There it is, P0300 code. Likely head #2 is bad.
 

I just got a 2006 H3 3.5 with 143K miles on it. At around 80K Miles the head was replaced by a Chevy dealer in Pittsburgh. Here I am, about 60K miles past that and having just bought the H3 Friday (2 days ago) I'm getting the famous and dreaded P0300 code that will typically end up being the valves not sealing. I need to do some testing to verify is not related to ignition before I pull the head and rebuild it.

Is there a tutorial anywhere on the R&R of the 3.5 head?

I signed an "As is. No warranty expressed or implied" document Friday when I bought the H3, I don't expect I have a leg to stand on with the dealer. I suspect that the fellow that traded it in knew what was going on and cleared the code the day he took it in.

I have access to the equipment to do the head work myself, so I'll pull the head and work it myself. I'm not paying $3000 for another time bomb when I can build my own time bomb for near nothing :-)

This is what I'm seeing on the Torque app.

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The P0300 code means that a cylinder(s) is misfiring or is randomly misfiring. Start by checking for intake leaks, intake gaskets are common cause of multi-cylinder misfiring. If no leaks are found the next step is to replace the spark plugs. If the problem persist more tests needs to be done to diagnose problem.
Possible causes
- Faulty spark plug (s)
- Faulty ignition coil (s)
- Clogged or faulty fuel injector (s)
- Intake air leak
- Fuel injectors harness is open or shorted
- Fuel Injectors circuit poor electrical connection
- Ignition coils harness is open or shorted
- Ignition coils circuit poor electrical connection
- Insufficient cylinders compression
- Incorrect fuel pressure

http://engine-codes.com/uploads/gmc/03-06-04-030F.pdf


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Easy does it Larry, chill that paranoia, a P0300 does not mean replace the head. As discussed, that usually is piddly little crap.

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I'm amazed at how many posts I find stating: "Cyl head needs replaced"... or another common post: "pcm needs replaced".

99% of the time it's NOT the head!... 99% of the time, it's NOT the pcm!

Instead of jumping to a conclusion, perform diagnostics and find the source of the problem, and save alot of money in the meantime.

The p0300 code for instance, check plugs, if fouled, replace them instead of reinstalling them=cheap fix. Inspect coils, if there is any sign of fluid intrusion, side arcing, etc., replace them instead of reinstalling them=cheap fix. Clean the injectors=cheap fix. Check manifold bolts and retorque=cheap fix. Decarbonize the engine=cheap fix. In the long run, performing the above maintenance=better performance and fuel economy=saving money at the pump. Oh and make sure that engine oil is clean and full.
Btw, there are many more steps/procedures that I would perform before even thinking of replacing the cyl. head.

I called Hummer and they made an appointment to have it looked at by a local Hummer approved GM dealer. I drop it off next Tuesday evening for Wednesday diagnostics.

Hey Larry, what was the out come after taking it to the dealer?

Interesting enough, I called the dealer and explained that I cleared the code and it hasn't come back. They asked me to just wait for it to come back and they would bring it in to read the computer and then follow it up with trouble shooting. Problem is, the light will not come on! I ran two tanks of Sunoco Ultra and in each tank I poured some Lucas injector cleaner in. Maybe I fixed it?