Hello got 94 z28 runs drives good but have ses light on scanned it and it says code 42 something about ground don't know where to start thanks in advance

 

Tough one to figure out. The code can only set when the engine is cranking. According to the factory service manual it does not turn on the SES light. When it sets, it disables the fuel injectors. The diagnostic counts pulses from the Opti, and looks for an IC event before the crankshaft rotates 84 times. Hence, it seems like it can be related to intermittent issues with the Opti signals.

Questions are - is your engine taking longer than normal to start? Does it display any other problems? And, why is the SES light on?

Here are the diagnostic description/diagnosis pages from the 1994 factory manual:

 

Thank you injuneer