code 43 on a gmc jimmy

I have a 1994 gmc jimmy thats has a problem, The engine code (self diagnoses) came up with a 43 code and by the def. is what it the prob is. "Knock sensor signal was either above 3.5 volts or below 1.5 volts for at least 20 seconds" what exactly does that mean the sensor is bad or something else is causing it to be bad. The engine idles rough and at times its hard to start (constent cranking) then starts. sometimes when starting up it puffs out black smoke (i know that mean that its running rich) and at other times it has a foul odor (i believe to be the cat. working) it purges but the motor never dies do you think any of you guys could help me out. my email address is thumper3494@hotmail.com if you could help i would really apreate it. thank keith horn