Chances are you are using this as a 2.5V voltage reference for the feedback on your regulator. One thing that isn't clear is exactly what failure you are seeing. Please specify the nature of the failure. Is your power bus out of spec? I really can't tell you what you need to know without knowing what problem you are having.
That said, if you are experiencing an out-of-spec issue, is the zener operating in spec? It may be and your power bus still may be off. The problem is the TL431 may not have a tight enough tolerance to meet your operating spec. Try the TL431A which has about a +/- 1% tolerance. Sorry, but I'm shooting in the dark here since you provided no details about the failure.
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