If you set the detector temperature by computer, and you set it to 250C but only get 235C by true and actual measurement with a thermometer, at what setting would you set the detector temperature to achieve a true temperature of 250C ? or 235C? or 275C? or 150C ?
Spend a hour or two and determine the actual temperatures achieved at different settings and document it.
Use the document as a calibration of your instrument.
Unless you wish to perform your own service, ie. replace controller boards, or determine why the sensors are shutting off the heater at the wrong temperature.
What else are you going to do, other than buying a new GC ?
If you get 235C when you type in 250C, and you don't want 235C , then don't type in 250C. Type something else instead.
You have to determine the proper setting to get the temperature you want.
Or call in the service department and pay for the work they will do.
Good luck,
Rod