Hoover windtunnel dirt finder light stays red. We've cleaned the elbow the roller the hose and have a clean bag. The vacuum is new only had it for 3 months.
I'm not sure about a 3 month old vac, I'd probably go the warranty route. However, for the older vacs thy start getting false positives. The sensor is simply a microphone hooked to a computer chip detecting sounds of dirt entering the bag.
To rig it to start working, you'll need to remove the tool accessory tray and elbow that enters the back of the vac, and attached to the bag. This will reveal the microphone at the elbow, and wire that runs into the bottom area of the vac. When on, a large amount of air comes from the bottom screw and area where the wires enter. My fix was to clean up the microphone contact points a little bit with a screwdriver, and then leave the vac running while putting in dampering in the hole at the bottom, and around the back of the elbow. Cotton balls and electrical tape at the bottom hole, and napkins around the outside of the elbow (wedged between the back of the vac assembly and elbow did the trick). The schematic for the vac is here: http<colon>//www<dot>vacsrus<dot>com/CartGenie/pg_U6485900<dot>asp
Look for micrphone.