I have a trane heating system and I have already clean the flame sensor and replace the thermostat. The unit its set at 70 but it will only bring temp to 63 max. The burners will light up and then shutdown when the fan starts. then, minutes latter the burners will come on again. I noticed that the small fan stays on all the time and a red light comes on on the board when the burners are on is all this normal? please help, do i still need to replace the flame sensor.

Have you checked the air filter?

Are all the registers and cold air returns open?