2005 Toyota Echo Mass Air Flow

To begin with I live in Quebec where it's cold ! So I bought this echo and realized it had a bad 9.5L/100km gas mileage. But, the car was going really well ! So I went to the garage. There was no check engine but said he found a code for a o2 sensor (2nd one). He repaired it.... But, after a few weeks, I didn't see any amelioration. I asked to a friend to check the car with a WIFI obd2 and his iphone and he realized that the information of the Intake Air Temperature (IAT) had an error. It was shown that the temperature was 254575.... degres. Which probably means there is a problem somewhere ! Plz help me !