Oven Tempeture

I HAVE A PROBLEM WITH MY OVEN NOT GETTING UP TO PRE-SET TEMPETURE.
WHEN I TURN THE OVEN ON AND SET THE TEMPETURE, IT SHOWS A INTITIAL TEMPETURE OF 185 DEGREES. TO TRY TO GET TO THE TEMPETURE YOU WANT, YOU HAVE TO SET IT AT 550 DEGREES AND IT MIGHT REACH 300.

I HAVE REPLACED THE OVEN IGNITOR THINKING THAT WAS THE PROBLEM.

I THICK IT MAYBE THE OVEN SENSOR.
:confused: CAN SOMEONE HELP ME OUT. THANKS