Trans problems.
 

I have a 93 blazer with a 5.7 tbi. I was told that it stoped shifting. When i checked it, I found it to be in limp mode. Only 2nd and 3rd gear. I disconnected the battery over night and drove, it shifted fine for a couple miles then back in limp mode. checked the codes and got two, a 12, test active. and a 43, knock sensor error. Could the knock sensor error be putting the trans in limp mode? Any help appreciated, Thanks.