i was tryin to take off my header but didnt feel like fighting to get the bolts off with a box wrench.. so i started my car to pull it into the garage an the 'check engine' light was on.. my exhaust isnt bolted onto the cat due to rust but its been like that for about a week and the light was never on.. only one bolt was loosened from the header but was put back in.. bad o2 sensor??