The sensor is behind the metal plate held by 3 screws on the rear of the lens assy. Remove the screws to access the sensor and the IR filter. Is is very unlikely that dust has entered here because it is sealed on the inside by a rubber gasket and clamped on the other side. Mine had dust below the IR filter that was blown out by extending the lens (with the lens assy removed) There are several openings around the lens that allow air to enter and dust to exit. The 3 screws on the sensor plate must be torqued well to make the camera focus at infinity with the zoom at minimum. (Found out the hard way) <grin>

I have a SD1300 IS and it has another case screw beneath a plastic cover just below the USB port.

Also step 4 shows 2 screws to remove on the lens that do not go through the metal frame. These should not be removed for lens removal.

It is necessary on mine to remove the 2 screws on the left side of the metal frame that attach to the lens assy.