Temperature is a measurement of the heat of something, the word "bad" is hard to apply to a fact of nature. Perhaps you meant something else by "bad", if so please make what you are asking clearer.
Bad temperature sending unit? Bad gauge?
YES
That is one of many possible causes.
They never go bad. Ever.
temperature gauge not working can be caused by a bad temp sensor in engine, a bad gauge in dash, or a bad fuse
All semiconductor are sensitive to temperature. Some vary greatly some not so great. Zeners are not one of class as greatly. Zeners do change voltage slightly as temperature vary like all active devices. But to use one as a temperature sensor is bad idea.
No. This is normal.
your a retarde.
what are the symptoms of a bad temperature switch
Bad gauge, bad sending unit, bad wiring.
Potenially a bad temperature sensor or bad contact on the wire connector. Test for set voltage resistance according to temperature.
Good