Season the outside of the roast on both sides with salt, pepper and garlic powder.
Set your pressure cooker to saute. When hot, add oil. When the oil begins to shimmer, brown your chuck roast on both sides.
Add the tomatoes with their juice, cola, broth, onions, celery and spaghetti sauce mix. Stir well until mix is dissolved.
Add the roast to the pressure cooker. Nestle the roast in a little bit. Set the pressure cooker to high for 55 minutes. Let the pressure come down naturally for AT LEAST 10.
Add the carrots and potatoes, set the pressure cooker to pressure for 5 minutes. Quick release the pressure when done.
If you want to make a 'gravy' out of the liquid, remove the roast and veggies to a serving platter. Add 2 tablespoons of the liquid in the pot with 1 tablespoon cold water. Mix. Add 2 tablespoons cornstarch to the cooled mixture and stir. Turn the pot to to saute. Add to the cornstarch mixture to the pot of liquid and stir until thickened.