Just got my car back from the dealership to fix a misfire problem on my car. Now it is showing DTC 18 on my scanmaster. What could it be?

 

A fault on the fuel injector circuits. One of your injectors has an open circuit, a shorted circuit, or just enough current leakage to tell the PCM that it isn't working correctly. The PCM can't tell you which injector it is. You have to figure that out yourself.

Maybe they forgot to plug one of the injectors back in - does the engine feel like its missing on one cylinder.

 

Is that a DTC 118?

 

He apparently has a 94 LT1. That's OBD-I, and the trouble codes are in the form DTC xx, where xx is a 2-digit number. There are no OBD-I PCM codes with 3-digits.

On the OBD-II PCMs, the trouble codes are in the format Pxxxx, with four numeric characters.