I have 403 code on truck, tested egr by applying pressure and stalled truck out. I replaced DPFE sensor but when I back probe the wireharness side of the plug the most voltage I can get is 1.4v that is after the truck is hot and with me driving the truck. When I'm driving if it starts to shutter I press down harder on the gas and it down shifts and doesnt shutter. Today when I had the voltmeter hooked up and did this the voltage dropped to .9v then went back to 1.4v. I checked all vacuum lines found one connection above pcv valve that was melted so replaced it and it fixed nothing. I checked all COPs and their resistance is in range but there seems to be a connection between cylinder 6 and the problem. When I disconnect this COP when the truck is running the engine skips and sounds the same. So I replaced this coil with another one but it still does the same thing. I place a spark tester between the plug and COP and you see a strong spark but and truck runs good but when you put the COP back on and screw it down it starts running bad again. I am starting to think it is arcing out when I put it on the plug. If anyone has any ideas on something else to test please tell me.

 

Inspect the coil very closely for cracks , then clean the plug boot and use a dab of dielectric grease on the plug end and spring. See if that helps.