It operates using the cruise control switches, so it should be just a programming change. (like setting an enable 'bit'?)
(the 'elevated idle', used to warm the engine and trans, is standard on all 6.6L Dmaxes.)
Fast Idle System
If equipped, this feature is available only with cruise control. The manual fast idle switch is operated using the cruise control buttons on the left side of the steering wheel.
This system can be used to increase engine idle speed whenever the following conditions are met:
. The parking brake is set.
. The brake pedal is not pressed.
. The vehicle must not be moving and the accelerator must not be pressed.
To control the fast idle:
. To enable the Fast Idle System, press and release the cruise control on/off button and ensure that the switch indicator light is lit.
. Press and release the cruise control SET- button. Engine speed will be held at approximately 1200 rpm.
One of the following actions will turn off the fast idle:
. Pressing the brake.
. Selecting the cruise control cancel button.
. Releasing the parking brake.
. Moving the transmission shift lever out of P (Park) or N (Neutral).
. Selecting the cruise control on/ off button when it was previously on.
. Pressing the cruise control SETbutton a second time.
. Pressing the accelerator more than one-quarter of the way down.
. Turning the ignition switch to the LOCK/OFF position.
2018 GMC 2500 L5P CCSB 4x4 Denali - Mineral Metallic; previous '11 LML, '10 LMM and 04.5 LLY