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I've seen this a number of times in the new ford and chevy DEF tanks. They have a fluid level sending unit (just like a fuel pump) that goes out and makes the truck enter limp mode because it thinks the truck is out of DEF.
What year, make, and model is your truck?
Do you have a scan tool to help nail down what error it is giving?
If not, that's fine.
Also, how low was the level when it first gave you the warning and you added to it?