Throwing codes W/O the check eng light on.
 

1999 SLK230/auto with about 49K miles, I was wondering if it would throw a code without triggering the check engine light.

NP

Yes they most certainly will. The rule for setting the light is anytime the system goes 1.5 times the FTP (federal test procedure) limits. For many issues the light won't come on till the second time a failure occurs. So in that case all single failures set what some manufacturers call pending codes, still readable.

Some fault codes are for things that don't effect emissions and never set a light.