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I've checked all four sensor's by removing them from the bumper, cleaned them, removed the connectors (no signs of corrosion) and re-installing them back to there original position.
I've also checked the main connector that is located on the side of the bumper, which connects all four sensors, and didn't find any corrosion.
There are a number of threads on the same topic here, but not much in the way of results to get it fixed. Most are just people like us with the issue. I think the best advice I've seen so far is go to a dealer and have them read the code. Not a something a ceap reader can do, it needs to have body diagnostics on it. Likely only a dealer can do it.
I bit the bullet on this while my truck was in for a transmission service. The dealer found a broken wire in the r/r tail light where the wiring harness going to the sensors is. $92 for that part of the repair, back in business.