Aactually it is a very low level read out, all the modules/nodes are checked to see if codes are set, but it won't tell you what the codes are, so doing it won't get you anywhere that the check engine light being on, or the message that gets desplayed in the DIM (drivers information module) wouldn't do. You have to know what the code is by number and then know how to verify the failure, in order to repair what might be the problem.