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question for anyone here.

I took took my 2021 Tahoe RST Diesel in for the first oil change/service and to fix the airbag recall. My Tahoe was just over 7,000 miles.. Anyways, its been about 3 days of having it back and one thing I noticed immediately, is that when driving the idle is bit higher then normal and more "throaty". Kinda louder as if you put an exhaust on the Tahoe. So my issue here is before taking it in for service, it only had this rougher idle a few times and would go away either driving slow and/or parking the vehicle and using it later on.

Prior to the service, I would be able to drive the city streets and stop at lights or stop signs and auto start/stop would engage but now this idle prevents that from happening. I am not sure if this is normal for a Diesel engine or what, but all I know is that before taking it in Tuesday I would be able to stop at a light and the idle would be low and the Tahoe quiet like if you put it in Park even when auto start/stop wasn't engaged, but now you stop and for sure auto start/stop won't work and its louder like if you are giving it a little gas.

So based on this idle issue it also affects the MPG as I have monitored the display while driving and comparing it to similar driving prior to getting the service. After ready the Duramax manual, it seems like Diesel Particulate Filter(DPF) cleaning as it has this bit of info

"During self-cleaning there will be a change in the exhaust sound and engine idle speed. Along with this, a burning smell and reduction of fuel economy may be noticed. This is normal"

So there is no burning smell, fuel economy does take a massive hit here like 20% or more. There also was never any message or any warning that would leave me to believe it was happening. Does anyone know if this automatically happens after an oil change/service or of maybe a service tech initiated it?
 
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