Missfire/cooling fan

I have a 91 chev. Cavalier 3.1 air, the cooling fan runs constant, I have unplugged the relay still runs, I unplug the coolant temp sensor still runs unplug both and it still runs, is it possible the ac pressure sensor is bad? What could this be, And I do have a second question same vehicle thanks to your site I found my no start condition was due to a faulty injector, I replaced the faulty and have a missfire not sure if I have another bad injector or something else, have rechecked all tests again, and everything checks good, I am thinking of a possible second injector, since my injectors and fuel rail are located under my plenum they are very difficult to access, any help on these 2 problems would be greatly appreciated. Before these problems arose the car ran awesome.