Yes with the 20D I get
Err 01 displayed and the corresponding info is
Communication between the camera and lens is faulty.
Clean the contacts
There is a chip in the 1.4x and 2x so the camera 'expects' the appropriate signals from the lens chip via the teleconverter chip.This is my interpretation.
If you really want to try it you would have to cover the contacts so the camera cannot communicate with the teleconverter.
Bert
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