No i didnt. I was totally (and i now realise stupidly) relying on the diagnostics that on-star was running for me.

 

So i finally got a call back from the dealer yesterday. The lifters were replaced and they did not find evidence of any other damage. They also filled it up with the mobile1 full synthetic.
I am either incredibly lucky that nothing else is damaged, or terribly screwed that they did not find anything that is actually damaged.

That being said, does anyone know of good technicians or service centers near cleveland, ohio where i can go for a second opinion? Or is there anything i can check on once i get it back?

 

check oil,,,and temp. the 2 things i watch, and especially testing a new used car.
3, anything dripping out the bottom at idle hot.

4 hot brakes after coasting to a stop.

 

Ok, so i got my hummer back a week ago. The knocking sound had gone. Dealer replaced lifters for cyl 4 and 6.
Ive been driving it for a week, almost 500 miles later i now get a P0420.

The week i got it back, after barely driving it a little the dash showed a change oil message. Reset it and it went away.

When i started up today morning, i could hear some sound from the engine like some squeaking. Drove it a little and the CEL came on with the p0420 code.

 

I did monitor all these for the last week. The outside temps are pretty low so the engine takes a while to heat up, but never crossed the halfway point.

 

The CHG Oil message is likely because they did not reset the OLMS, but you did now.

If I were you, I would check the oil, check the oil, check the oil every day for a long while to make sure the level is constant.

That P0420 tells me you could have oil contamination to the CAT. Got any smoke? Watch that oil level..... check the oil, check the oil...

 

At the time the change oil appeared it did smoke a little, but i attributed that to the weather. I drove in constant rains for 2 hours and the weather was cold.
However, ever since i got back from the dealer, there was a funny smell from the engine and exhaust. I was to post the details this weekend about it and the CEL came up today !

Also, i noticed that it doesnt rev up as fast as it used to when i got it. Earlier when merging on interstates i stepped on the pedal - it dropped a gear, the v8 vroomes up in no time. Now it still picks up speed but there is no vroom....kinda goes up gently. Does drop a gear, but then quickly up shifts and gently speeds up.

 

oil level looking good so far. Dropped it at dealer today. Lets see what they come up with.
Surprisingly, the cruise control worked even with the CEL on!

 

The service center rep said its carbon build up in the exhaust. They will be cleaning it up and thats all. The o2 and other sensors look good. Keeping my fingers crossed.

 

Got my hummer back. Dealer didnt charge me this time. So far its running good, but ive been doing only city driving.

Thank you to all those who provided suggestions and inputs.