it probably an old code stored in the ecu try a reset go out for a drive then check for faults again. i would be suprised if its there after the reset.
Have you ever tried to start the car with the coil,coil lead etc disconected this would log this code.
Had this myself after an oil change i like to leave the car cranking over to build some oil pressure without starting so i pull the coil lead so it doesnt start and this logs the code 21.
Thanks
Paul