Oven Temp

CRE9800ACE s/n 13005280HY Control panel shows temp reaching set degrees but using two different thermostats it is showing 100 degrees cooler. Ex. set for 500 reaching 400. This is after I reset thermostat by 35 degrees. My question is can it be the sensor or is there an internal thermostat or a control board. How do I check to determine where the problem is? This the only issue I have had with this 15 year old range.