1994 Service Engine Soon Light with C12 and H13 Codes
 

94 C4 corvette with 83k miles on it. For the past couple of years, I get the service engine light soon that comes on after about 10 minutes of driving. I have used the jumper method on the OBDII for pins 4 and 12 to identify the following:
Module 1: C12
Module 4: H14

Based upon input from corvette central, these types of codes and others like it come up with the SYS shows up on the LCD display. I don't ever see the SYS show up but the Service Engine Soon Light does on dash mid-console display center. Based upon corvette central, C12 implies it is the current code with no diagnostics which is typically a good thing. H14 is history with 14 being DIC switches shorted to ground.

Based upon this it does not appear to be recognized current problem.

First question is the jumper method to identify codes applicable when the Service Engine Light Soon light comes on, or only when the SYS comes up on the LCD display.

Secondly, is there a different way to determine what is causing the Service Light to come on vs what I have doing. I have a engine code reader, but does not read this era.

Lastly, assuming it is related to my service light being one, the car and engine seem to run fine. It is only when I stop the car do I smell an order from the exhaust that I would categorize as not typical. Lige a potent sulfur smell - sulfur may not be the right smell but something that is that odd or different would categorize it. Exhaust that is not "clean" if that can even make sense. i.e. it does not smell light normal exhaust, but like something is not burning correctly.

Any help would be appreciated.