Without looking at an identical model, I can't confirm or deny that the only thing inside the incubator is the heating element but I can pretty much guarantee that the temperature sensor is inside the incubator and if it also reads humidity, the hygrometer probe will also be inside the incubator. The sensors tend to develop a coating over time which inhibits their ability to transmit accurate temperatures and humidity.The only "thing" inside my bator is the heating element thing.
I did not wash that part when I washed it...should I have?
The humidity element can also become coated so a careful wiping of it should be helpful.