Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
The temp sender may be located in a place where the transmission is much more dense and holds more heat. The pan likely has much more fluid in it than the sensor location which helps drop the temperature and there will be some heat loss through the pan itself. Do you have the ability to check the trans temp through the engine and transmission computers in data stream? What are you attempting to accomplish?
Good chance they are guessing if they can't give a solid reason. It is a reasonable guess I suppose if the sensor and the harness have been replaced. I would be guessing connection or wiring harness corrosion problem. Are they willing to put any guarantee on the fix? Like no charge if it does not fix it?