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, Flshing Hi 3rd Bk Lt, MDS "ON" Lt, Airtab Vortex Gens, Debadged, Fumoto Drain Valve, BT Catch Can, Chopped EGR Tube, SRT8 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 90mm T-Body, SOS Ign Coils w/.060 spark plug gaps, SRT Reps with 265/45&295/40X20's.