temp guage not working

[flipsta510fazn]

hi i have a 92 celica st 1.6L when i bought the car last year it was great but the temp guage never worked acquritly it was always in the middle right when i start it in the morning so last 6 months before driving a 400mile trip i tried tampering with it with no results and it just went to c and stayed there eversince yesterday i bought a new sensor and went to the junk yard to get another guage and replaced it and still no difference again i change the temp sensor and the guage i also change the other sensor that kicks the fan on i dont know anymore im thinking of maybee just putting an aftermarket guage and just run it directly...i tried unpluging the temp sensor while car is running the check engine light went on how can i go about taking the sensor out and putting an aftermarket sensor for the guage on for the engine light to not come on

also i use to get about 32-35 mpg and now i only get 25 to a gallon i change the spark plugs, egr, wires, fuel filter no vacume leak change water pump and timing belt no gas smell in car so meaning i guess not running rich right
let me know what you guys can suggest for both issues

[PittsburghCeli]

Must be the wire between the gauge and sensor. Try running it directly.

Have you recently been putting premium in your tank? Premium gets worse gas mileage than 87.

[flipsta510fazn]

i been runing 87 the whole time but yea im going to run it directly soon
how do i know which one is + and - and which one is lead wire for sensor