There's a fuse (or fuses, can't remember) for the O2 sensor heater circuits located in the fuse block by the battery. That's most likely your problem if it's just the 31 and 51 codes you're getting.
Went through the same thing a number of years ago where I replaced the sensors and still was getting the codes. Found and replaced the fuse and everything went back to normal.
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'17 Titan Bone Stock
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