I have a 97 Chevy K1500 with 5.0L and 205K miles. I have only had the truck for about a year but for the most part it is running really well. I just noticed that at some times when I key on, the Service Engine Soon (SES) light is lit during bulb check and other times it is not. Can someone please confirm whether it is supposed to be lit on key on during the bulb check? If so, other than a burnt bulb, what could cause it to drop out?