My Hoover Windtunnel has a red light when area is dirty and changes to green when clean. Now both lights stay on all the time. Why

SOURCE: vacuum

If it is blowing air, then it is still clogged up somewhere, you didn't give your model number, but some of these have removable covers on each side and on the back. Also make sure that all covers are back on securely.

SOURCE: red light stays on

It sounds like the plates inside the vacuum that detect the dirt have become dirty. I don't remember where they are, but I think they're accessible if you remove a certain hose. Again, I don't know which hose it is.

SOURCE: hoover windtunnel dirt finder light stays red

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.