code 51 after installing modified o2 sensor
 

I installed a 93 o2 sensor on my 91 p/u. I had to solder the old harness clip onto the 93 sensor. I started the truck and it is showing a code 51(switch code). What could cause this? Im thinking it might be the blk. Wires that I soldered? There were two blk. Wires on the sensor. Does it matter which order the blk wires are in?