT Catch Can, Chop EGR Tube, SRT-8 CAI, K & N Filter, Sonnax Trans Line Press Booster, SC Tuner W/Adv Trans Tune App, DS Tuner, Cust tubular hdrs w/3 in Magnaflow/Xlerator AP Exh Sys, Bilsteins, SRT Springs/Sways, Cust Eng Cvr, SRT8 frt/rear bumpers, FRI Sidewinder Cam/6.1 valvetrain, SLP 25% UD Pulley, BBK true 85mm T-Body, SOS Ign Coils w/.060 spark plug gaps.