Transmission range sensor

This can be a number of issues, first i would make sure that there are no oil leaks that may have depleted the oil level, do this quick check first, report back if this is ok.

I check for leaks at the motor and transmission. No leaks

next thing to do is get a line pressure test done, this will see if pump and governor are working properly, not sure if your transmission is wired for live data read, but if it is this will also help the mechanic to see if all shift solenoids are working correctly, also the Converter lock up clutch may be permanently engaged and the car is trying to drive off in top gear.