Ok, looks like you dont have the UF3 high idle option for this truck. if the truck has the high idle system it works by using the cruise button.
There has been some questions regarding High Idle, Elevated Idle and PTO. The below information is to help provide additional information as to what each of the options are:
Elevated Idle- Is a standard option on all 6.6L Diesel Engines, which elevates the engine idle speed from base idle to 1050 rpms when outside temperatures are below 32°F (0°C), and the engine coolant temperature is below 150°F (65°C.) This feature enhances heater performance by raising the engine coolant temperature faster. This feature can be turned on or off, please refer to the "Duramax Diesel Supplement" Owners Manual for more information.
PTO (power take-off)- Is available as an option (RPO PTO) only on 3500 Chassis Cab models equipped with the 6.6L Diesel Engine RPO LMM and Allison 6-speed transmission RPO MW7. The PTO allows the user to create an auxiliary power source for running add-on equipment, such as salt spreaders, snow plows, winches and lift buckets. The dash mounted PTO switch is used to turn on the PTO and controls engine speed to values higher than normal base idle. For more information see the "Duramax Diesel Supplement" Owners Manual or online at "www.gmupfitter.com".
High Idle- Is available as an option (RPO UF3) on certain 2008 models with cruise control. This system can be used to increase your engine idle speed for what ever reason an owner wishes, more generator output at idle, belt driven add on equipment, etc. The manual fast idle switch is operated using the cruise control buttons located on the left hand side of the steering wheel
If the vehicle has this feature it is available only with cruise control. The manual fast idle switch is operated using the cruise control buttons located on the left hand side of the steering wheel.
This system can be used to increase engine idle speed whenever the following conditions are met:
| • ||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, press and release the Cruise Control On/Off button and ensure that the LED is lit.|
| • ||For the preset Fast Idle, press and release the cruise set switch. This will set the preset fast idle speed.|
| • ||For the variable fast idle, press and hold the accelerator at the desired RPM, then press and release the cruise control cruise set switch to set the desired idle speed.|
When the fast idle is active the Driver Information Center (DIC) will display "FAST IDLE ON."
One of the following actions will turn off the fast idle:
| • ||Selecting the Cancel button.|
| • ||Releasing the Parking Brake.|
| • ||The transmission shifter is moved out of P (Park) or N (Neutral).|
| • ||Selecting the cruise control on/off button when it was previously on.|