dirt sensor lights?

I have a Kenmore Vacuum Cleaner, Model 116.25812, Serial #D50502889. The dirt sensor light system consists of two red lights and two green lights which provide an indication of the amount of dirt particles that are passing through the sensor on its way to the dust bag. The other day while vacuuming my carpet, the system stopped functioning properly & the red lights remain on constantly. I checked my owner's manual which states that the passage of dirt through the air passage system can coat the lens causing the green light to come on & stay on - never switching to red. When this happens the lens must be cleaned. I cleaned the lens as shown in the manual even though on my machine the red light remains on not the green lights. This did not correct the problem. Do you have any suggestions as to what the problem may be? Thank you for your time.