error code 21, 31, & 41

i have a 1990 pontiac boniville with a little over 246000 miles on it.it has a 3800 v6 in it and the codes have just started today.On the code 21 what does it mean by signal high on it and is there a way to fix it without getting a new sensor. On the code 31 im not even sure i have that one but it comes up with the other codes and im just wondering what its for, what to do to fix it,and where its located on the car . the code 41 what does it mean and what to do to fix it.