I removed the old oxygen sensor and tried to put in the new one, but it won't on 1991 GMC 1500 Pickup

fit????Why and what do I do now since it won't pass inspection because h20 is too high??

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3 answers
Threads damaged.Get appropriate size tap and re-thread.
Someone sold you the wrong sensor...Ask for a upstream if the sensor is on the manifold...
wrong part