Serpentine belt replacement is an easily performed operation. Before you remove old belt(s) - Make a diagram of belt routing. When working in close quarters of the engine compartment it's easy to forget the routing of the belt(s). Save yourself the headache & time and make a diagram of the belt routing. If not, it's like doing a Rubiks' Cube

Pop the hood of the car and locate the serpentine belt routing diagram. The diagram on the 2003 Chevrolet Impala is on top of the fan shroud. The diagram will be a picture of all of the accessory pulleys and how the serpentine belt is routed among the pulleys. This diagram will give you the exact direction for routing the new belt onto the pulleys. If you cannot find a serpentine belt routing diagram, take a notepad and pen and draw out the exact routing diagram of the old belt across the pulleys.

2 Locate the serpentine belt tensioner. It is located below the alternator on the left front side of the engine. The tensioner on the 3.4 liter, 3.5 liter and the 3.9 liter engines will require a 3/8 ratchet to release the belt tensioner. The 3.8 liter engine will require a ratchet and a 15 mm socket for the belt tensioner. Rotate the belt tensioner clockwise to loosen and remove the belt from the belt tensioner pulley.

3 Slide the old belt from the belt tensioner pulley. Remove the belt from the remaining pulleys and pull the belt from the engine compartment.

4 Route the new belt across the crankshaft pulley first. The crankshaft pulley is the big pulley on the bottom of the engine in the center. Then route the serpentine belt across the remaining pulleys as directed by the serpentine belt routing diagram. Leave the belt tensioner pulley for last.

5 Rotate the belt tensioner clockwise again to relieve the tension and slide the new belt across the top of the belt tensioner pulley. Release the belt tensioner and the belt will tighten up as the tensioner applies pressure to the belt. Make sure the new belt is properly seated in all the pulleys.

6 Crank the engine and inspect the belt again to make sure that it stays seated in the pulleys.

Read more: How to Replace a Serpentine Belt on a 2003 Chevrolet Impala ' eHow.co.uk http://www.ehow.co.uk/how_6120665_replace-belt-2003-chevrolet-impala.html#ixzz1Yi6QLAjb

These instructions will help you . and also the picture below will help you more.
1
Locate the belt-routing diagram, printed on the fan shroud or possibly applied to the underside of the hood. This diagram shows you how to properly route the new belt, as well as identifying the location of the various pulleys, including the serpentine belt tensioner.
2
Move the belt tensioner off the Sable's belt using the serpentine belt tool. The tool connects to the side of the tensioner's arm and essentially acts as a handle for use in moving the tensioner. You can purchase a serpentine belt tool at most auto-part retailers.
3
Slip the belt away from the tensioner with your free hand and release the pressure on the tensioner with the tool. Pull the belt off the rest of the pulleys by hand and then remove it from the engine well.
4
Grab the old belt and new belt with one hand and put your foot through the other end of the belts. Stretch them both to their full length and check to ensure their both the same size. The old belt may have up to one-inch of extra length due to normal stretching that occurs during the belt's service life.
5
Wind the new belt, rib side to the pulley, around the crankshaft pulley. Consult the routing diagram and route the belt to the next pulley on the diagram. Continue to route and then check the diagram until you have the belt wound around every pulley except the belt tensioner pulley.
6
Double check to make sure the belt routes exactly as shown on the diagram and then move the tensioner aside once more so you can route the belt over the tensioner pulley as shown in the diagram. Move the tensioner back into place slowly and remove the tool.

Good luck

1. 1 Check to ensure the Sienna's engine is cold, then make sure the Sienna is in park, the ignition is in the off position and the key is removed from the ignition to prevent any possibility of the engine engaging during the serpentine belt replacement.
   
2. 2 Open the hood and locate the serpentine belt routing diagram sticker on the side of the engine casing. Study it for a moment before removing the existing belt. It is essential the new belt be installed exactly as the diagram indicates.
   
3. 3 Find the tensioner pulley, the main pulley that controls the serpentine belt, and fit a 1/2-inch socket wrench over the self-tensioner on the outside of the pulley.
   
4. 4 Pull the wrench to the left to move the pulley forward, which loosens the belt, then remove the belt.
   
5. 5 Compare the old serpentine belt with the new, replacement belt for length, width and number of grooves on the belt's underside. Be certain the replacement belt is the right part.
   
6. 6 Fit the new belt on to each pulley according to the serpentine belt routing diagram on the manufacturer's sticker, or according to the drawing made prior to removing the old belt. Fit the new belt on to the tensioner pulley last, again applying pressure on the pulley's self-tensioner with the half-inch socket wrench to loosen the pulley.
   
7. 7 Release the pressure on the self tensioner and allow the tensioner pulley to ease back into place, thereby tightening the new serpentine belt with the appropriate tension.
   
8. 8 Remove the wrench and close the hood.
   

Thank you and good luck,
Lee

There should be a factory diagram on the top of the original radiator shroud. This will show the belt routing. The easiest way to change the belt is to use a 5/8 socket on the pulley bolt on the tension wheel, attach a breaker bar or ratchet, stand on the passenger side and pull the handle to you. If you need more leverage, I like to put a large box end wrench on the handle, it works and is easier to find than a small cheater pipe. You can easily lever the tension pulley up to release the belt tension. You will need an extra pair of hands to pull the old belt out and place the new belt, following the routing guide.

2.0L Engines
See Figures 11, 12 and 13
The accessory drive belt for 2.0L engines has no provision for manual belt adjustment, since these engines use an automatic belt tensioner. Drive belt slack is taken up by the automatic tensioner.
Movement of the automatic tensioner assembly during engine operation is not a sign of a malfunctioning tensioner. This movement is the tensioner self-adjusting and is required to maintain constant belt tension.
The belt tensioner has a belt length indicator. If the indicator marks are not between the MIN and MAX marks, the belt must be replaced.
Fig. 11: Drive belt routing-2.0L SOHC engine with A/C Fig. 12: Drive belt routing-2.0L SOHC engine without A/C Fig. 13: Drive belt routing-2.0L DOHC engine