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Random Cylinder misfire P0300 Code!


Probationary Member
Replaced Plugs, and Coil packs!
Still throws the engine light!? New fuel pump 2 years ago as well! No rough idle and no surges or power loss?! I'm about at wits end??!!! Any help is greatly appreciated!!!! Thanks in advance, 2006 H3 won't pass emissions cause a this!!!!


Well-Known Member
Palmdale, CA
I had the same problem.

The 2006 3.5L had a problem with cycler 1 & 5 due to the seals wearing out on the coils packs and allowing water to enter. I just recently fixed this but needed to do redo it like 3 times.
1. The pcv intake hose ripped and oil entered coil pack 5 (cylinder nearest to the fire wall)
2. Water entered coil pack 1 (cylinder near the front of the engine).
1. Make sure you purchase the correct coil packs 2006 3.5L
2. Make sure you purchase the correct spark plugs 2006 3.5L and make sure they are gapped (super important). You will misfire if they are not.
3. Throughly clean the area where the coil packs are. This is an important if it’s not clean the new coils cannot seal correctly and moisture will enter.
4. After cleaning, vacuum the area as well as the inside the cylinder to ensure no debris or moisture is present. (You will misfire if debris or moisture is present)
5. Clean the inside of the cylinder again but leave the towel in the cylinder for 10 minutes. Let it sit and absorb any potential moisture and vacuum one more time. ( I burned two spark plugs and a new coil pack because of moisture still present) do it right the first time. Unfortunately it took me 3 times
6. Reinstall everything

Happy Hummer

Well-Known Member
Okay. That is a good start.
I would do a compression test for each cylinder.
I would also test for fuel rail pressure.
I'd make certain that each fuel injector was contributing and that my hose under the rear of the resonator was not dry rotted or damaged.

The above recommendation can007 is also some extra info .


Well-Known Member
United States
I had that years ago and it turned out to be a loose intake manifold. It was frustrating trying to track down, so I wish you luck finding the issue quickly.