Cannot pass emissions

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My check engine light has been on for several months. My car will not pass emissions. Several things are wrong with the car - the engine light is on, the air bag light is on, the horn won't work, the cruise control won't work, and the light showing the car is in drive doesn't work. Are these related? The mechanic said it could be the O2 sensor, knock sensor, catalytic convertor, or something else with the fuel line. He said it would cost at least $600, probably more. Any ideas? I need to get my tags and cannot until it passes emissions.