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I was wondering if anyone has had any experience with an intermittent P0300 code (multiple cylinder misfire) on a Chevy Express 3500 6.0 or any vehicle with the 6.0 Vortec 6000. I tried changing the plugs and wires to no avail. The code will appear only on the highway when the vehicle is decelerating. A couple days later it will go away. But when I get on the highway again it usually comes back on. I had a mechanic friend of mine check the vehicle computer and the misfires he found were on the 1 and the 6 cylinder. Several thousand misfires on these cylinders were recorded. On his advice I took it to the dealer to check it out. There diagnostics indicated misfires on all cylinders especially the #7. The technician did not think it was ignition coils or ignition sysem he thought there may be a problem with a clogged catalytic converter. This dealer could not lift the vehicle due to the fact that it has dual rear wheels and a utility box on the back. I am taking the van to another dealer who can lift it and perform a cat back pressure test. However I am not having any driveability or power problems with the vehicle which would normally be associated with a cat problem. Anybody have any thoughts on this issue.
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