Why wouldnt my truck start, Cat 3126??

I run my truck out of the shop the other day, shut it off and had some other work to do. I got ready to bring it back in after about a hour. I forgot to wait till check engine light was out. (it was warm outside) It just cranked over and never would start. I waited about 30 min. went and tried it again, still no start, waited about a hour and still wouldn't start, just spin over. I finally had to do the No-No just to get it back inside for the night. I gave it a small shot of either. Fired right up. I checked the codes today and it has a 34 code which is a RPM fault sensor code. Where is this sensor located at and will it affect the way it runs. (having issues there too) Cat # CKM6522, 2003 KenworthT300.