The fan belt on a Toyota Avensis is actually part of the serpentine belt. The serpentine belt is a long belt that connects multiple vehicle components together. As the belt ages, it will begin to crack and deteriorate. The belt will "glaze" over or show other signs of deterioration, like worn or missing teeth. When this happens, replace the fan belt on your Avensis.
Open the bonnet and locate the tension pulley in the centre of the pulley system on the front of the engine. It will be a small pulley with a nut welded to the end of it, sitting in the centre of the pulley. The pulley will not appear to be driving any vehicle component.
Turn the pulley clockwise with a socket wrench to remove tension on the pulley. On the Avensis, the nut requires a 19mm socket. You may need to slide a break over bar over the end of the socket wrench to get leverage to remove tension on the pulley.
Slide the belt off the tension pulley.
Remove the belt from the pulley system. This will take the belt off the Avensis fan, which is driven off the water pump.
Install the new belt according to the belt path stamped onto the underside of the Avensis hood. The specific belt path must be followed, or the pulley system and the vehicle's components will not function properly and you could end up damaging several critical engine components.
Orient the belt so that there is slack near the tension pulley. Turn the pulley clockwise again to allow the belt to slide over the pulley. Once the fan belt is over the pulley, release the tensioner and apply tension to the belt. The tensioner will automatically apply the correct tension.