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Gas mileage drops after replacing coolant temperature sensor

I have a 1996 Nissan Altima with 170K. The car had been running perfectly other than a shaky coolant temperature gauge (the needle is shaky sometimes). I replaced the coolant temperature sensor three weeks ago (bought from AdvanceAutoParts), now the coolant temperature gauge problem seems to be fixed but the gas mileage has dropped from 25MPG to 20MPG (city driving).

Is it because a faulty sensor or the computer needs to be reset?

Thank you.

My 87 Ranger has a single wire sending unit for the temp gauge and a 2 wire sending unit for the PCM/computer.

A quick look at the wiring diagrams will tell you if you have the same as my Ranger or if you only have a single sender for temp gauge & PCM.

OR Plan B

Call a Nissan dealer parts dept & ask if they show one or 2 sending units.