Yes, the key trick will work most of the times. I was able to read a few codes by turn the key back and forth with engine off. You can also go to an auto parts store like Auto Zone and they will read the codes for you for free.
I needed to clear the codes from my truck so I bought a code reader to do this. I check for codes once a month or so and note if any are present so far with the Smarty installed I have not seen any since the install.
Jim W.