Chevrolet Low Cab Forward 4500 2019 Series Owner's Manual page 172

Actions that Can Lead to a Breakdown with an Automatic Transmission
Actions that can lead to a breakdown
Stopping the vehicle on an uphill road with the selector lever in a position
other than N or P, the accelerator pedal pressed, and the brakes not
applied.
Pressing the accelerator pedal and brake pedal at the same time while the
vehicle is in a gear other than N or P.
Placing the selector lever into any gear with the accelerator pedal pressed
and the engine speed high.
Placing the selector lever into P before the vehicle has completely stopped.
Turning off the engine control switch while driving.
Driving with the selector lever in the N position on a long downward slope
(this is dangerous due to the lack of engine brake).
Driving and Operating
Breakdown symptoms
The transmission fluid overheats.
The transmission gears are
overloaded.
The parking mechanism will be
damaged.
The transmission is not properly
lubricated.
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