I'm not really an expert when it comes to this, but I would say your issue is about the same... My idle was not really bad... Starting the engine up was fine too. The headlights and heater bogging down when stopping was a problem. I assume that cleaning the MAF and throttle body would really help your issue. It did mine even though I was showing codes as opposed to you not having any. Hopefully someone with more experience will chime in.

Cheers,
Matt

I replaced my thermostat once the code P0128 came up and still engine light comes on. So I did the steps on this site H3 Throttle body and MAF sensor cleaning (up to step 5 (I couldnt get the vacuum hose on the intake loose so I left it as is) to clean the MAF sensor, and now I get different codes.(P0449) Would these codes have anything to do with messing with this vacuum hose or other sensors? TIA

A quick search of the forum database would have revealed to you a few possibilities that could be related to your recent maintenance: https://www.hummerforums.com/forum/h...de-help-14854/

Otherwise, inspect the evap. solenoid in question. They are known to crack and erode surrounding the attached breather port.