1997 3500 Chevy Vortec 454

I have a 1997 Chevy 3500 with a Vortec 454. The check engine light came on about two months ago. I guess you can say I made a dumb mistake and never had it checked. We have a Suburban that the check engine light has been coming on for over two years. When I talked to a mechanic about it he said that it was the gas/air sensor and it really would not hurt anything, so we have left it alone and sure enough no problem. So being as it was I figured the Truck was doing about the same thing since the truck has 135,000 miles. No changes in performance and all gauges worked fine. Finally, this weekend the truck died and now I have no fire to the plugs. All fuses check out OK and the reading is showing RPM but the code P1345 is coming up. So any idea on what it might be and how to solve the problem. Thanks Keith