I believe the check engine light has to light and stay lit to pull any kind of codes...there aren't stored codes...at least in my 1990 300E that's the case....I have had service centers say that, yes, there may be latent error codes stored in the module but that's a all bullcrap......If you don't have a check engine light just figure it out...probably a tune up would work...that's what I did...worked great