Bank 1 sensor 1 should be switching continuously between rich and lean (that's how you know it's working) sensor 2 (the one after the cat) should be steady (that's how you know the cat is working). This is what the computer looks for and if it doesn't see a pre-programed parameter of switching and a steady #2 sensor it throws a code that SHOULD tell you what is wrong. The thing is, if you have a vac leak you can run lean and it's not a faulty sensor, the computer does not monitor the vac system. Get the vac system smoked, if its tight with no leaks, replace the sensor. They can get old and start slow switching which can also throw that code.