I have a 2003 Silverado 2500 HD with a 6L and get a misfire code, #P0300, dealer told me to change wires, plugs and run injector cleaner through it for a few tank fulls which I have done all 3 but still get the code. The Check Engine light will blink only when I am on the highway. I can drive all day without the light blinking but as soon as I get on the highway is when it will happen. Anybody got any suggestions???

 

what kind of plugs did you use and did you make sure you put them in there snug? to where all of them are firin?

 

check to see if your plug wires are arching. i try to use all gm parts and ive had a set of new plug wires start arching a couple of days after i put them in.

 

AC Delco and they are all in tight, don't understand why it only happens on the highway

 

Check fuel pressure and volume. These can cause fits at highway speeds.

 

i found it easier to find an arching plug wire after dark. even then u have to look close. another thing u can try is going to autozone and renting a diagnostic tool to check if one of your coilpacks is going bad.