This quick video is perhaps the best one to help you diagnose your faulty radiator. Do as it says step by step
http://www.youtube.com/watch?v=wqku_Rc_YuI
in my case it was a stuck down pin under the thermostat control. The symptoms were cold in and out pipes and cold radiator. All other radiators were working fine. the black lug popped off fine to reveal the pin and slight tapping allowed the pin to come out further about 1/2 inch.

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