2004 Chevy Silverado Z71 4wd 5.3L. Truck has never had a starting problem until I got it back from the transmission shop who replaced rear main seal, the plate the seal is installed in and the camshaft sensor. Now this transmission shop is one of the best in the business and honest to a fault, so I have no problem with them. Here's the problem. Turn the key and the engine turns but does not start. Try it again and it starts as usual. And it does this probably 95% of the time now. Sometimes if you keep the engine spinning long enough (5 secs) it will backfire or what sounds like backfiring. When I confronted them with this problem they thought it was the camshaft sensor. They explained that on initial start the camshaft sensor is in play and if a no start happens and crankshaft sensor takes over, or something to that effect. They wound up replacing it to no avail and put a known good one from another truck in,same result.They also replaced the pigtail the sensor plugs into. It's odd that this problem just happened after work was performed but I guess it could happen. They gave me a automotive shop's number and they'll pick up the charges if it's anything they did but I'd like to fix this myself if anyone has an idea what could be causing this.