Hi, this is my first time posting here. I own a 2003 envoy sle with 120k miles, regular maintenance. I love this truck. It's great in the snow. Recently I've had a problem with the ses(service engine soon) light blinking, a very rough idle and when my guy put it on the computer code s0300 came up which is a misfire in cylindar 1, cyl5 and cyl4. I changed the plugs at about 115k, ac delco iridium. My mechanic stumbled across a technical service bulletin #06-06-04-048b which described why this happens. It turns out the weather seal close to the windshield under the hood is corroded and was cheap allowing water and snow to leak down into the camshaft and corroding the coil packs. For a $20 part this is going to cost me to replace my 6 coil packs plus 6 spark plugs. We removed the coil packs and discovered they are slightly corroded causing a misfire. I called gmc corporate at 866-790-5600. I have an appt at my local gmc dealer which is going to cost me $85 for a diagnosis. I'm hoping they'll concur with this tsb and gmc will pick up the tab for my crap weather seal and the fact my 6 coil packs and plugs need to be replaced. Funny how a crappy weather seal can cause a $600 fix.